Dynamixel torque control using current feedback

[Gray Fox]

Hi. I've been recently introduced to Variable Stiffness Actuators and Force/Impedance control in robot manipulators arms. For this pourpose, a feedback of the torque (or equivalently the current) is needed. Dynamixel MX-106 seems to provide current measurements, however, in the tech specs it says:

<<Notes: Current load is inferred from the internal torque value, not from Torque sensor etc. For that reason, it cannot be used to measure weight or torque; however, it must be used only to detect which direction the force works.>>


(http://support.robotis.com/en/produc...ies/mx-106.htm)

A very similar question was already stated here: http://forums.trossenrobotics.com/sh...rrent-feedback

However, the response is not enough for me. Has anyone tried contolling a Dynamixel servo in force/torque? Is there any not too complex way to measure externally the current? I do not require too much accuracy in the measurements.

Thanks in advance.

[jwatte]

The "current" register on the MX-64 and MX-106 servos should be sufficient for what you need. (Register 0x44/0x45)
The "present load" that the documentation talks about is register 0x28/0x29, and is, I believe, a mirror of the current PWM output value for the motor H-bridge.

If you want to measure current, you can also measure it through the power line. The base draw is in the 40-80 mA range, anything above that goes to the motor. You would not want to daisy-chain other servos after the servo you measure, though; you'd instead use a multi-tap bus with a current sensor on each of the taps.

Given that the stall current of those servos is about 5A, you could use an ACS714 sensor for example (range -5A - 5A.) Pololu has a nice break-out board for this sensor if you want to quickly get going: http://www.pololu.com/product/1185

[Gray Fox]

Thank you so much for the info, jwatte, that was what I needed.

[kert]

Given that the stall current of those servos is about 5A, you could use an ACS714 sensor for example (range -5A - 5A.) Pololu has a nice break-out board for this sensor if you want to quickly get going: http://www.pololu.com/product/1185

You should be able to do the same thing with a much much cheaper appropriately sized current sense resistor, as you will be measuring the DC power line going into the servo.

The challenge of course is having enough ADC channels available for all your servos. However, the MCU _within_ dynamixel already probably has a few ( unused ? ) ADC channels ?

[urko_18]

Hi everyone!

I am also trying to do the control of the torque with the current, in order to create a colaborative robot arm (with 2 dynamixel Mx-64 and other 2 dynamixel 106). But I have not been able to get the current values with the library that I´m using (<ax12.h>).
Did anybody know the program code to obtain those values?

I probe codes like:

dxlGetPresentCurrent
dxlgetPresentcurrent
dxlgetCurrent
regData8 = dxlGetCurrent(SERVO_ID[3]);

...

and nothing...

Thank you!

Urko.