ardupilot/libraries/AP_RangeFinder/RangeFinder.cpp

// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
/*
   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include "RangeFinder.h"
#include "AP_RangeFinder_analog.h"
#include "AP_RangeFinder_PulsedLightLRF.h"
#include "AP_RangeFinder_MaxsonarI2CXL.h"
#include "AP_RangeFinder_PX4.h"
#include "AP_RangeFinder_PX4_PWM.h"
#include "AP_RangeFinder_BBB_PRU.h"
#include "AP_RangeFinder_LightWareI2C.h"
#include "AP_RangeFinder_LightWareSerial.h"

// table of user settable parameters
const AP_Param::GroupInfo RangeFinder::var_info[] = {
    // @Param: _TYPE
    // @DisplayName: Rangefinder type
    // @Description: What type of rangefinder device that is connected
    // @Values: 0:None,1:Analog,2:APM2-MaxbotixI2C,3:APM2-PulsedLightI2C,4:PX4-I2C,5:PX4-PWM,6:BBB-PRU,7:LightWareI2C,8:LightWareSerial
    // @User: Standard
    AP_GROUPINFO("_TYPE",    0, RangeFinder, _type[0], 0),

    // @Param: _PIN
    // @DisplayName: Rangefinder pin
    // @Description: Analog pin that rangefinder is connected to. Set this to 0..9 for the APM2 analog pins. Set to 64 on an APM1 for the dedicated 'airspeed' port on the end of the board. Set to 11 on PX4 for the analog 'airspeed' port. Set to 15 on the Pixhawk for the analog 'airspeed' port.
    // @Values: -1:Not Used, 0:APM2-A0, 1:APM2-A1, 2:APM2-A2, 3:APM2-A3, 4:APM2-A4, 5:APM2-A5, 6:APM2-A6, 7:APM2-A7, 8:APM2-A8, 9:APM2-A9, 11:PX4-airspeed port, 15:Pixhawk-airspeed port, 64:APM1-airspeed port
    // @User: Standard
    AP_GROUPINFO("_PIN",     1, RangeFinder, _pin[0], -1),

    // @Param: _SCALING
    // @DisplayName: Rangefinder scaling
    // @Description: Scaling factor between rangefinder reading and distance. For the linear and inverted functions this is in meters per volt. For the hyperbolic function the units are meterVolts.
    // @Units: meters/Volt
    // @Increment: 0.001
    // @User: Standard
    AP_GROUPINFO("_SCALING", 2, RangeFinder, _scaling[0], 3.0f),

    // @Param: _OFFSET
    // @DisplayName: rangefinder offset
    // @Description: Offset in volts for zero distance for analog rangefinders. Offset added to distance in centimeters for PWM and I2C Lidars
    // @Units: Volts
    // @Increment: 0.001
    // @User: Standard
    AP_GROUPINFO("_OFFSET",  3, RangeFinder, _offset[0], 0.0f),

    // @Param: _FUNCTION
    // @DisplayName: Rangefinder function
    // @Description: Control over what function is used to calculate distance. For a linear function, the distance is (voltage-offset)*scaling. For a inverted function the distance is (offset-voltage)*scaling. For a hyperbolic function the distance is scaling/(voltage-offset). The functions return the distance in meters.
    // @Values: 0:Linear,1:Inverted,2:Hyperbolic
    // @User: Standard
    AP_GROUPINFO("_FUNCTION", 4, RangeFinder, _function[0], 0),

    // @Param: _MIN_CM
    // @DisplayName: Rangefinder minimum distance
    // @Description: Minimum distance in centimeters that rangefinder can reliably read
	// @Units: centimeters
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO("_MIN_CM",  5, RangeFinder, _min_distance_cm[0], 20),

    // @Param: _MAX_CM
    // @DisplayName: Rangefinder maximum distance
    // @Description: Maximum distance in centimeters that rangefinder can reliably read
	// @Units: centimeters
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO("_MAX_CM",  6, RangeFinder, _max_distance_cm[0], 700),

