|
|
Home |
|
|
Low bandwidth occurs when the servo system tuning supports only modest response rates. The servo responds to rapid commands in a sluggish manner. The following figures demonstrate problems typical of low bandwidth. At left, the system response (below) is sluggish when commanded to respond to a square velocity command (above). Notice that the response is equivalent on the positive and negative edges of the command. (If sluggish response is seen only when the system is commanded to go zero speed, the problem may be slow settling.)
Velocity vs. time for a typical slow response system (left) compared to rapid response (right) in a servo system. The command is shown above and the response below. The time scale is 0.02 sec/div.
If you suspect that you have low bandwidth, subject the machine to rapidly changing commands and observe the response with higher servo gains. Raise the gains a moderate amount (20%). If you see improvement in the servo response with higher gains, it is likely that your machine is experiencing low bandwidth. If you are using a PI or PDFF (PI+) velocity loop, increase the velocity-loop propotional gain (KV on the ServoStar S, CD, and SC). If you are using a PID position loop, raise the position loop derivative gain.
You may see instability as you increase servo gains. Low servo gains are a good cure for stability problems of all kinds. Unfortunately, low servo gains also cause low bandwith. It is common when raising gains to simlutaeously cure low-bandwidth problems and worsen instability. If you see instability, you may still be on the right track. If you need the responsiveness offered by higher gains, you will probably have to cure the instability in some way other than lowering servo gains. See Investigation of Instability for more information.
The ideal command for this test is a square wave velocity command. A square wave velocity command is equivalent to commanding unlimited acceleration. Some positioners do not allow such a command. If you cannot command a square wave, command small incremental moves (50 mSec each) with the maximum acceleration rates allowable. High acceleration rates inject high frequencies into the servo system, and are best at exposing stability problems. Be sure to limit the amplitude of the command to avoid saturation of the current loop. Anytime you are observing servo response, you should keep the commanded current below the maximum. Saturating the current controller masks stability problems.
| Rate your level of concern about this problem: | |