diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_ADIS1647x.h b/libraries/AP_InertialSensor/AP_InertialSensor_ADIS1647x.h
index 6c2a6d2851ad7..5dc97edb2aac1 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_ADIS1647x.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_ADIS1647x.h
@@ -66,8 +66,6 @@ class AP_InertialSensor_ADIS1647x : public AP_InertialSensor_Backend {
         Delta32    =3
     } opmode;
 
-    uint8_t accel_instance;
-    uint8_t gyro_instance;
     enum Rotation rotation;
     uint8_t drdy_pin;
 
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_BMI055.h b/libraries/AP_InertialSensor/AP_InertialSensor_BMI055.h
index 5b5773a3272f0..6622906e2dd49 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_BMI055.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_BMI055.h
@@ -62,8 +62,6 @@ class AP_InertialSensor_BMI055 : public AP_InertialSensor_Backend {
     AP_HAL::OwnPtr<AP_HAL::Device> dev_accel;
     AP_HAL::OwnPtr<AP_HAL::Device> dev_gyro;
 
-    uint8_t accel_instance;
-    uint8_t gyro_instance;
     enum Rotation rotation;
     uint8_t temperature_counter;
 };
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_BMI088.h b/libraries/AP_InertialSensor/AP_InertialSensor_BMI088.h
index d8820529aa88a..005ddc2615daf 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_BMI088.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_BMI088.h
@@ -79,8 +79,6 @@ class AP_InertialSensor_BMI088 : public AP_InertialSensor_Backend {
     AP_HAL::OwnPtr<AP_HAL::Device> dev_gyro;
     AP_HAL::Device::PeriodicHandle gyro_periodic_handle;
 
-    uint8_t accel_instance;
-    uint8_t gyro_instance;
     enum Rotation rotation;
     uint8_t temperature_counter;
     enum DevTypes _accel_devtype;
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_BMI160.cpp b/libraries/AP_InertialSensor/AP_InertialSensor_BMI160.cpp
index 8659d053ffe1b..777ab3cf0e30d 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_BMI160.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_BMI160.cpp
@@ -201,8 +201,8 @@ void AP_InertialSensor_BMI160::start()
 
     _dev->get_semaphore()->give();
 
-    if (!_imu.register_accel(_accel_instance, BMI160_ODR_TO_HZ(BMI160_ODR), _dev->get_bus_id_devtype(DEVTYPE_BMI160)) ||
-        !_imu.register_gyro(_gyro_instance, BMI160_ODR_TO_HZ(BMI160_ODR),   _dev->get_bus_id_devtype(DEVTYPE_BMI160))) {
+    if (!_imu.register_accel(accel_instance, BMI160_ODR_TO_HZ(BMI160_ODR), _dev->get_bus_id_devtype(DEVTYPE_BMI160)) ||
+        !_imu.register_gyro(gyro_instance, BMI160_ODR_TO_HZ(BMI160_ODR),   _dev->get_bus_id_devtype(DEVTYPE_BMI160))) {
         return;
     }
 
@@ -213,8 +213,8 @@ void AP_InertialSensor_BMI160::start()
 
 bool AP_InertialSensor_BMI160::update()
 {
-    update_accel(_accel_instance);
-    update_gyro(_gyro_instance);
+    update_accel(accel_instance);
+    update_gyro(gyro_instance);
     return true;
 }
 
@@ -420,11 +420,11 @@ void AP_InertialSensor_BMI160::_read_fifo()
         accel *= _accel_scale;
         gyro *= _gyro_scale;
 
-        _rotate_and_correct_accel(_accel_instance, accel);
-        _rotate_and_correct_gyro(_gyro_instance, gyro);
+        _rotate_and_correct_accel(accel_instance, accel);
+        _rotate_and_correct_gyro(gyro_instance, gyro);
 
-        _notify_new_accel_raw_sample(_accel_instance, accel);
-        _notify_new_gyro_raw_sample(_gyro_instance, gyro);
+        _notify_new_accel_raw_sample(accel_instance, accel);
+        _notify_new_gyro_raw_sample(gyro_instance, gyro);
     }
 
     if (excess) {
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_BMI160.h b/libraries/AP_InertialSensor/AP_InertialSensor_BMI160.h
index 77aac9eb08f3b..fe5940015ab3f 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_BMI160.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_BMI160.h
@@ -120,10 +120,7 @@ class AP_InertialSensor_BMI160 : public AP_InertialSensor_Backend {
     AP_HAL::OwnPtr<AP_HAL::Device> _dev;
     enum Rotation _rotation;
 
-    uint8_t _accel_instance;
     float _accel_scale;
-
-    uint8_t _gyro_instance;
     float _gyro_scale;
 
     AP_HAL::DigitalSource *_int1_pin;
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_BMI270.cpp b/libraries/AP_InertialSensor/AP_InertialSensor_BMI270.cpp
index 0d3bb850718d5..41776d0143e9f 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_BMI270.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_BMI270.cpp
@@ -212,14 +212,14 @@ void AP_InertialSensor_BMI270::start()
 
     _dev->get_semaphore()->give();
 
-    if (!_imu.register_accel(_accel_instance, BMI270_BACKEND_SAMPLE_RATE, _dev->get_bus_id_devtype(DEVTYPE_BMI270)) ||
-        !_imu.register_gyro(_gyro_instance, BMI270_BACKEND_SAMPLE_RATE, _dev->get_bus_id_devtype(DEVTYPE_BMI270))) {
+    if (!_imu.register_accel(accel_instance, BMI270_BACKEND_SAMPLE_RATE, _dev->get_bus_id_devtype(DEVTYPE_BMI270)) ||
+        !_imu.register_gyro(gyro_instance, BMI270_BACKEND_SAMPLE_RATE, _dev->get_bus_id_devtype(DEVTYPE_BMI270))) {
         return;
     }
 
     // setup sensor rotations from probe()
-    set_gyro_orientation(_gyro_instance, _rotation);
-    set_accel_orientation(_accel_instance, _rotation);
+    set_gyro_orientation(gyro_instance, _rotation);
+    set_accel_orientation(accel_instance, _rotation);
 
     /* Call read_fifo() at 1600Hz */
     periodic_handle = _dev->register_periodic_callback(BACKEND_PERIOD_US, FUNCTOR_BIND_MEMBER(&AP_InertialSensor_BMI270::read_fifo, void));
@@ -227,8 +227,8 @@ void AP_InertialSensor_BMI270::start()
 
 bool AP_InertialSensor_BMI270::update()
 {
-    update_accel(_accel_instance);
-    update_gyro(_gyro_instance);
+    update_accel(accel_instance);
+    update_gyro(gyro_instance);
     return true;
 }
 
@@ -364,8 +364,8 @@ void AP_InertialSensor_BMI270::fifo_reset()
     // flush and reset FIFO
     write_register(BMI270_REG_CMD, BMI270_CMD_FIFOFLUSH);
 