    // @Param: _STOP_PIN
    // @DisplayName: Rangefinder stop pin
    // @Description: Digital pin that enables/disables rangefinder measurement for an analog rangefinder. A value of -1 means no pin. If this is set, then the pin is set to 1 to enable the rangefinder and set to 0 to disable it. This can be used to ensure that multiple sonar rangefinders don't interfere with each other.
    // @Values: -1:Not Used,50:Pixhawk AUXOUT1,51:Pixhawk AUXOUT2,52:Pixhawk AUXOUT3,53:Pixhawk AUXOUT4,54:Pixhawk AUXOUT5,55:Pixhawk AUXOUT6,111:PX4 FMU Relay1,112:PX4 FMU Relay2,113:PX4IO Relay1,114:PX4IO Relay2,115:PX4IO ACC1,116:PX4IO ACC2
    // @User: Standard
    AP_GROUPINFO("_STOP_PIN", 7, RangeFinder, _stop_pin[0], -1),

    // @Param: _SETTLE
    // @DisplayName: Rangefinder settle time
    // @Description: The time in milliseconds that the rangefinder reading takes to settle. This is only used when a STOP_PIN is specified. It determines how long we have to wait for the rangefinder to give a reading after we set the STOP_PIN high. For a sonar rangefinder with a range of around 7m this would need to be around 50 milliseconds to allow for the sonar pulse to travel to the target and back again.
    // @Units: milliseconds
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO("_SETTLE", 8, RangeFinder, _settle_time_ms[0], 0),

    // @Param: _RMETRIC
    // @DisplayName: Ratiometric
    // @Description: This parameter sets whether an analog rangefinder is ratiometric. Most analog rangefinders are ratiometric, meaning that their output voltage is influenced by the supply voltage. Some analog rangefinders (such as the SF/02) have their own internal voltage regulators so they are not ratiometric.
    // @Values: 0:No,1:Yes
    // @User: Standard
    AP_GROUPINFO("_RMETRIC", 9, RangeFinder, _ratiometric[0], 1),

    // @Param: _PWRRNG
    // @DisplayName: Powersave range
    // @Description: This parameter sets the estimated terrain distance in meters above which the sensor will be put into a power saving mode (if available). A value of zero means power saving is not enabled
    // @Units: meters
    // @Range: 0 32767
    // @User: Standard
    AP_GROUPINFO("_PWRRNG", 10, RangeFinder, _powersave_range, 0),

    // @Param: _GNDCLEAR
    // @DisplayName: Distance (in cm) from the range finder to the ground
    // @Description: This parameter sets the expected range measurement(in cm) that the range finder should return when the vehicle is on the ground.
    // @Units: centimeters
    // @Range: 0 127
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO("_GNDCLEAR", 11, RangeFinder, _ground_clearance_cm[0], RANGEFINDER_GROUND_CLEARANCE_CM_DEFAULT),

    // 10..12 left for future expansion

#if RANGEFINDER_MAX_INSTANCES > 1
    // @Param: 2_TYPE
    // @DisplayName: Second Rangefinder type
    // @Description: What type of rangefinder device that is connected
    // @Values: 0:None,1:Analog,2:APM2-MaxbotixI2C,3:APM2-PulsedLightI2C,4:PX4-I2C,5:PX4-PWM,6:BBB-PRU,7:LightWareI2C,8:LightWareSerial
    // @User: Advanced
    AP_GROUPINFO("2_TYPE",    12, RangeFinder, _type[1], 0),

    // @Param: 2_PIN
    // @DisplayName: Rangefinder pin
    // @Description: Analog pin that rangefinder is connected to. Set this to 0..9 for the APM2 analog pins. Set to 64 on an APM1 for the dedicated 'airspeed' port on the end of the board. Set to 11 on PX4 for the analog 'airspeed' port. Set to 15 on the Pixhawk for the analog 'airspeed' port.
    // @Values: -1:Not Used, 0:APM2-A0, 1:APM2-A1, 2:APM2-A2, 3:APM2-A3, 4:APM2-A4, 5:APM2-A5, 6:APM2-A6, 7:APM2-A7, 8:APM2-A8, 9:APM2-A9, 11:PX4-airspeed port, 15:Pixhawk-airspeed port, 64:APM1-airspeed port
    // @User: Advanced
    AP_GROUPINFO("2_PIN",     13, RangeFinder, _pin[1], -1),

