# Thread: Torque limit and moving speed for AX-12A

1. Abacus
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## Torque limit and moving speed for AX-12A

Given a certain torque limit (0x22,0x23) and certain moving speed (0x20,0x21), if the distance between the initial position and the goal position (0x1E,0x1F) is larger than the slope (let us say 128), the trajectory of the movement is always seemed to be 3 phase: After a little delay, a quick acceleration-phase to a certain velocity, an uniform-velocity-phase (the velocity is supposed to be the moving speed), a quick deceleration-phase when close to the goal position.

There are two strange behaviors that I cannot understand:

1. Fix the torque limit and change the moving speed (10%, 20%,30%...100%), always from the same initial position to the same goal position, however, the uniform-velocity-phase value (derivative )are not proportional to the set value, why?

1. Fix the moving speed to 0 ( or 1023 for 100% maximal velocity), always from the same initial position to the same goal position, change the torque limit (10%, 20%,30%...100%), there will always be uniform-velocity-phase with different derivatives, but smaller than the 100% moving speed limit for torque limit not to big, this uniform-velocity-phase extends into the Slope range (0X1C=128), until very close to the goal position. So at the end of the uniform-velocity-phase the output torque is not the limit since it decreases in Slope range, Why the output torque maintains this uniform-velocity-phase while it can still accelerate the speed?

Looking forward for some answers or suggestions.

2. ## Re: Torque limit and moving speed for AX-12A

Hi!

First element of an answer : "torque limit" is actually the maximum PWM value that can be used on the motor driver. That explains why the turn rate in the uniform-velocity-phase will be affected in non-trivial way by a change in this value.

3. Abacus
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## Re: Torque limit and moving speed for AX-12A

Do you mean that the AX-12 uses step motor and controls it using PWM, one pulse pushes the shaft advance by one step?

4. ## Re: Torque limit and moving speed for AX-12A

No, the motor is a brushed DC motor. The PWM applied to the driver has a similar effect to varying the DC voltage applied to the motor, and brushed DC motors have non-linear corellation between voltage, torque and turn rate.

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