-    notify_accel_fifo_reset(_accel_instance);
-    notify_gyro_fifo_reset(_gyro_instance);
+    notify_accel_fifo_reset(accel_instance);
+    notify_gyro_fifo_reset(gyro_instance);
 }
 
 /*
@@ -383,8 +383,8 @@ void AP_InertialSensor_BMI270::read_fifo(void)
 
     uint8_t len[2];
     if (!read_registers(BMI270_REG_FIFO_LENGTH_LSB, len, 2)) {
-        _inc_accel_error_count(_accel_instance);
-        _inc_gyro_error_count(_gyro_instance);
+        _inc_accel_error_count(accel_instance);
+        _inc_gyro_error_count(gyro_instance);
         return;
     }
     uint16_t fifo_length = len[0] + (len[1]<<8);
@@ -403,8 +403,8 @@ void AP_InertialSensor_BMI270::read_fifo(void)
 
     uint8_t data[fifo_length];
     if (!read_registers(BMI270_REG_FIFO_DATA, data, fifo_length)) {
-        _inc_accel_error_count(_accel_instance);
-        _inc_gyro_error_count(_gyro_instance);
+        _inc_accel_error_count(accel_instance);
+        _inc_gyro_error_count(gyro_instance);
         return;
     }
 
@@ -462,14 +462,14 @@ void AP_InertialSensor_BMI270::read_fifo(void)
         temperature_counter = 0;
         uint8_t tbuf[2];
         if (!read_registers(BMI270_REG_TEMPERATURE_LSB, tbuf, 2)) {
-            _inc_accel_error_count(_accel_instance);
-            _inc_gyro_error_count(_gyro_instance);
+            _inc_accel_error_count(accel_instance);
+            _inc_gyro_error_count(gyro_instance);
         } else {
             uint16_t tval = tbuf[0] | (tbuf[1] << 8);
             if (tval != 0x8000) {   // 0x8000 is invalid
                 int16_t klsb = static_cast<int16_t>(tval);
                 float temp_degc = klsb * 0.002f + 23.0f;
-                _publish_temperature(_accel_instance, temp_degc);
+                _publish_temperature(accel_instance, temp_degc);
             }
         }
     }
@@ -487,8 +487,8 @@ void AP_InertialSensor_BMI270::parse_accel_frame(const uint8_t* d)
 
     accel *= scale;
 
-    _rotate_and_correct_accel(_accel_instance, accel);
-    _notify_new_accel_raw_sample(_accel_instance, accel);
+    _rotate_and_correct_accel(accel_instance, accel);
+    _notify_new_accel_raw_sample(accel_instance, accel);
 }
 
 void AP_InertialSensor_BMI270::parse_gyro_frame(const uint8_t* d)
@@ -502,8 +502,8 @@ void AP_InertialSensor_BMI270::parse_gyro_frame(const uint8_t* d)
     Vector3f gyro(xyz[0], xyz[1], xyz[2]);
     gyro *= scale;
 
-    _rotate_and_correct_gyro(_gyro_instance, gyro);
-    _notify_new_gyro_raw_sample(_gyro_instance, gyro);
+    _rotate_and_correct_gyro(gyro_instance, gyro);
+    _notify_new_gyro_raw_sample(gyro_instance, gyro);
 }
 
 bool AP_InertialSensor_BMI270::hardware_init()
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_BMI270.h b/libraries/AP_InertialSensor/AP_InertialSensor_BMI270.h
index fb21ca0f57de5..0e16d230945c7 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_BMI270.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_BMI270.h
@@ -115,8 +115,6 @@ class AP_InertialSensor_BMI270 : public AP_InertialSensor_Backend {
 
     enum Rotation _rotation;
 
-    uint8_t _accel_instance;
-    uint8_t _gyro_instance;
     uint8_t temperature_counter;
 
     static const uint8_t maximum_fifo_config_file[];
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_Backend.h b/libraries/AP_InertialSensor/AP_InertialSensor_Backend.h
index e51d9362b80b5..b86b9bb57fe15 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_Backend.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_Backend.h
@@ -141,6 +141,10 @@ class AP_InertialSensor_Backend
     //Default Clip Limit
     float _clip_limit = 15.5f * GRAVITY_MSS;
 
+    // instance numbers of accel and gyro data
+    uint8_t gyro_instance;
+    uint8_t accel_instance;
+
     void _rotate_and_correct_accel(uint8_t instance, Vector3f &accel) __RAMFUNC__;
     void _rotate_and_correct_gyro(uint8_t instance, Vector3f &gyro) __RAMFUNC__;
 
@@ -281,6 +285,14 @@ class AP_InertialSensor_Backend
         _imu._accel_orientation[instance] = rotation;
     }
 
+    uint8_t get_gyro_instance() const {
+        return gyro_instance;
+    }
+
+    uint8_t get_accel_instance() const {
+        return accel_instance;
+    }
+
     // increment clipping counted. Used by drivers that do decimation before supplying
     // samples to the frontend
     void increment_clip_count(uint8_t instance) {
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_ExternalAHRS.h b/libraries/AP_InertialSensor/AP_InertialSensor_ExternalAHRS.h
index 1af9e4c033c9f..cc3853aba261b 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_ExternalAHRS.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_ExternalAHRS.h
@@ -21,8 +21,6 @@ class AP_InertialSensor_ExternalAHRS : public AP_InertialSensor_Backend
     bool get_output_banner(char* banner, uint8_t banner_len) override;
 
 private:
-    uint8_t gyro_instance;
-    uint8_t accel_instance;
     const uint8_t serial_port;
     bool started;
 };
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_Invensense.cpp b/libraries/AP_InertialSensor/AP_InertialSensor_Invensense.cpp
index 50ff76205accd..18eb0a2080fa5 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_Invensense.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_Invensense.cpp
@@ -202,8 +202,8 @@ void AP_InertialSensor_Invensense::_fifo_reset(bool log_error)
     _dev->set_speed(AP_HAL::Device::SPEED_HIGH);
     _last_stat_user_ctrl = user_ctrl | BIT_USER_CTRL_FIFO_EN;
 
-    notify_accel_fifo_reset(_accel_instance);
-    notify_gyro_fifo_reset(_gyro_instance);
+    notify_accel_fifo_reset(accel_instance);
+    notify_gyro_fifo_reset(gyro_instance);
 }
 
 void AP_InertialSensor_Invensense::_fast_fifo_reset()
@@ -211,8 +211,8 @@ void AP_InertialSensor_Invensense::_fast_fifo_reset()
     fast_reset_count++;
     _register_write(MPUREG_USER_CTRL, _last_stat_user_ctrl | BIT_USER_CTRL_FIFO_RESET);
 
-    notify_accel_fifo_reset(_accel_instance);
-    notify_gyro_fifo_reset(_gyro_instance);
+    notify_accel_fifo_reset(accel_instance);
+    notify_gyro_fifo_reset(gyro_instance);
 }
 