    // @Param: 2_SCALING
    // @DisplayName: Rangefinder scaling
    // @Description: Scaling factor between rangefinder reading and distance. For the linear and inverted functions this is in meters per volt. For the hyperbolic function the units are meterVolts.
    // @Units: meters/Volt
    // @Increment: 0.001
    // @User: Advanced
    AP_GROUPINFO("2_SCALING", 14, RangeFinder, _scaling[1], 3.0f),

    // @Param: 2_OFFSET
    // @DisplayName: rangefinder offset
    // @Description: Offset in volts for zero distance
    // @Units: Volts
    // @Increment: 0.001
    // @User: Advanced
    AP_GROUPINFO("2_OFFSET",  15, RangeFinder, _offset[1], 0.0f),

    // @Param: 2_FUNCTION
    // @DisplayName: Rangefinder function
    // @Description: Control over what function is used to calculate distance. For a linear function, the distance is (voltage-offset)*scaling. For a inverted function the distance is (offset-voltage)*scaling. For a hyperbolic function the distance is scaling/(voltage-offset). The functions return the distance in meters.
    // @Values: 0:Linear,1:Inverted,2:Hyperbolic
    // @User: Advanced
    AP_GROUPINFO("2_FUNCTION",  16, RangeFinder, _function[1], 0),

    // @Param: 2_MIN_CM
    // @DisplayName: Rangefinder minimum distance
    // @Description: Minimum distance in centimeters that rangefinder can reliably read
	// @Units: centimeters
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("2_MIN_CM",  17, RangeFinder, _min_distance_cm[1], 20),

    // @Param: 2_MAX_CM
    // @DisplayName: Rangefinder maximum distance
    // @Description: Maximum distance in centimeters that rangefinder can reliably read
	// @Units: centimeters
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("2_MAX_CM",  18, RangeFinder, _max_distance_cm[1], 700),

    // @Param: 2_STOP_PIN
    // @DisplayName: Rangefinder stop pin
    // @Description: Digital pin that enables/disables rangefinder measurement for an analog rangefinder. A value of -1 means no pin. If this is set, then the pin is set to 1 to enable the rangefinder and set to 0 to disable it. This can be used to ensure that multiple sonar rangefinders don't interfere with each other.
    // @Values: -1:Not Used,50:Pixhawk AUXOUT1,51:Pixhawk AUXOUT2,52:Pixhawk AUXOUT3,53:Pixhawk AUXOUT4,54:Pixhawk AUXOUT5,55:Pixhawk AUXOUT6,111:PX4 FMU Relay1,112:PX4 FMU Relay2,113:PX4IO Relay1,114:PX4IO Relay2,115:PX4IO ACC1,116:PX4IO ACC2
    // @User: Advanced
    AP_GROUPINFO("2_STOP_PIN", 19, RangeFinder, _stop_pin[1], -1),

    // @Param: 2_SETTLE
    // @DisplayName: Sonar settle time
    // @Description: The time in milliseconds that the rangefinder reading takes to settle. This is only used when a STOP_PIN is specified. It determines how long we have to wait for the rangefinder to give a reading after we set the STOP_PIN high. For a sonar rangefinder with a range of around 7m this would need to be around 50 milliseconds to allow for the sonar pulse to travel to the target and back again.
    // @Units: milliseconds
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("2_SETTLE", 20, RangeFinder, _settle_time_ms[1], 0),

    // @Param: 2_RMETRIC
    // @DisplayName: Ratiometric
    // @Description: This parameter sets whether an analog rangefinder is ratiometric. Most analog rangefinders are ratiometric, meaning that their output voltage is influenced by the supply voltage. Some analog rangefinders (such as the SF/02) have their own internal voltage regulators so they are not ratiometric.
    // @Values: 0:No,1:Yes
    // @User: Advanced
    AP_GROUPINFO("2_RMETRIC", 21, RangeFinder, _ratiometric[1], 1),

    // @Param: 2_GNDCLEAR
    // @DisplayName: Distance (in cm) from the second range finder to the ground
    // @Description: This parameter sets the expected range measurement(in cm) that the second range finder should return when the vehicle is on the ground.
    // @Units: centimeters
    // @Range: 0 127
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("2_GNDCLEAR", 22, RangeFinder, _ground_clearance_cm[1], RANGEFINDER_GROUND_CLEARANCE_CM_DEFAULT),
#endif

