Negative value of current, rpm and position are wrongly read

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1

Software

Dynamixel SDK with Python

Control Environment

I’m using the dynamixel XM540-W270-T with U2D2 and Power board connected to windows 11, python with Dynamixel SDK.

Software Description

When I read the present current in python of the motor I get back the wrong value. Same for the speed and position.

Belowe you can see how I read the values:

dxl_present_current, dxl_comm_result, dxl_error = packetHandler.read2ByteTxRx(portHandler, MOTOR_ID, 126)

dxl_present_position, dxl_comm_result, dxl_error = packetHandler.read4ByteTxRx(portHandler, MOTOR_ID, 132)

dxl_present_rpm, dxl_comm_result, dxl_error = packetHandler.read4ByteTxRx(portHandler, MOTOR_ID, 128)

Issue Description

When the value are positive I receive the correct value, but if the value is negative (negative velocity, negative current or negative position (in extended position control mode)) the value that I get is completely wrong.
If I use the dynamixel wizard 2.0 I get the correct value in any case.
I think is a problem of the read2ByteTxRx and read4ByteTxRx function in the Dynamixel SDK library, I think they handle in the wrong way negative value.

How can I solve this problem?

2

As documented on the eManual page for the XM540-W270-T, negative values are represented using two’s compliment.

To read the correct values in your code you will need to process negative numbers according to this encoding method.

3

the dynamixel SDK library should not already handle these types of things? (Appear to be not the case)

4

Hey. I’m running in a similar issue I guess, not sure. I cannot take a twos compliment of a number that’s not binary and the current value I’m receiving are in the range of 65523. Did you end up figuring it out ?

5

The “integer” received from ROBOTIS hardware via DXL-SDK function calls such as read2ByteTxRx is already in its 2-complement format. So let’s say that we are dealing with a parameter being returned from the Control Table as a 16-bit integer using a 2-complement coding scheme, meaning that its “real” value is between -32768 and + 32767 (and not between 0 and 65535), for example, the Acc_X of the IMU in the Controller CM-550, see screen capture below:

image

So inside your Python/C/C++ code, a possible pseudo-code approach for conversion into “regular” positive or negative integers can be as follows:

if (Acc_X > 32767) // NEGATIVE VALUES for Acc_X
Acc_X = (65536 - Acc_X) * -1
else // POSITIVE VALUES for Acc_X so use it as received
Acc_X = Acc_X

so “65535” represents “-1” and “65523” represents “-13”. Also “99” represents “99”!

1 Like
6

Thanks a lot for a detailed response. I had tried this approach but the control table for the XC330M288T motor does not have these parameters so im not sure what range to use for the logic.
i will try this logic and get back .
Thanks a lot !!