 
@@ -224,7 +224,7 @@ bool AP_InertialSensor_Invensense::_has_auxiliary_bus()
 void AP_InertialSensor_Invensense::start()
 {
     // pre-fetch instance numbers for checking fast sampling settings
-    if (!_imu.get_gyro_instance(_gyro_instance) || !_imu.get_accel_instance(_accel_instance)) {
+    if (!_imu.get_gyro_instance(gyro_instance) || !_imu.get_accel_instance(accel_instance)) {
         return;
     }
 
@@ -320,8 +320,8 @@ void AP_InertialSensor_Invensense::start()
         break;
     }
 
-    if (!_imu.register_gyro(_gyro_instance, 1000, _dev->get_bus_id_devtype(gdev)) ||
-        !_imu.register_accel(_accel_instance, 1000, _dev->get_bus_id_devtype(adev))) {
+    if (!_imu.register_gyro(gyro_instance, 1000, _dev->get_bus_id_devtype(gdev)) ||
+        !_imu.register_accel(accel_instance, 1000, _dev->get_bus_id_devtype(adev))) {
         return;
     }
 
@@ -329,12 +329,12 @@ void AP_InertialSensor_Invensense::start()
     _set_filter_register();
 
     // update backend sample rate
-    _set_accel_raw_sample_rate(_accel_instance, _accel_backend_rate_hz);
-    _set_gyro_raw_sample_rate(_gyro_instance, _gyro_backend_rate_hz);
+    _set_accel_raw_sample_rate(accel_instance, _accel_backend_rate_hz);
+    _set_gyro_raw_sample_rate(gyro_instance, _gyro_backend_rate_hz);
 
     // indicate what multiplier is appropriate for the sensors'
     // readings to fit them into an int16_t:
-    _set_raw_sample_accel_multiplier(_accel_instance, multiplier_accel);
+    _set_raw_sample_accel_multiplier(accel_instance, multiplier_accel);
 
     // set sample rate to 1000Hz and apply a software filter
     // In this configuration, the gyro sample rate is 8kHz
@@ -426,8 +426,8 @@ void AP_InertialSensor_Invensense::start()
     _dev->set_speed(AP_HAL::Device::SPEED_HIGH);
 
     // setup sensor rotations from probe()
-    set_gyro_orientation(_gyro_instance, _rotation);
-    set_accel_orientation(_accel_instance, _rotation);
+    set_gyro_orientation(gyro_instance, _rotation);
+    set_accel_orientation(accel_instance, _rotation);
 
     // setup scale factors for fifo data after downsampling
     _fifo_accel_scale = _accel_scale / _accel_fifo_downsample_rate;
@@ -447,7 +447,7 @@ void AP_InertialSensor_Invensense::start()
 bool AP_InertialSensor_Invensense::get_output_banner(char* banner, uint8_t banner_len) {
     if (_fast_sampling) {
         snprintf(banner, banner_len, "IMU%u: fast sampling enabled %.1fkHz/%.1fkHz",
-            _gyro_instance, _gyro_backend_rate_hz * _gyro_fifo_downsample_rate * 0.001, _gyro_backend_rate_hz * 0.001);
+            gyro_instance, _gyro_backend_rate_hz * _gyro_fifo_downsample_rate * 0.001, _gyro_backend_rate_hz * 0.001);
         return true;
     }
     return false;
@@ -459,10 +459,10 @@ bool AP_InertialSensor_Invensense::get_output_banner(char* banner, uint8_t banne
  */
 bool AP_InertialSensor_Invensense::update() /* front end */
 {
-    update_accel(_accel_instance);
-    update_gyro(_gyro_instance);
+    update_accel(accel_instance);
+    update_gyro(gyro_instance);
 
-    _publish_temperature(_accel_instance, _temp_filtered);
+    _publish_temperature(accel_instance, _temp_filtered);
 
     if (fast_reset_count) {
         // check if we have reported in the last 1 seconds or
@@ -472,13 +472,13 @@ bool AP_InertialSensor_Invensense::update() /* front end */
         if (now - last_fast_reset_count_report_ms > 5000U) {
             last_fast_reset_count_report_ms = now;
             char param_name[sizeof("IMUxx_RST")];
-            snprintf(param_name, sizeof(param_name), "IMU%u_RST", MIN(_gyro_instance,9));
+            snprintf(param_name, sizeof(param_name), "IMU%u_RST", MIN(gyro_instance,9));
             gcs().send_named_float(param_name, fast_reset_count);
         }
 #endif
 #if HAL_LOGGING_ENABLED
         if (last_fast_reset_count != fast_reset_count) {
-            AP::logger().Write_MessageF("IMU%u fast fifo reset %u", _gyro_instance, fast_reset_count);
+            AP::logger().Write_MessageF("IMU%u fast fifo reset %u", gyro_instance, fast_reset_count);
             last_fast_reset_count = fast_reset_count;
         }
 #endif
@@ -602,7 +602,7 @@ bool AP_InertialSensor_Invensense::_accumulate(uint8_t *samples, uint8_t n_sampl
                 return false;
             } else {
                 if (!hal.scheduler->in_expected_delay()) {
-                    debug("temp reset IMU[%u] %d %d", _accel_instance, _raw_temp, t2);
+                    debug("temp reset IMU[%u] %d %d", accel_instance, _raw_temp, t2);
                 }
                 _fifo_reset(true);
                 return false;
@@ -615,11 +615,11 @@ bool AP_InertialSensor_Invensense::_accumulate(uint8_t *samples, uint8_t n_sampl
                         -int16_val(data, 6));
         gyro *= _gyro_scale;
 
-        _rotate_and_correct_accel(_accel_instance, accel);
-        _rotate_and_correct_gyro(_gyro_instance, gyro);
+        _rotate_and_correct_accel(accel_instance, accel);
+        _rotate_and_correct_gyro(gyro_instance, gyro);
 
-        _notify_new_accel_raw_sample(_accel_instance, accel, 0, fsync_set);
-        _notify_new_gyro_raw_sample(_gyro_instance, gyro);
+        _notify_new_accel_raw_sample(accel_instance, accel, 0, fsync_set);
+        _notify_new_gyro_raw_sample(gyro_instance, gyro);
 
         _temp_filtered = _temp_filter.apply(temp);
     }
@@ -652,7 +652,7 @@ bool AP_InertialSensor_Invensense::_accumulate_sensor_rate_sampling(uint8_t *sam
                 ret = false;
             } else {
                 if (!hal.scheduler->in_expected_delay()) {
-                    debug("temp reset IMU[%u] %d %d", _accel_instance, _raw_temp, t2);
+                    debug("temp reset IMU[%u] %d %d", accel_instance, _raw_temp, t2);
                 }
                 _fifo_reset(true);
                 ret = false;
@@ -674,14 +674,14 @@ bool AP_InertialSensor_Invensense::_accumulate_sensor_rate_sampling(uint8_t *sam
             _accum.accel += _accum.accel_filter.apply(a);
 
             Vector3f a2 = a * _accel_scale;
-            _notify_new_accel_sensor_rate_sample(_accel_instance, a2);
+            _notify_new_accel_sensor_rate_sample(accel_instance, a2);
 