    // @Param: _ADDR
    // @DisplayName: Bus address of sensor
    // @Description: This sets the bus address of the sensor, where applicable. Used for the LightWare I2C sensor to allow for multiple sensors on different addresses. A value of 0 disables the sensor.
    // @Range: 0 127
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO("_ADDR", 23, RangeFinder, _address[0], 0),

#if RANGEFINDER_MAX_INSTANCES > 1
    // @Param: 2_ADDR
    // @DisplayName: Bus address of 2nd rangefinder
    // @Description: This sets the bus address of the sensor, where applicable. Used for the LightWare I2C sensor to allow for multiple sensors on different addresses. A value of 0 disables the sensor.
    // @Range: 0 127
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("2_ADDR", 24, RangeFinder, _address[1], 0),
#endif

#if RANGEFINDER_MAX_INSTANCES > 2
    // @Param: 3_TYPE
    // @DisplayName: Second Rangefinder type
    // @Description: What type of rangefinder device that is connected
    // @Values: 0:None,1:Analog,2:APM2-MaxbotixI2C,3:APM2-PulsedLightI2C,4:PX4-I2C,5:PX4-PWM,6:BBB-PRU,7:LightWareI2C,8:LightWareSerial
    AP_GROUPINFO("3_TYPE",    25, RangeFinder, _type[2], 0),

    // @Param: 3_PIN
    // @DisplayName: Rangefinder pin
    // @Description: Analog pin that rangefinder is connected to. Set this to 0..9 for the APM2 analog pins. Set to 64 on an APM1 for the dedicated 'airspeed' port on the end of the board. Set to 11 on PX4 for the analog 'airspeed' port. Set to 15 on the Pixhawk for the analog 'airspeed' port.
    // @Values: -1:Not Used, 0:APM2-A0, 1:APM2-A1, 2:APM2-A2, 3:APM2-A3, 4:APM2-A4, 5:APM2-A5, 6:APM2-A6, 7:APM2-A7, 8:APM2-A8, 9:APM2-A9, 11:PX4-airspeed port, 15:Pixhawk-airspeed port, 64:APM1-airspeed port
    AP_GROUPINFO("3_PIN",     26, RangeFinder, _pin[2], -1),

    // @Param: 3_SCALING
    // @DisplayName: Rangefinder scaling
    // @Description: Scaling factor between rangefinder reading and distance. For the linear and inverted functions this is in meters per volt. For the hyperbolic function the units are meterVolts.
    // @Units: meters/Volt
    // @Increment: 0.001
    AP_GROUPINFO("3_SCALING", 27, RangeFinder, _scaling[2], 3.0f),

    // @Param: 3_OFFSET
    // @DisplayName: rangefinder offset
    // @Description: Offset in volts for zero distance
    // @Units: Volts
    // @Increment: 0.001
    AP_GROUPINFO("3_OFFSET",  28, RangeFinder, _offset[2], 0.0f),

    // @Param: 3_FUNCTION
    // @DisplayName: Rangefinder function
    // @Description: Control over what function is used to calculate distance. For a linear function, the distance is (voltage-offset)*scaling. For a inverted function the distance is (offset-voltage)*scaling. For a hyperbolic function the distance is scaling/(voltage-offset). The functions return the distance in meters.
    // @Values: 0:Linear,1:Inverted,2:Hyperbolic
    AP_GROUPINFO("3_FUNCTION",  29, RangeFinder, _function[2], 0),

    // @Param: 3_MIN_CM
    // @DisplayName: Rangefinder minimum distance
    // @Description: Minimum distance in centimeters that rangefinder can reliably read
	// @Units: centimeters
    // @Increment: 1
    AP_GROUPINFO("3_MIN_CM",  30, RangeFinder, _min_distance_cm[2], 20),

    // @Param: 3_MAX_CM
    // @DisplayName: Rangefinder maximum distance
    // @Description: Maximum distance in centimeters that rangefinder can reliably read
	// @Units: centimeters
    // @Increment: 1
    AP_GROUPINFO("3_MAX_CM",  31, RangeFinder, _max_distance_cm[2], 700),

