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View Full Version : [Question(s)] Reading Dynamixel's position in Wheel Mode



pauljurczak
10-28-2011, 06:31 AM
Hello,

After some googling I still don't have a clear answer. It seems that higher end models (MX-28 and EX-106+) will return a valid position value (overflowing) in wheel mode. All other models have a dead zone and this caution note in their manuals: "If it is set to Wheel Mode, the value cannot be used to measure the moving distance and the rotation frequency".

I'm most interested in AX-12A behavior. Can someone verify that they read valid position values in wheel mode (except when in dead zone) from AX-12A?

Thank you,
Paul

tician
10-28-2011, 08:57 AM
See here (http://www.robotis.com/xe/qna_en/81764).

The MX-28 (http://support.robotis.com/en/product/dynamixel/mx_series/mx-28.htm#Actuator_Address_24) has a single magnetic encoder that produces an absolute position for the entire 360 degrees of rotation, in both wheel and servo modes, with values from 0 to 4095.
The EX-106+ (http://support.robotis.com/en/product/dynamixel/ex_series/ex-106.htm#Actuator_Address_24) has a rotary encoder for wheel mode that does not produce an absolute position value, only a relative value (a 16-bit counter that increments for CCW rotation and decrements for CW rotation with each count being ~0.06 degrees). It also has an absolute position sensor (standard potentiometer?) that only produces a valid value when in servo mode (with the same dead zone as all other non-MX series servos).
The AX-12A (http://support.robotis.com/en/product/dynamixel/ax_series/dxl_ax_actuator.htm#Actuator_Address_24), like all other dynamixels not described above, cannot produce a valid position value when in wheel mode (it is even stated in a big yellow box preceded by a graphic exclamation point).

pauljurczak
10-28-2011, 09:26 AM
Thank you for clearing this up. BTW, the thread you linked to contains this from Robotis guy: "... the new MX-28 (and other MX series) ...". Do you have any information about other MX series?

tician
10-28-2011, 03:04 PM
No idea about anything yet, but it is not unexpected that they would shift more of their existing products to the newer controller/sensor boards. No more worn potentiometers (contactless magnetic encoder), higher dynamixel bus data rates (72MHz STM32 vs 16MHz of the ATmega8), and 32-bit math for position and speed control (32-bit ARM Cortex-M3 core versus 8-bit AVR core of the ATmega8).