             _accum.accel_count++;
 
             if (_accum.accel_count % _accel_fifo_downsample_rate == 0) {
                 _accum.accel *= _fifo_accel_scale;
-                _rotate_and_correct_accel(_accel_instance, _accum.accel);
-                _notify_new_accel_raw_sample(_accel_instance, _accum.accel, 0, false);
+                _rotate_and_correct_accel(accel_instance, _accum.accel);
+                _notify_new_accel_raw_sample(accel_instance, _accum.accel, 0, false);
                 _accum.accel.zero();
                 _accum.accel_count = 0;
                 // we assume that the gyro rate is always >= and a multiple of the accel rate
@@ -696,20 +696,20 @@ bool AP_InertialSensor_Invensense::_accumulate_sensor_rate_sampling(uint8_t *sam
                    -int16_val(data, 6));
 
         Vector3f g2 = g * _gyro_scale;
-        _notify_new_gyro_sensor_rate_sample(_gyro_instance, g2);
+        _notify_new_gyro_sensor_rate_sample(gyro_instance, g2);
 
         _accum.gyro += g;
 
         if (_accum.gyro_count % _gyro_fifo_downsample_rate == 0) {
             _accum.gyro *= _fifo_gyro_scale;
-            _rotate_and_correct_gyro(_gyro_instance, _accum.gyro);
-            _notify_new_gyro_raw_sample(_gyro_instance, _accum.gyro);
+            _rotate_and_correct_gyro(gyro_instance, _accum.gyro);
+            _notify_new_gyro_raw_sample(gyro_instance, _accum.gyro);
             _accum.gyro.zero();
         }
     }
 
     if (clipped) {
-        increment_clip_count(_accel_instance);
+        increment_clip_count(accel_instance);
     }
 
     if (ret) {
@@ -788,7 +788,7 @@ void AP_InertialSensor_Invensense::_read_fifo()
         if (_fast_sampling) {
             if (!_accumulate_sensor_rate_sampling(rx, n)) {
                 if (!hal.scheduler->in_expected_delay() && !_enable_fast_fifo_reset) {
-                    debug("IMU[%u] stop at %u of %u", _accel_instance, n_samples, bytes_read/MPU_SAMPLE_SIZE);
+                    debug("IMU[%u] stop at %u of %u", accel_instance, n_samples, bytes_read/MPU_SAMPLE_SIZE);
                 }
                 break;
             }
@@ -831,8 +831,8 @@ void AP_InertialSensor_Invensense::_read_fifo()
     AP_HAL::Device::checkreg reg;
     if (!_dev->check_next_register(reg)) {
         log_register_change(_dev->get_bus_id(), reg);
-        _inc_gyro_error_count(_gyro_instance);
-        _inc_accel_error_count(_accel_instance);
+        _inc_gyro_error_count(gyro_instance);
+        _inc_accel_error_count(accel_instance);
     }
     _dev->set_speed(AP_HAL::Device::SPEED_HIGH);
 }
@@ -890,7 +890,7 @@ void AP_InertialSensor_Invensense::_set_filter_register(void)
     _gyro_to_accel_sample_ratio = 2;
     _gyro_backend_rate_hz = _accel_backend_rate_hz =  1000;
     
-    if (enable_fast_sampling(_accel_instance)) {
+    if (enable_fast_sampling(accel_instance)) {
         _fast_sampling = _dev->bus_type() == AP_HAL::Device::BUS_TYPE_SPI;
         if (_fast_sampling) {
             // constrain the gyro rate to be at least the loop rate
@@ -919,11 +919,11 @@ void AP_InertialSensor_Invensense::_set_filter_register(void)
             }
 
             // for logging purposes set the oversamping rate
-            _set_accel_oversampling(_accel_instance, _accel_fifo_downsample_rate);
-            _set_gyro_oversampling(_gyro_instance, _gyro_fifo_downsample_rate);
+            _set_accel_oversampling(accel_instance, _accel_fifo_downsample_rate);
+            _set_gyro_oversampling(gyro_instance, _gyro_fifo_downsample_rate);
 
-            _set_accel_sensor_rate_sampling_enabled(_accel_instance, true);
-            _set_gyro_sensor_rate_sampling_enabled(_gyro_instance, true);
+            _set_accel_sensor_rate_sampling_enabled(accel_instance, true);
+            _set_gyro_sensor_rate_sampling_enabled(gyro_instance, true);
 
             /* set divider for internal sample rate to 0x1F when fast
              sampling enabled. This reduces the impact of the slave
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_Invensense.h b/libraries/AP_InertialSensor/AP_InertialSensor_Invensense.h
index 7f77e35e48aaf..522dfd527d3c3 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_Invensense.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_Invensense.h
@@ -115,10 +115,6 @@ class AP_InertialSensor_Invensense : public AP_InertialSensor_Backend
 
     int16_t _raw_temp;
     
-    // instance numbers of accel and gyro data
-    uint8_t _gyro_instance;
-    uint8_t _accel_instance;
-
     float temp_sensitivity = 1.0f/340; // degC/LSB
     float temp_zero = 36.53f; // degC
     
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev2.cpp b/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev2.cpp
index 1e8be9d2b56a1..b8279d339fd62 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev2.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev2.cpp
@@ -155,8 +155,8 @@ void AP_InertialSensor_Invensensev2::_fifo_reset()
     _dev->set_speed(AP_HAL::Device::SPEED_HIGH);
     _last_stat_user_ctrl = user_ctrl | BIT_USER_CTRL_FIFO_EN;
 
-    notify_accel_fifo_reset(_accel_instance);
-    notify_gyro_fifo_reset(_gyro_instance);
+    notify_accel_fifo_reset(accel_instance);
+    notify_gyro_fifo_reset(gyro_instance);
 }
 
 bool AP_InertialSensor_Invensensev2::_has_auxiliary_bus()
@@ -167,7 +167,7 @@ bool AP_InertialSensor_Invensensev2::_has_auxiliary_bus()
 void AP_InertialSensor_Invensensev2::start()
 {
     // pre-fetch instance numbers for checking fast sampling settings
-    if (!_imu.get_gyro_instance(_gyro_instance) || !_imu.get_accel_instance(_accel_instance)) {
+    if (!_imu.get_gyro_instance(gyro_instance) || !_imu.get_accel_instance(accel_instance)) {
         return;
     }
 
@@ -207,8 +207,8 @@ void AP_InertialSensor_Invensensev2::start()
         break;
     }
 
-    if (!_imu.register_gyro(_gyro_instance, 1125, _dev->get_bus_id_devtype(gdev)) ||
-        !_imu.register_accel(_accel_instance, 1125, _dev->get_bus_id_devtype(adev))) {
+    if (!_imu.register_gyro(gyro_instance, 1125, _dev->get_bus_id_devtype(gdev)) ||
+        !_imu.register_accel(accel_instance, 1125, _dev->get_bus_id_devtype(adev))) {
         return;
     }
 
