There are a couple of things that might be going on:
1) The control loop can output a a value between -10V and +10V. This voltage is then converted by your amplifer to a corresponding current. This current is what actually produces the torque on your motor. If the maximum current of your amp (which would correspond to +10V from your controller)is what it takes to produce 17 lb-ft, then you are at the maximum torque for your system. This may result in not being able to hold position or velocity if more than 17 lb-ft is required. You can check this by calling "Read DAC.flx" and seeing if you are outputting a constant value of 32,767, which corresponds with a value of 10V from the controller. The way to fix this problem is to get an amplifer with a larger power r
ating, so that a lower voltage output, such as 5V, corresponds to to the current needed for 17 lb-ft of torque.
2) The next problem again has to do with your amplifer. Most amplifiers/motors have two Maximum Current levels specified. One is the peak output, and the other is a continuous output. Typically the peak output can only be held for a couple of seconds. The amplifier will then cut back to the continuous output maximum. So what you might see is that the motor holds it maximum torque briefly, but then loses control. As soon as you go below the continuous output maximum, you can usually go back to the peak max, but again only for a short time. Depending on the type of moves, this may result in behavior that seems irradic. Again the solution would be to get a amplifer with a higher power rating.