    // @Param: 3_STOP_PIN
    // @DisplayName: Rangefinder stop pin
    // @Description: Digital pin that enables/disables rangefinder measurement for an analog rangefinder. A value of -1 means no pin. If this is set, then the pin is set to 1 to enable the rangefinder and set to 0 to disable it. This can be used to ensure that multiple sonar rangefinders don't interfere with each other.
    // @Values: -1:Not Used,50:Pixhawk AUXOUT1,51:Pixhawk AUXOUT2,52:Pixhawk AUXOUT3,53:Pixhawk AUXOUT4,54:Pixhawk AUXOUT5,55:Pixhawk AUXOUT6,111:PX4 FMU Relay1,112:PX4 FMU Relay2,113:PX4IO Relay1,114:PX4IO Relay2,115:PX4IO ACC1,116:PX4IO ACC2
    AP_GROUPINFO("3_STOP_PIN", 32, RangeFinder, _stop_pin[2], -1),

    // @Param: 3_SETTLE
    // @DisplayName: Sonar settle time
    // @Description: The time in milliseconds that the rangefinder reading takes to settle. This is only used when a STOP_PIN is specified. It determines how long we have to wait for the rangefinder to give a reading after we set the STOP_PIN high. For a sonar rangefinder with a range of around 7m this would need to be around 50 milliseconds to allow for the sonar pulse to travel to the target and back again.
    // @Units: milliseconds
    // @Increment: 1
    AP_GROUPINFO("3_SETTLE", 33, RangeFinder, _settle_time_ms[2], 0),

    // @Param: 3_RMETRIC
    // @DisplayName: Ratiometric
    // @Description: This parameter sets whether an analog rangefinder is ratiometric. Most analog rangefinders are ratiometric, meaning that their output voltage is influenced by the supply voltage. Some analog rangefinders (such as the SF/02) have their own internal voltage regulators so they are not ratiometric.
    // @Values: 0:No,1:Yes
    AP_GROUPINFO("3_RMETRIC", 34, RangeFinder, _ratiometric[2], 1),

    // @Param: 3_GNDCLEAR
    // @DisplayName: Distance (in cm) from the second range finder to the ground
    // @Description: This parameter sets the expected range measurement(in cm) that the second range finder should return when the vehicle is on the ground.
    // @Units: centimeters
    // @Range: 0 127
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("3_GNDCLEAR", 35, RangeFinder, _ground_clearance_cm[2], RANGEFINDER_GROUND_CLEARANCE_CM_DEFAULT),
#endif

#if RANGEFINDER_MAX_INSTANCES > 3
    // @Param: 3_ADDR
    // @DisplayName: Bus address of 2nd rangefinder
    // @Description: This sets the bus address of the sensor, where applicable. Used for the LightWare I2C sensor to allow for multiple sensors on different addresses. A value of 0 disables the sensor.
    // @Range: 0 127
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("3_ADDR", 36, RangeFinder, _address[2], 0),
    
    // @Param: 4_TYPE
    // @DisplayName: Second Rangefinder type
    // @Description: What type of rangefinder device that is connected
    // @Values: 0:None,1:Analog,2:APM2-MaxbotixI2C,3:APM2-PulsedLightI2C,4:PX4-I2C,5:PX4-PWM,6:BBB-PRU,7:LightWareI2C,8:LightWareSerial
    AP_GROUPINFO("4_TYPE",    37, RangeFinder, _type[3], 0),

    // @Param: 4_PIN
    // @DisplayName: Rangefinder pin
    // @Description: Analog pin that rangefinder is connected to. Set this to 0..9 for the APM2 analog pins. Set to 64 on an APM1 for the dedicated 'airspeed' port on the end of the board. Set to 11 on PX4 for the analog 'airspeed' port. Set to 15 on the Pixhawk for the analog 'airspeed' port.
    // @Values: -1:Not Used, 0:APM2-A0, 1:APM2-A1, 2:APM2-A2, 3:APM2-A3, 4:APM2-A4, 5:APM2-A5, 6:APM2-A6, 7:APM2-A7, 8:APM2-A8, 9:APM2-A9, 11:PX4-airspeed port, 15:Pixhawk-airspeed port, 64:APM1-airspeed port
    AP_GROUPINFO("4_PIN",     38, RangeFinder, _pin[3], -1),

    // @Param: 4_SCALING
    // @DisplayName: Rangefinder scaling
    // @Description: Scaling factor between rangefinder reading and distance. For the linear and inverted functions this is in meters per volt. For the hyperbolic function the units are meterVolts.
    // @Units: meters/Volt
    // @Increment: 0.001
    AP_GROUPINFO("4_SCALING", 39, RangeFinder, _scaling[3], 3.0f),