@@ -219,12 +219,12 @@ void AP_InertialSensor_Invensensev2::start()
     _register_write(INV2REG_FSYNC_CONFIG, FSYNC_CONFIG_EXT_SYNC_AZ, true);
 #endif
     // update backend sample rate
-    _set_accel_raw_sample_rate(_accel_instance, _accel_backend_rate_hz);
-    _set_gyro_raw_sample_rate(_gyro_instance, _gyro_backend_rate_hz);
+    _set_accel_raw_sample_rate(accel_instance, _accel_backend_rate_hz);
+    _set_gyro_raw_sample_rate(gyro_instance, _gyro_backend_rate_hz);
 
     // indicate what multiplier is appropriate for the sensors'
     // readings to fit them into an int16_t:
-    _set_raw_sample_accel_multiplier(_accel_instance, multiplier_accel);
+    _set_raw_sample_accel_multiplier(accel_instance, multiplier_accel);
 
     // set sample rate to 1.125KHz
     _register_write(INV2REG_GYRO_SMPLRT_DIV, 0, true);
@@ -238,8 +238,8 @@ void AP_InertialSensor_Invensensev2::start()
     _dev->set_speed(AP_HAL::Device::SPEED_HIGH);
 
     // setup sensor rotations from probe()
-    set_gyro_orientation(_gyro_instance, _rotation);
-    set_accel_orientation(_accel_instance, _rotation);
+    set_gyro_orientation(gyro_instance, _rotation);
+    set_accel_orientation(accel_instance, _rotation);
 
     // setup scale factors for fifo data after downsampling
     _fifo_accel_scale = _accel_scale / _accel_fifo_downsample_rate;
@@ -259,7 +259,7 @@ void AP_InertialSensor_Invensensev2::start()
 bool AP_InertialSensor_Invensensev2::get_output_banner(char* banner, uint8_t banner_len) {
     if (_fast_sampling) {
         snprintf(banner, banner_len, "IMU%u: fast sampling enabled %.1fkHz/%.1fkHz",
-            _gyro_instance, _gyro_backend_rate_hz * _gyro_fifo_downsample_rate * 0.001, _gyro_backend_rate_hz * 0.001);
+            gyro_instance, _gyro_backend_rate_hz * _gyro_fifo_downsample_rate * 0.001, _gyro_backend_rate_hz * 0.001);
         return true;
     }
     return false;
@@ -270,10 +270,10 @@ bool AP_InertialSensor_Invensensev2::get_output_banner(char* banner, uint8_t ban
  */
 bool AP_InertialSensor_Invensensev2::update()
 {
-    update_accel(_accel_instance);
-    update_gyro(_gyro_instance);
+    update_accel(accel_instance);
+    update_gyro(gyro_instance);
 
-    _publish_temperature(_accel_instance, _temp_filtered);
+    _publish_temperature(accel_instance, _temp_filtered);
 
     return true;
 }
@@ -341,7 +341,7 @@ bool AP_InertialSensor_Invensensev2::_accumulate(uint8_t *samples, uint8_t n_sam
         int16_t t2 = int16_val(data, 6);
         if (!_check_raw_temp(t2)) {
             if (!hal.scheduler->in_expected_delay()) {
-                debug("temp reset IMU[%u] %d %d", _accel_instance, _raw_temp, t2);
+                debug("temp reset IMU[%u] %d %d", accel_instance, _raw_temp, t2);
             }
             _fifo_reset();
             return false;
@@ -353,11 +353,11 @@ bool AP_InertialSensor_Invensensev2::_accumulate(uint8_t *samples, uint8_t n_sam
                         -int16_val(data, 5));
         gyro *= GYRO_SCALE;
 
-        _rotate_and_correct_accel(_accel_instance, accel);
-        _rotate_and_correct_gyro(_gyro_instance, gyro);
+        _rotate_and_correct_accel(accel_instance, accel);
+        _rotate_and_correct_gyro(gyro_instance, gyro);
 
-        _notify_new_accel_raw_sample(_accel_instance, accel, 0, fsync_set);
-        _notify_new_gyro_raw_sample(_gyro_instance, gyro);
+        _notify_new_accel_raw_sample(accel_instance, accel, 0, fsync_set);
+        _notify_new_gyro_raw_sample(gyro_instance, gyro);
 
         _temp_filtered = _temp_filter.apply(temp);
     }
@@ -386,7 +386,7 @@ bool AP_InertialSensor_Invensensev2::_accumulate_sensor_rate_sampling(uint8_t *s
         int16_t t2 = int16_val(data, 6);
         if (!_check_raw_temp(t2)) {
             if (!hal.scheduler->in_expected_delay()) {
-                debug("temp reset IMU[%u] %d %d", _accel_instance, _raw_temp, t2);
+                debug("temp reset IMU[%u] %d %d", accel_instance, _raw_temp, t2);
             }
             _fifo_reset();
             ret = false;
@@ -406,14 +406,14 @@ bool AP_InertialSensor_Invensensev2::_accumulate_sensor_rate_sampling(uint8_t *s
             _accum.accel += _accum.accel_filter.apply(a);
 
             Vector3f a2 = a * _accel_scale;
-            _notify_new_accel_sensor_rate_sample(_accel_instance, a2);
+            _notify_new_accel_sensor_rate_sample(accel_instance, a2);
 
             _accum.accel_count++;
 
             if (_accum.accel_count % _accel_fifo_downsample_rate == 0) {
                 _accum.accel *= _fifo_accel_scale;
-                _rotate_and_correct_accel(_accel_instance, _accum.accel);
-                _notify_new_accel_raw_sample(_accel_instance, _accum.accel, 0, false);
+                _rotate_and_correct_accel(accel_instance, _accum.accel);
+                _notify_new_accel_raw_sample(accel_instance, _accum.accel, 0, false);
                 _accum.accel.zero();
                 _accum.accel_count = 0;
                 // we assume that the gyro rate is always >= and a multiple of the accel rate
@@ -428,20 +428,20 @@ bool AP_InertialSensor_Invensensev2::_accumulate_sensor_rate_sampling(uint8_t *s
                    -int16_val(data, 5));
 
         Vector3f g2 = g * GYRO_SCALE;
-        _notify_new_gyro_sensor_rate_sample(_gyro_instance, g2);
+        _notify_new_gyro_sensor_rate_sample(gyro_instance, g2);
 
