Gripper Integration
This document describes how to integrate the gripper
Grippers are a commonly used mechanism for UAV delivery applications. Although there are multiple types of interfaces (e.g. PWM, CAN) for different products in the market, the most commonly used interface is PWM. The integration of PWM controlled gripper hardware is explained in the following section.
Connecting a PWM-controlled Gripper
For grippers actuated via PWM signal, the cabling comprises of three lines: Power, Ground and Signal. A typical connector is shown in the image below:
In the image above the wire colors have the following meanings:
Brown
Ground
Red
Power
Yellow
PWM Signal
Compatibility checks
Before proceeding, double-check the following requirements:
Signal line voltage level: Check the datasheet of your gripper mechanism to find the voltage level of the signal line. And make sure this is compatible with the voltage level of the FMU pins, exposed on the Pixhawk Adapter Board. As of writing, the FMU pins have 3.3V logic level, so gripper should not drive the signal line above 3.3V!
Power requirements of gripper: Check the mechanism's datasheet for the power-line voltage level requirements. Based on the voltage the gripper can be either connected directly to the Power module and use the battery voltage, or connected to a separate 5V line. Alternatively a custom voltage regulator can also be used to output any other voltage required.
Connect the PWM cable to Pixhawk Adapter Board
Make sure to connect the ground and signal pins correctly to the adapter board and rely on the G-V-S labels printed on the PCB.
Power Supply
If your gripper mechanism receives power through the power line of PWM cable (which is the most cases), you must supply the power to the Power line of the PWM pinouts of the Pixhawk Adapter Board from the source chosen as described above.
This is done by using one of the PWM pinouts to connect to the power source via the Power and Ground lines.
Writing a Mixer for the Gripper
Gripper Mixer for the AUX output
# AUX1 pin is used for Payload deploy (Gripper / Winch)
# It uses 'Control Group 6 (Payload), channel 0' to avoid conflict with other flight controls
# More about control group 6: https://docs.px4.io/v1.13/en/concept/mixing.html#control-group-6-first-payload
# Read more about Summing mixer here: https://docs.px4.io/v1.13/en/concept/mixing.html#summing-mixer
M: 1
O: 10000 10000 0 -10000 10000
S: 1 4 10000 10000 0 -10000 10000Above is a copy of the Auterion PX4's pre-existing gripper mixer (gripper.aux.mix). As indicated by its name, this mixer is only available on AUX outputs.
To enable this mixer, add the following line to your airframe file. This will automatically utilize the gripper.aux.mix file and load it as a AUX mixer:
# Configure the Gripper Mixer for AUX channels for Payload delivery feature
set MIXER_AUX gripperWith the above setting, AUX1 pin will provide the PWM (1000 ~ 2000 us Pulse-Width) output for the Gripper mechanism.
Custom Mixer for Gripping Mechanism
In case you want to use a different pin than AUX1, or a MAIN output for gripping mechanism, you will need to modify the mixer yourself. Take a look at the section on writing a custom mixer to see how.
Then modify the mixer and airframe files accordingly to map the delivery mechanism to the correct output. For further information visit the PX4's Mixer documentation for understanding how summing mixer works, the mixer needed for the gripper mechanism.
Here is an example of a custom mixer and airframe file that will use AUX3 as an output for the Gripper mechanism:
# AUX1 pin is not used, use Null mixer (Z)
Z:
# AUX2 pin is not used, use Null mixer (Z)
Z:
# AUX3 pin is used for Payload deploy (Gripper / Winch)
# It uses 'Control Group 6 (Payload), channel 0' to avoid conflict with other flight controls
# More about control group 6: https://docs.px4.io/v1.13/en/concept/mixing.html#control-group-6-first-payload
# Read more about Summing mixer here: https://docs.px4.io/v1.13/en/concept/mixing.html#summing-mixer
M: 1
O: 10000 10000 0 -10000 10000
S: 1 4 10000 10000 0 -10000 10000# Configure the Gripper Mixer for AUX channels for Payload delivery feature
set MIXER_AUX gripper_customNotes on loading the Mixer
Although mentioned above already, there are 2 crucial steps to getting the mixer loaded properly and working.
The mixer needs to be loaded in the airframe file as described in Creating Airframe Configurations for APX4.
And then it must be uploaded to the vehicle as described here: Creating Airframe Configurations for APX4.
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