Motor tuning is crucial with the DDM mounts, and regrettably it used to be a trial-and-error thing to do to get the mount working properly. First of all you need to make sure the Balancing is as good as possible.
A little theory...
The thing with direct drive motors is that, at least on the ASA mounts, they are driven by a PID control loop, which uses the on-axis encoders to hold the position or get to a new one.
First of all there are some settings under Drive -> Servo-Setup affecting how tightly AUtoslew tries to hold the position and when it will throw an error, giving up (like when your scope hits the pier):
Then there is a more detailed setup affecting the motor parameters itself under the Tune Manual button:
There are the P, I and D values which control the behaviour of the mount at three different speed settings, so you have to optimize your setup for six different sets of PID values:
- RA siderial speed
- RA Medium Speed
- RA Full Speed
- DEC siderial speed
- DEC Medium Speed
- DEC Full Speed
So what does the single values do?
- D: The higher the more the mount tries to hold the position. It makes for stiff movements.
- I: The higher the more the mount tends to oscillate when getting close to position
- P: Behaves similar to I
There is also a Noise Filter which is a good idea to keep on, otherwise the mount might tend to oscillate more. Please note that vibrations can occour when both axis' move at the same time - you should test for that, too. But more on that later.
A second set of parameters affects the speed and acceleration/deceleration rates of the mount, and can be found under Drive -> Slew Speed in Autoslew:
You can tune this to make slews faster or slower. Especially with big, heavy systems you might need to lower the values a bit to avoid position errors.
Tuning with Autoslew
Before tuning please make sure your sistem is stable, all screws are tightened, and it is well balanced. An unbalanced system will yield you plenty of errors with a seemingly unknown cause. Trust us, balance is VERY important
If you go under Drive -> Servo-Setup and click on the TUNE Manual button you will see the following window.
There are a few sections worth mentioning:
- At the top you have a graph which shows the error curve while the motors are active
- on the left side you can select which axis you are working on, so either RA or DEC
- There is a Quick select scope section to quickly load "good enough" default values for most systems
- The Tuning parameters are in the middle
- On the right you see the Test Speed settings, together with the Toggle checkbox and the Go and STOP buttons
Alright, now to tune your settings you should start with a somewhat working configuration - either by running an automatic tuning first, or by loading one configuration using the Quick Select Scope area.
Once done, start with RA, select Siderial Speed, check Toggle and hit Go. The scope will move in RA in siderial speed and change direction after a few seconds. You will see the curve on the upper part of the window. Now if you want to tune your parameters, move the sliders of P, I and D accordingly, then hit Go again. Do this until you are satisfied. Switch to the DE axis and repeat the process. Once satisfied try your luck with the higher speeds.
If you go under Drive -> Servo-Setup and click on the TUNE Auto button you will see the following window.
This is now pretty straightforward.
- Move the scope (motors will be powered down) to a position where you want to optimize the settings for. For a start, point your scope to the pole with the counterweights pointing down.
- Hit the Start Fast tuning button. The mount will move a bit in all directions and Autoslew will try to figure out better values for PID on both axes.
- If successful, you can try to press the Start High Performance Tuning button, which is supposed to refine your settings.
- When done, click on Save to Ini to save these new settings.
Usually it makes sense to do some manual tuning afterwards to get your system spot on.