         _accum.gyro += g;
 
         if (_accum.gyro_count % _gyro_fifo_downsample_rate == 0) {
             _accum.gyro *= _fifo_gyro_scale;
-            _rotate_and_correct_gyro(_gyro_instance, _accum.gyro);
-            _notify_new_gyro_raw_sample(_gyro_instance, _accum.gyro);
+            _rotate_and_correct_gyro(gyro_instance, _accum.gyro);
+            _notify_new_gyro_raw_sample(gyro_instance, _accum.gyro);
             _accum.gyro.zero();
         }
     }
 
     if (clipped) {
-        increment_clip_count(_accel_instance);
+        increment_clip_count(accel_instance);
     }
 
     if (ret) {
@@ -519,7 +519,7 @@ void AP_InertialSensor_Invensensev2::_read_fifo()
         if (_fast_sampling) {
             if (!_accumulate_sensor_rate_sampling(rx, n)) {
                 if (!hal.scheduler->in_expected_delay()) {
-                    debug("IMU[%u] stop at %u of %u", _accel_instance, n_samples, bytes_read/INV2_SAMPLE_SIZE);
+                    debug("IMU[%u] stop at %u of %u", accel_instance, n_samples, bytes_read/INV2_SAMPLE_SIZE);
                 }
                 break;
             }
@@ -542,8 +542,8 @@ void AP_InertialSensor_Invensensev2::_read_fifo()
     AP_HAL::Device::checkreg reg;
     if (!_dev->check_next_register(reg)) {
         log_register_change(_dev->get_bus_id(), reg);
-        _inc_gyro_error_count(_gyro_instance);
-        _inc_accel_error_count(_accel_instance);
+        _inc_gyro_error_count(gyro_instance);
+        _inc_accel_error_count(accel_instance);
     }
     _dev->set_speed(AP_HAL::Device::SPEED_HIGH);
 }
@@ -608,7 +608,7 @@ void AP_InertialSensor_Invensensev2::_set_filter_and_scaling(void)
     _gyro_fifo_downsample_rate = _accel_fifo_downsample_rate = 1;
     _gyro_backend_rate_hz = _accel_backend_rate_hz =  1125;
     
-    if (enable_fast_sampling(_accel_instance)) {
+    if (enable_fast_sampling(accel_instance)) {
         _fast_sampling = _dev->bus_type() == AP_HAL::Device::BUS_TYPE_SPI;
         if (_fast_sampling) {
             // constrain the gyro rate to be at least the loop rate
@@ -631,11 +631,11 @@ void AP_InertialSensor_Invensensev2::_set_filter_and_scaling(void)
             _accel_backend_rate_hz *= MIN(fast_sampling_rate, 4);
 
             // for logging purposes set the oversamping rate
-            _set_accel_oversampling(_accel_instance, _accel_fifo_downsample_rate);
-            _set_gyro_oversampling(_gyro_instance, _gyro_fifo_downsample_rate);
+            _set_accel_oversampling(accel_instance, _accel_fifo_downsample_rate);
+            _set_gyro_oversampling(gyro_instance, _gyro_fifo_downsample_rate);
 
-            _set_accel_sensor_rate_sampling_enabled(_accel_instance, true);
-            _set_gyro_sensor_rate_sampling_enabled(_gyro_instance, true);
+            _set_accel_sensor_rate_sampling_enabled(accel_instance, true);
+            _set_gyro_sensor_rate_sampling_enabled(gyro_instance, true);
 
             /* set divider for internal sample rate to 0x1F when fast
              sampling enabled. This reduces the impact of the slave
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev2.h b/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev2.h
index 9becc1d60e644..b6e21d0034d1d 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev2.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev2.h
@@ -103,10 +103,6 @@ class AP_InertialSensor_Invensensev2 : public AP_InertialSensor_Backend
 
     int16_t _raw_temp;
     
-    // instance numbers of accel and gyro data
-    uint8_t _gyro_instance;
-    uint8_t _accel_instance;
-
     float temp_sensitivity = 1.0f/333.87f; // degC/LSB
     float temp_zero = 21; // degC
     
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev3.h b/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev3.h
index aa9971db0d48e..b3946aa81dd47 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev3.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_Invensensev3.h
@@ -74,10 +74,6 @@ class AP_InertialSensor_Invensensev3 : public AP_InertialSensor_Backend
     bool accumulate_samples(const struct FIFOData *data, uint8_t n_samples);
     bool accumulate_highres_samples(const struct FIFODataHighRes *data, uint8_t n_samples);
 
-    // instance numbers of accel and gyro data
-    uint8_t gyro_instance;
-    uint8_t accel_instance;
-
     // reset FIFO configure1 register
     uint8_t fifo_config1;
 
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_L3G4200D.cpp b/libraries/AP_InertialSensor/AP_InertialSensor_L3G4200D.cpp
index 0d97d71f8448d..82a19e2261656 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_L3G4200D.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_L3G4200D.cpp
@@ -250,8 +250,8 @@ bool AP_InertialSensor_L3G4200D::_init_sensor(void)
  */
 void AP_InertialSensor_L3G4200D::start(void)
 {
-    if (!_imu.register_gyro(_gyro_instance, 800, _dev_gyro->get_bus_id_devtype(DEVTYPE_L3G4200D)) ||
-        !_imu.register_accel(_accel_instance, 800, _dev_accel->get_bus_id_devtype(DEVTYPE_L3G4200D))) {
+    if (!_imu.register_gyro(gyro_instance, 800, _dev_gyro->get_bus_id_devtype(DEVTYPE_L3G4200D)) ||
+        !_imu.register_accel(accel_instance, 800, _dev_accel->get_bus_id_devtype(DEVTYPE_L3G4200D))) {
         return;
     }
 
@@ -276,8 +276,8 @@ void AP_InertialSensor_L3G4200D::_set_filter_frequency(uint8_t filter_hz)
  */
 bool AP_InertialSensor_L3G4200D::update(void)
 {
-    update_gyro(_gyro_instance);
-    update_accel(_accel_instance);
+    update_gyro(gyro_instance);
+    update_accel(accel_instance);
 
     return true;
 }
@@ -313,8 +313,8 @@ void AP_InertialSensor_L3G4200D::_accumulate_gyro (void)
                 // Adjust for chip scaling to get radians/sec
                 //hal.console->printf("gyro %f \r\n",gyro.x); 
                 gyro *= L3G4200D_GYRO_SCALE_R_S;
-                _rotate_and_correct_gyro(_gyro_instance, gyro);
-                _notify_new_gyro_raw_sample(_gyro_instance, gyro);
+                _rotate_and_correct_gyro(gyro_instance, gyro);
+                _notify_new_gyro_raw_sample(gyro_instance, gyro);
             }
         }
     }
@@ -341,8 +341,8 @@ void AP_InertialSensor_L3G4200D::_accumulate_accel (void)
                 Vector3f accel = Vector3f(buffer[i][0], -buffer[i][1], -buffer[i][2]);
                 // Adjust for chip scaling to get m/s/s
                 accel *= ADXL345_ACCELEROMETER_SCALE_M_S;
-                _rotate_and_correct_accel(_accel_instance, accel);
-                _notify_new_accel_raw_sample(_accel_instance, accel);
+                _rotate_and_correct_accel(accel_instance, accel);
+                _notify_new_accel_raw_sample(accel_instance, accel);
             }
         }
     } 
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_L3G4200D.h b/libraries/AP_InertialSensor/AP_InertialSensor_L3G4200D.h
index 535613571d49e..b97b430f6df16 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_L3G4200D.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_L3G4200D.h
@@ -52,9 +52,5 @@ class AP_InertialSensor_L3G4200D : public AP_InertialSensor_Backend
     LowPassFilter2pVector3f _gyro_filter;
 
