Throttle Arming in Rover

A software safety feature that requires the throttle to be explicitly armed by a driver is now available for Rover. It was introduced in version 3.0.0 and it is enabled by default. The idea is to prevent an unexpected throttle up and having the rover randomly accelerate away. It also performs a number of system checks to ensure the Rover ArduPilot system is healthy and ready to go.

Parameters governing throttle arming are introduced in the ARMING section of the parameters wiki page. This page discusses throttle arming in greater detail and walks through the typical procedures for using this safety feature.

A Safety Note About Arming

A safety feature of PX4 / Pixhawk is that all servo output is kept at at minimum values until the safety button is pushed. This has the nice effect of (usually) disabling electric motors and it is quite possible to use only this safety feature for propeller safety. However:

  1. This feature is not available for the APM.
  2. Generally during pre-arm checks it is desirable to verify servo operation (e.g., to check steering is deflecting in the expected direction). This can more safely be done with the throttle disabled. When the safety button is pushed all servos are enabled, including the motor – unless this additional software throttle arming safety is used.

A note about APM1 and channel 8

The APM1 has a hardware multiplexor on channel 8 which forces pass-thru of the first 4 channels if channel 8 is above 1750. This means the arming code is bypassed if you use channel 8 for your mode switch. So if you want to use arming on an APM1 then you should change FLTMODE_CH to another channel (say channel 5) and setup your transmitter/receiver to put the mode switch on that other channel.

IMPORTANT: RC Transmitter Calibration

It is essential that your RC radio transmitter be calibrated correctly before continuing. Please see the Calibrate your RC input wiki page if you don’t know how to calibrate your radio.

Warning

During RC calibration throttle ups are possible, even with safeties enabled and you should therefore ensure the rover’s wheels are not touching the ground when you do the calibration.

Note that if you have RCMAP_THROTTLE set to something other than 3, then the RCn_MIN etc values used will be the one for the channel you have selected as the throttle channel.

When calibrating your RC input you should also be careful to set the deadzone (DZ) value of the throttle (usually RC3_DZ) to a non-zero value when in normal control. If you have the RC3_DZ set to 0 and your transmitter doesn’t output exactly the same value as RC3_TRIM you won’t be able to arm using the steering as the APM will think you are at a non-zero throttle level.

Simplest Solution: Use Only ARMING_REQUIRE

The simplest way to use the throttle arming feature is to require the user to request the throttle to arm. ARMING_REQUIRE has three possible values, 0, 1 and 2 (the default value is 1, enabled). They have the following effect:

  • ARMING_REQUIRE=0: No effect. Throttle arming safety is not employed.
  • ARMING_REQUIRE=1: Before the user arms throttle, send trim PWM to the throttle channel (which is usually channel 3). Therefore the RC3_TRIM PWM value is sent on the servo output rail.
  • ARMING_REQUIRE=2: Before the user arms throttle, send no PWM signal to the throttle channel (usually channel 3). Some ESCs are not happy with this and will continuously beep, some will not. Some users may prefer this setting as it ensures no signal is sent to the ESC when disarmed.

When the ARMING_REQUIRE parameter is set to 1 or 2 (enabled), all that is required to arm the throttle is to either:

  1. Arm the throttle via the ground control software.

    OR

  2. Arm the throttle by applying full right rudder input for several seconds.

To arm throttle in Mission Planner, use the Flight Data screen, then select the Actions tab. This provides an “Arm/Disarm” button that can be used to arm and disarm the throttle.

docs/../images/armingButtonMissPlan.jpg

Location of the Arm/Disarm buttonin Mission Planner (button circled in red near the bottom of the image).

Alternatively, apply full right steering to your RC transmitter for several seconds to arm the throttle. Whichever method you choose should result in a message from your ground control software stating that throttle arming was successful. Mission planner displays a message in the artificial horizon on the HUD:

docs/../images/armingArmedMissPlan.jpg

Message on the artificial horizon inMission Planner stating that arming was successful.

Increased Safety: Perform System Checks Before Arming Throttle

By default the ARMING_CHECK parameter is set to 1 so the autopilot controller performs system health checks before arming throttle when a user attempts to arm. You can enable/disable any check with a bitmask. See wiki documentation on the ARMING_CHECK parameter for more information.

One thing to be aware of if you typically do not drive with a ground control station: it will be difficult to determine why your autopilot is not arming if you are not connected to a ground control station when arming. The ARMING_CHECK parameter should probably be left at 0 when without a ground control station.

The following are possible system health messages that may return if ARMING_CHECK is enabled and the autopilot rejects a request to arm the throttle:

  • Message: “Hardware Safety Switch.” Solution: push the hardware safety switch on the PX4 or Pixhawk (does not apply to APM).
  • Message: “Battery failsafe on.” Solution: Ensure your battery is charged. If it is, ensure your battery failsafe values are set correctly. For more information on failsafes, see the Failsafe Functions wiki page.
  • Message: “Radio failsafe on.” Solution: Ensure that the RC transmitter is able to communicate with the RC receiver. For more information on this failsafe, see the documentation on the Throttle Failsafe.
  • Message: “Bad GPS Pos.” Solution: Need to get a 3D fix with the GPS receiver. After ensuring your GPS receiver is functioning properly, ensure nothing on the rover or in the immediate environment is interfering with GPS satellite signals.
  • Message: “No GPS detected. Solution: ensure your GPS receiver is functioning.
  • Message: “No compass detected.” A rover needs a compass. You’ll need to ensure your compass is installed and healthy.
  • Message: “Compass not calibrated.” Solution: Calibrate compass. In Mission Planner this is accomplished in the Initial Setup screen, menu item Manadtory Hardware > Compass.
  • Message: “Compass not healthy.” Solution: Ensure you do not have the compass installed near something that can induce a magnetic field, such as the motor. You also may try re-calibrating the compass.