Old Actuators
WARNING: Servo Problems
It is important to note that under certain circumstances the servo can critically damage itself. It is a $300 part and we have had ten of them fail. Most of the failed servos were used for braking; this poor performance was the motivation for changing to the solenoid braking system. It appears that the servo needs its power turned on only AFTER providing it a PWM signal. Further, providing it a signal corresponding to its current position will prevent it from trying to move during startup. The low-level firmware (early 2019) has been adapted to perform this, with an optional external relay board.
So, worth noting that the linear actuators claim a duty cycle of 25%. I’ve just identified the driver chip, which has a duty cycle of 20% at 2A — with no thermal shutdown or safety. If you use the actuator continuously, it will just die. The driver is the Toshiba TB6612FNG, available from digikey.com TB6612FNG, C,8, EL Toshiba Semiconductor and Storage | Integrated Circuits (ICs) | DigiKeyMotor Driver Power MOSFET Parallel 24-SSOP}}
Thinking about this some more; once we’re using a PID we can use the deltas — cap them to prevent long-distance moves, and add them to an accumulator to keep track of total distance commanded. If the accumulator fills, we enter an error state and can’t steer. Then reduce the accumulator at a fixed rate, so that steering availability returns over time … or, probably easier… we could just use a different motor driver. Wire the driver to the DC motor in the servo that failed, and use the external feedback instead of the potentiometer inside the actuator.
- Ben Rockhold (December 2018)
Rotary servos
The ELF uses a i00600 Torxis rotary servo.