    // @Param: 4_OFFSET
    // @DisplayName: rangefinder offset
    // @Description: Offset in volts for zero distance
    // @Units: Volts
    // @Increment: 0.001
    AP_GROUPINFO("4_OFFSET",  40, RangeFinder, _offset[3], 0.0f),

    // @Param: 4_FUNCTION
    // @DisplayName: Rangefinder function
    // @Description: Control over what function is used to calculate distance. For a linear function, the distance is (voltage-offset)*scaling. For a inverted function the distance is (offset-voltage)*scaling. For a hyperbolic function the distance is scaling/(voltage-offset). The functions return the distance in meters.
    // @Values: 0:Linear,1:Inverted,2:Hyperbolic
    AP_GROUPINFO("4_FUNCTION",  41, RangeFinder, _function[3], 0),

    // @Param: 4_MIN_CM
    // @DisplayName: Rangefinder minimum distance
    // @Description: Minimum distance in centimeters that rangefinder can reliably read
	// @Units: centimeters
    // @Increment: 1
    AP_GROUPINFO("4_MIN_CM",  42, RangeFinder, _min_distance_cm[3], 20),

    // @Param: 4_MAX_CM
    // @DisplayName: Rangefinder maximum distance
    // @Description: Maximum distance in centimeters that rangefinder can reliably read
	// @Units: centimeters
    // @Increment: 1
    AP_GROUPINFO("4_MAX_CM",  43, RangeFinder, _max_distance_cm[3], 700),

    // @Param: 4_STOP_PIN
    // @DisplayName: Rangefinder stop pin
    // @Description: Digital pin that enables/disables rangefinder measurement for an analog rangefinder. A value of -1 means no pin. If this is set, then the pin is set to 1 to enable the rangefinder and set to 0 to disable it. This can be used to ensure that multiple sonar rangefinders don't interfere with each other.
    // @Values: -1:Not Used,50:Pixhawk AUXOUT1,51:Pixhawk AUXOUT2,52:Pixhawk AUXOUT3,53:Pixhawk AUXOUT4,54:Pixhawk AUXOUT5,55:Pixhawk AUXOUT6,111:PX4 FMU Relay1,112:PX4 FMU Relay2,113:PX4IO Relay1,114:PX4IO Relay2,115:PX4IO ACC1,116:PX4IO ACC2
    AP_GROUPINFO("4_STOP_PIN", 44, RangeFinder, _stop_pin[3], -1),

    // @Param: 4_SETTLE
    // @DisplayName: Sonar settle time
    // @Description: The time in milliseconds that the rangefinder reading takes to settle. This is only used when a STOP_PIN is specified. It determines how long we have to wait for the rangefinder to give a reading after we set the STOP_PIN high. For a sonar rangefinder with a range of around 7m this would need to be around 50 milliseconds to allow for the sonar pulse to travel to the target and back again.
    // @Units: milliseconds
    // @Increment: 1
    AP_GROUPINFO("4_SETTLE", 45, RangeFinder, _settle_time_ms[3], 0),

    // @Param: 4_RMETRIC
    // @DisplayName: Ratiometric
    // @Description: This parameter sets whether an analog rangefinder is ratiometric. Most analog rangefinders are ratiometric, meaning that their output voltage is influenced by the supply voltage. Some analog rangefinders (such as the SF/02) have their own internal voltage regulators so they are not ratiometric.
    // @Values: 0:No,1:Yes
    AP_GROUPINFO("4_RMETRIC", 46, RangeFinder, _ratiometric[3], 1),

    // @Param: 4_GNDCLEAR
    // @DisplayName: Distance (in cm) from the second range finder to the ground
    // @Description: This parameter sets the expected range measurement(in cm) that the second range finder should return when the vehicle is on the ground.
    // @Units: centimeters
    // @Range: 0 127
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("4_GNDCLEAR", 47, RangeFinder, _ground_clearance_cm[3], RANGEFINDER_GROUND_CLEARANCE_CM_DEFAULT),