     enum Rotation _rotation; 
-
-    // gyro and accel instances
-    uint8_t _gyro_instance;
-    uint8_t _accel_instance;
 };
 #endif // __AP_INERTIAL_SENSOR_L3G4200D2_H__
\ No newline at end of file
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS0.cpp b/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS0.cpp
index e6d91738cd4e1..4c1919c734bb9 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS0.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS0.cpp
@@ -512,19 +512,19 @@ bool AP_InertialSensor_LSM9DS0::_hardware_init()
  */
 void AP_InertialSensor_LSM9DS0::start(void)
 {
-    if (!_imu.register_gyro(_gyro_instance, 760, _dev_gyro->get_bus_id_devtype(DEVTYPE_GYR_L3GD20)) ||
-        !_imu.register_accel(_accel_instance, 1000, _dev_accel->get_bus_id_devtype(DEVTYPE_ACC_LSM303D))) {
+    if (!_imu.register_gyro(gyro_instance, 760, _dev_gyro->get_bus_id_devtype(DEVTYPE_GYR_L3GD20)) ||
+        !_imu.register_accel(accel_instance, 1000, _dev_accel->get_bus_id_devtype(DEVTYPE_ACC_LSM303D))) {
         return;
     }
 
     if (whoami_g == LSM9DS0_G_WHOAMI_H) {
-        set_gyro_orientation(_gyro_instance, _rotation_gH);
+        set_gyro_orientation(gyro_instance, _rotation_gH);
     } else {
-        set_gyro_orientation(_gyro_instance, _rotation_g);
+        set_gyro_orientation(gyro_instance, _rotation_g);
     }
-    set_accel_orientation(_accel_instance, _rotation_a);
+    set_accel_orientation(accel_instance, _rotation_a);
     
-    _set_accel_max_abs_offset(_accel_instance, 5.0f);
+    _set_accel_max_abs_offset(accel_instance, 5.0f);
 
     /* start the timer process to read samples */
     _dev_gyro->register_periodic_callback(1000, FUNCTOR_BIND_MEMBER(&AP_InertialSensor_LSM9DS0::_poll_data, void));
@@ -700,11 +700,11 @@ void AP_InertialSensor_LSM9DS0::_poll_data()
     AP_HAL::Device::checkreg reg;
     if (!_dev_gyro->check_next_register(reg)) {
         log_register_change(_dev_gyro->get_bus_id(), reg);
-        _inc_gyro_error_count(_gyro_instance);
+        _inc_gyro_error_count(gyro_instance);
     }
     if (!_dev_accel->check_next_register(reg)) {
         log_register_change(_dev_accel->get_bus_id(), reg);
-        _inc_accel_error_count(_accel_instance);
+        _inc_accel_error_count(accel_instance);
     }
 }
 
@@ -740,8 +740,8 @@ void AP_InertialSensor_LSM9DS0::_read_data_transaction_a()
     Vector3f accel_data(raw_data.x, -raw_data.y, -raw_data.z);
     accel_data *= _accel_scale;
 
-    _rotate_and_correct_accel(_accel_instance, accel_data);
-    _notify_new_accel_raw_sample(_accel_instance, accel_data, AP_HAL::micros64());
+    _rotate_and_correct_accel(accel_instance, accel_data);
+    _notify_new_accel_raw_sample(accel_instance, accel_data, AP_HAL::micros64());
 
     // read temperature every 10th sample
     if (_temp_counter++ >= 10) {
@@ -770,15 +770,15 @@ void AP_InertialSensor_LSM9DS0::_read_data_transaction_g()
 
     gyro_data *= _gyro_scale;
 
-    _rotate_and_correct_gyro(_gyro_instance, gyro_data);
-    _notify_new_gyro_raw_sample(_gyro_instance, gyro_data, AP_HAL::micros64());
+    _rotate_and_correct_gyro(gyro_instance, gyro_data);
+    _notify_new_gyro_raw_sample(gyro_instance, gyro_data, AP_HAL::micros64());
 }
 
 bool AP_InertialSensor_LSM9DS0::update()
 {
-    update_gyro(_gyro_instance);
-    update_accel(_accel_instance);
-    _publish_temperature(_accel_instance, _temperature);
+    update_gyro(gyro_instance);
+    update_accel(accel_instance);
+    _publish_temperature(accel_instance, _temperature);
 
     return true;
 }
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS0.h b/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS0.h
index 29fff90270541..18244897a24ee 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS0.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS0.h
@@ -97,8 +97,6 @@ class AP_InertialSensor_LSM9DS0 : public AP_InertialSensor_Backend
     float _accel_scale;
     int _drdy_pin_num_a;
     int _drdy_pin_num_g;
-    uint8_t _gyro_instance;
-    uint8_t _accel_instance;
     float _temperature;
     uint8_t _temp_counter;
     LowPassFilter2pFloat _temp_filter;
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS1.cpp b/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS1.cpp
index df73ecb62fe04..d95cfce0f0fcc 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS1.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS1.cpp
@@ -321,8 +321,8 @@ void AP_InertialSensor_LSM9DS1::_fifo_reset()
 
     _dev->set_speed(AP_HAL::Device::SPEED_HIGH);
 
-    notify_accel_fifo_reset(_accel_instance);
-    notify_gyro_fifo_reset(_gyro_instance);
+    notify_accel_fifo_reset(accel_instance);
+    notify_gyro_fifo_reset(gyro_instance);
 }
 
 /*
@@ -330,15 +330,15 @@ void AP_InertialSensor_LSM9DS1::_fifo_reset()
  */
 void AP_InertialSensor_LSM9DS1::start(void)
 {
-    if (!_imu.register_gyro(_gyro_instance, 952, _dev->get_bus_id_devtype(DEVTYPE_GYR_LSM9DS1)) ||
-        !_imu.register_accel(_accel_instance, 952, _dev->get_bus_id_devtype(DEVTYPE_ACC_LSM9DS1))) {
+    if (!_imu.register_gyro(gyro_instance, 952, _dev->get_bus_id_devtype(DEVTYPE_GYR_LSM9DS1)) ||
+        !_imu.register_accel(accel_instance, 952, _dev->get_bus_id_devtype(DEVTYPE_ACC_LSM9DS1))) {
         return;
     }
 
-    set_accel_orientation(_accel_instance, _rotation);
-    set_gyro_orientation(_gyro_instance, _rotation);
+    set_accel_orientation(accel_instance, _rotation);
+    set_gyro_orientation(gyro_instance, _rotation);
 
-    _set_accel_max_abs_offset(_accel_instance, 5.0f);
+    _set_accel_max_abs_offset(accel_instance, 5.0f);
 
     /* start the timer process to read samples */
     _dev->register_periodic_callback(1000, FUNCTOR_BIND_MEMBER(&AP_InertialSensor_LSM9DS1::_poll_data, void));
@@ -431,7 +431,7 @@ void AP_InertialSensor_LSM9DS1::_poll_data()
     AP_HAL::Device::checkreg reg;
     if (!_dev->check_next_register(reg)) {
         log_register_change(_dev->get_bus_id(), reg);
-        _inc_accel_error_count(_accel_instance);
+        _inc_accel_error_count(accel_instance);
     }
 }
 