    // @Param: 4_ADDR
    // @DisplayName: Bus address of 2nd rangefinder
    // @Description: This sets the bus address of the sensor, where applicable. Used for the LightWare I2C sensor to allow for multiple sensors on different addresses. A value of 0 disables the sensor.
    // @Range: 0 127
    // @Increment: 1
    // @User: Advanced
    AP_GROUPINFO("4_ADDR", 48, RangeFinder, _address[3], 0),
#endif
    
    AP_GROUPEND
};

RangeFinder::RangeFinder(AP_SerialManager &_serial_manager) :
    primary_instance(0),
    num_instances(0),
    estimated_terrain_height(0),
    serial_manager(_serial_manager)
{
    AP_Param::setup_object_defaults(this, var_info);

    // init state and drivers
    memset(state,0,sizeof(state));
    memset(drivers,0,sizeof(drivers));
}

/*
  initialise the RangeFinder class. We do detection of attached range
  finders here. For now we won't allow for hot-plugging of
  rangefinders.
 */
void RangeFinder::init(void)
{
    if (num_instances != 0) {
        // init called a 2nd time?
        return;
    }
    for (uint8_t i=0; i<RANGEFINDER_MAX_INSTANCES; i++) {
        detect_instance(i);
        if (drivers[i] != NULL) {
            // we loaded a driver for this instance, so it must be
            // present (although it may not be healthy)
            num_instances = i+1;
        }
        // initialise pre-arm check variables
        state[i].pre_arm_check = false;
        state[i].pre_arm_distance_min = 9999;  // initialise to an arbitrary large value
        state[i].pre_arm_distance_max = 0;

        // initialise status
        state[i].status = RangeFinder_NotConnected;
        state[i].range_valid_count = 0;
    }
}

/*
  update RangeFinder state for all instances. This should be called at
  around 10Hz by main loop
 */
void RangeFinder::update(void)
{
    for (uint8_t i=0; i<num_instances; i++) {
        if (drivers[i] != NULL) {
            if (_type[i] == RangeFinder_TYPE_NONE) {
                // allow user to disable a rangefinder at runtime
                state[i].status = RangeFinder_NotConnected;
                state[i].range_valid_count = 0;
                continue;
            }
            drivers[i]->update();
            update_pre_arm_check(i);
        }
    }

    // work out primary instance - first sensor returning good data
    for (int8_t i=num_instances-1; i>=0; i--) {
        if (drivers[i] != NULL && (state[i].status == RangeFinder_Good)) {
            primary_instance = i;
        }
    }
}
    
/*
  detect if an instance of a rangefinder is connected. 
 */
void RangeFinder::detect_instance(uint8_t instance)
{
    uint8_t type = _type[instance];
#if CONFIG_HAL_BOARD == HAL_BOARD_PX4 || CONFIG_HAL_BOARD == HAL_BOARD_VRBRAIN
    if (type == RangeFinder_TYPE_PLI2C || 
        type == RangeFinder_TYPE_MBI2C) {
        // I2C sensor types are handled by the PX4Firmware code
        type = RangeFinder_TYPE_PX4;
    }
#endif
    if (type == RangeFinder_TYPE_PLI2C) {
        if (AP_RangeFinder_PulsedLightLRF::detect(*this, instance)) {
            state[instance].instance = instance;
            drivers[instance] = new AP_RangeFinder_PulsedLightLRF(*this, instance, state[instance]);
            return;
        }
    } 
    if (type == RangeFinder_TYPE_MBI2C) {
        if (AP_RangeFinder_MaxsonarI2CXL::detect(*this, instance)) {
            state[instance].instance = instance;
            drivers[instance] = new AP_RangeFinder_MaxsonarI2CXL(*this, instance, state[instance]);
            return;
        }
    }
    if (type == RangeFinder_TYPE_LWI2C) {
        if (AP_RangeFinder_LightWareI2C::detect(*this, instance)) {
            state[instance].instance = instance;
            drivers[instance] = new AP_RangeFinder_LightWareI2C(*this, instance, state[instance]);
            return;
        }
    } 
#if CONFIG_HAL_BOARD == HAL_BOARD_PX4  || CONFIG_HAL_BOARD == HAL_BOARD_VRBRAIN
    if (type == RangeFinder_TYPE_PX4) {
        if (AP_RangeFinder_PX4::detect(*this, instance)) {
            state[instance].instance = instance;
            drivers[instance] = new AP_RangeFinder_PX4(*this, instance, state[instance]);
            return;
        }
    }
    if (type == RangeFinder_TYPE_PX4_PWM) {
        if (AP_RangeFinder_PX4_PWM::detect(*this, instance)) {
            state[instance].instance = instance;
            drivers[instance] = new AP_RangeFinder_PX4_PWM(*this, instance, state[instance]);
            return;
        }
    }
#endif
#if CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_BBBMINI
    if (type == RangeFinder_TYPE_BBB_PRU) {
        if (AP_RangeFinder_BBB_PRU::detect(*this, instance)) {
            state[instance].instance = instance;
            drivers[instance] = new AP_RangeFinder_BBB_PRU(*this, instance, state[instance]);
            return;
        }
    }
#endif
    if (type == RangeFinder_TYPE_LWSER) {
        if (AP_RangeFinder_LightWareSerial::detect(*this, instance, serial_manager)) {
            state[instance].instance = instance;
            drivers[instance] = new AP_RangeFinder_LightWareSerial(*this, instance, state[instance], serial_manager);
            return;
        }
    } 
    if (type == RangeFinder_TYPE_ANALOG) {
        // note that analog must be the last to be checked, as it will
        // always come back as present if the pin is valid
        if (AP_RangeFinder_analog::detect(*this, instance)) {
            state[instance].instance = instance;
            drivers[instance] = new AP_RangeFinder_analog(*this, instance, state[instance]);
            return;
        }
    }
}