@@ -465,8 +465,8 @@ void AP_InertialSensor_LSM9DS1::_read_data_transaction_x(uint16_t samples)
     Vector3f accel_data(raw_data.x, raw_data.y, -raw_data.z);
     accel_data *= _accel_scale;
 
-    _rotate_and_correct_accel(_accel_instance, accel_data);
-    _notify_new_accel_raw_sample(_accel_instance, accel_data);
+    _rotate_and_correct_accel(accel_instance, accel_data);
+    _notify_new_accel_raw_sample(accel_instance, accel_data);
 }
 
 /*
@@ -501,14 +501,14 @@ void AP_InertialSensor_LSM9DS1::_read_data_transaction_g(uint16_t samples)
     Vector3f gyro_data(raw_data.x, raw_data.y, -raw_data.z);
     gyro_data *= _gyro_scale;
 
-    _rotate_and_correct_gyro(_gyro_instance, gyro_data);
-    _notify_new_gyro_raw_sample(_gyro_instance, gyro_data);
+    _rotate_and_correct_gyro(gyro_instance, gyro_data);
+    _notify_new_gyro_raw_sample(gyro_instance, gyro_data);
 }
 
 bool AP_InertialSensor_LSM9DS1::update()
 {
-    update_gyro(_gyro_instance);
-    update_accel(_accel_instance);
+    update_gyro(gyro_instance);
+    update_accel(accel_instance);
 
     return true;
 }
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS1.h b/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS1.h
index f52859c1bbd96..7997476abac39 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS1.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_LSM9DS1.h
@@ -68,7 +68,5 @@ class AP_InertialSensor_LSM9DS1 : public AP_InertialSensor_Backend
     float _gyro_scale;
     float _accel_scale;
     int _drdy_pin_num_xg;
-    uint8_t _gyro_instance;
-    uint8_t _accel_instance;
     enum Rotation _rotation;
 };
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_RST.cpp b/libraries/AP_InertialSensor/AP_InertialSensor_RST.cpp
index 860ae4aa261db..fa061c4078125 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_RST.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_RST.cpp
@@ -336,13 +336,13 @@ bool AP_InertialSensor_RST::_init_sensor(void)
  */
 void AP_InertialSensor_RST::start(void)
 {
-    if (!_imu.register_gyro(_gyro_instance, 800, _dev_gyro->get_bus_id_devtype(DEVTYPE_GYR_I3G4250D)) ||
-        !_imu.register_accel(_accel_instance, 1000, _dev_accel->get_bus_id_devtype(DEVTYPE_ACC_IIS328DQ))) {
+    if (!_imu.register_gyro(gyro_instance, 800, _dev_gyro->get_bus_id_devtype(DEVTYPE_GYR_I3G4250D)) ||
+        !_imu.register_accel(accel_instance, 1000, _dev_accel->get_bus_id_devtype(DEVTYPE_ACC_IIS328DQ))) {
         return;
     }
 
-    set_gyro_orientation(_gyro_instance, _rotation_g);
-    set_accel_orientation(_accel_instance, _rotation_a);
+    set_gyro_orientation(gyro_instance, _rotation_g);
+    set_accel_orientation(accel_instance, _rotation_a);
 
     // start the timer process to read samples
     _dev_gyro->register_periodic_callback(1150, FUNCTOR_BIND_MEMBER(&AP_InertialSensor_RST::gyro_measure, void));
@@ -354,8 +354,8 @@ void AP_InertialSensor_RST::start(void)
  */
 bool AP_InertialSensor_RST::update(void)
 {
-    update_gyro(_gyro_instance);
-    update_accel(_accel_instance);
+    update_gyro(gyro_instance);
+    update_accel(accel_instance);
 
     return true;
 }
@@ -376,8 +376,8 @@ void AP_InertialSensor_RST::gyro_measure(void)
     if (_dev_gyro->read_registers(GYRO_OUT_X_L | ADDR_INCREMENT, (uint8_t *)raw_data, sizeof(raw_data))) {
         gyro = Vector3f(raw_data[0], raw_data[1], raw_data[2]);
         gyro *= _gyro_scale;
-        _rotate_and_correct_gyro(_gyro_instance, gyro);
-        _notify_new_gyro_raw_sample(_gyro_instance, gyro);
+        _rotate_and_correct_gyro(gyro_instance, gyro);
+        _notify_new_gyro_raw_sample(gyro_instance, gyro);
     }
 }
 
@@ -397,8 +397,8 @@ void AP_InertialSensor_RST::accel_measure(void)
     if (_dev_accel->read_registers(ACCEL_OUT_X_L | ADDR_INCREMENT, (uint8_t *)raw_data, sizeof(raw_data))) {
         accel = Vector3f(raw_data[0], raw_data[1], raw_data[2]);
         accel *= _accel_scale;
-        _rotate_and_correct_accel(_accel_instance, accel);
-        _notify_new_accel_raw_sample(_accel_instance, accel);
+        _rotate_and_correct_accel(accel_instance, accel);
+        _notify_new_accel_raw_sample(accel_instance, accel);
     }
 }
 
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_RST.h b/libraries/AP_InertialSensor/AP_InertialSensor_RST.h
index 90d76d3e39376..0efaa6cbbef86 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_RST.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_RST.h
@@ -47,9 +47,6 @@ class AP_InertialSensor_RST : public AP_InertialSensor_Backend
     float _gyro_scale;
     float _accel_scale;
 
-    // gyro and accel instances
-    uint8_t _gyro_instance;
-    uint8_t _accel_instance;
     enum Rotation _rotation_g;
     enum Rotation _rotation_a;
 };
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_SCHA63T.h b/libraries/AP_InertialSensor/AP_InertialSensor_SCHA63T.h
index 3ffc203a26ece..64110a1260404 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_SCHA63T.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_SCHA63T.h
@@ -84,7 +84,5 @@ class AP_InertialSensor_SCHA63T : public AP_InertialSensor_Backend
     AP_HAL::OwnPtr<AP_HAL::Device> dev_uno;
     AP_HAL::OwnPtr<AP_HAL::Device> dev_due;
 
-    uint8_t accel_instance;
-    uint8_t gyro_instance;
     enum Rotation rotation;
 };
diff --git a/libraries/AP_InertialSensor/AP_InertialSensor_SITL.h b/libraries/AP_InertialSensor/AP_InertialSensor_SITL.h
index ac95bac811e25..856f40ab003a7 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor_SITL.h
+++ b/libraries/AP_InertialSensor/AP_InertialSensor_SITL.h
@@ -45,8 +45,6 @@ class AP_InertialSensor_SITL : public AP_InertialSensor_Backend
     const uint16_t gyro_sample_hz;
     const uint16_t accel_sample_hz;
 
-    uint8_t gyro_instance;
-    uint8_t accel_instance;
     uint64_t next_gyro_sample;
     uint64_t next_accel_sample;
     float gyro_time;