// query status
RangeFinder::RangeFinder_Status RangeFinder::status(uint8_t instance) const
{
    // sanity check instance
    if (instance >= RANGEFINDER_MAX_INSTANCES) {
        return RangeFinder_NotConnected;
    }

    if (drivers[instance] == NULL || _type[instance] == RangeFinder_TYPE_NONE) {
        return RangeFinder_NotConnected;
    }

    return state[instance].status;
}

// true if sensor is returning data
bool RangeFinder::has_data(uint8_t instance) const
{
    // sanity check instance
    if (instance >= RANGEFINDER_MAX_INSTANCES) {
        return RangeFinder_NotConnected;
    }
    return ((state[instance].status != RangeFinder_NotConnected) && (state[instance].status != RangeFinder_NoData));
}

/*
  returns true if pre-arm checks have passed for all range finders
  these checks involve the user lifting or rotating the vehicle so that sensor readings between
  the min and 2m can be captured
 */
bool RangeFinder::pre_arm_check() const
{
    for (uint8_t i=0; i<num_instances; i++) {
        // if driver is valid but pre_arm_check is false, return false
        if ((drivers[i] != NULL) && (_type[i] != RangeFinder_TYPE_NONE) && !state[i].pre_arm_check) {
            return false;
        }
    }
    return true;
}

/*
  set pre-arm checks to passed if the range finder has been exercised through a reasonable range of movement
      max distance sensed is at least 50cm > min distance sensed
      max distance < 200cm
      min distance sensed is within 10cm of ground clearance or sensor's minimum distance
 */
void RangeFinder::update_pre_arm_check(uint8_t instance)
{
    // return immediately if already passed or no sensor data
    if (state[instance].pre_arm_check || state[instance].status == RangeFinder_NotConnected || state[instance].status == RangeFinder_NoData) {
        return;
    }

    // update min, max captured distances
    state[instance].pre_arm_distance_min = min(state[instance].distance_cm, state[instance].pre_arm_distance_min);
    state[instance].pre_arm_distance_max = max(state[instance].distance_cm, state[instance].pre_arm_distance_max);

    // Check that the range finder has been exercised through a realistic range of movement
    if (((state[instance].pre_arm_distance_max - state[instance].pre_arm_distance_min) > RANGEFINDER_PREARM_REQUIRED_CHANGE_CM) &&
         (state[instance].pre_arm_distance_max < RANGEFINDER_PREARM_ALT_MAX_CM) &&
         ((int16_t)state[instance].pre_arm_distance_min < (max(_ground_clearance_cm[instance],min_distance_cm(instance)) + 10)) &&
         ((int16_t)state[instance].pre_arm_distance_min > (min(_ground_clearance_cm[instance],min_distance_cm(instance)) - 10))) {
        state[instance].pre_arm_check = true;
    }
}