Modified Servo - 360 degree rotation
Birdseye
Posts: 3
I'm using four servos in my camera system and the PSC-USB (#28823) to control the servos (GWS).
I shall be grateful if someone can explain why the I'm having the following experience with the servo that has been modified for continuous rotation. This servo provides the 360 degree horizontal rotation for the camera pan/tilt cradle.
Experience:
When I turn on the power, the servo rotates non-stop. Through trial and error I manage to make it stop at, or near, the position 1480 and the "offset" slider at the extreme right position. To rotate clockwise I slide the servo position bar to a number greater than 1480 and the larger this number is, the faster the rotation. The reverse is true for ani-clockwise rotation. To stop at the desired position, I type in the number 1480 and press ENTER and the servo stops and holds that position.
When the power to the PSC-USB is turned off and then on again, the non-stop rotation happens even though the servo position is at 1480 and the "offset" slider is to the extreme right. To slow it down to a complete stop, I slide the position bar down below 1480 and then up again almost immediately. With this movement, the speed of the rotation slows down and it will stop moving when I find the new magical number. This 'stop number' is usually slightly different from the previous number.
Any advise how I may properly handle the situation with this modified servo will be greatly appreciated. As the camera will be situated on top of a mast 40-50 ft above ground level, this spinning must be avoided.
I shall be grateful if someone can explain why the I'm having the following experience with the servo that has been modified for continuous rotation. This servo provides the 360 degree horizontal rotation for the camera pan/tilt cradle.
Experience:
When I turn on the power, the servo rotates non-stop. Through trial and error I manage to make it stop at, or near, the position 1480 and the "offset" slider at the extreme right position. To rotate clockwise I slide the servo position bar to a number greater than 1480 and the larger this number is, the faster the rotation. The reverse is true for ani-clockwise rotation. To stop at the desired position, I type in the number 1480 and press ENTER and the servo stops and holds that position.
When the power to the PSC-USB is turned off and then on again, the non-stop rotation happens even though the servo position is at 1480 and the "offset" slider is to the extreme right. To slow it down to a complete stop, I slide the position bar down below 1480 and then up again almost immediately. With this movement, the speed of the rotation slows down and it will stop moving when I find the new magical number. This 'stop number' is usually slightly different from the previous number.
Any advise how I may properly handle the situation with this modified servo will be greatly appreciated. As the camera will be situated on top of a mast 40-50 ft above ground level, this spinning must be avoided.
Comments
A simple change in temperature can cause the servo to require a new "magic" number to stop.
Servos have a "dead band" in which they will not move. I don't know if it would be possible to make the dead-band wider.
A stepper motor would probably be a better choice for what your doing.
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"SX-Video·Module"·available from Parallax for only $28.95 http://www.parallax.com/detail.asp?product_id=30012
"SX-Video OSD module"·available from Parallax for only·$49.95 http://www.parallax.com/detail.asp?product_id=30015
Product web site: www.sxvm.com
Available now! Cheap 4-digit LED display with driver IC·www.hc4led.com
"I reject your reality, and substitute my own." Mythbusters
·
Thank you for your comment.
You are indeed correct - there's a "dead band". I know about this dead band through the trial and error process but , frankly, I didn't know this temperature issue you mentioned so I hope the band will be sufficient for the temperature changes that may arise.
At one time I was considering the purchase of a pan-tilt device which uses stepper motors like you suggested but the thing was quite expensive and it did not cater for the zoom and shutter functions. I decided to DIY complete with shutter and zoom functions. I use servos because I felt there are ready solutions like the PSC-USB and compatibility with RC systems should I venture in that direction. Perhaps It would have been easier if an unmodified servo was used instead but I a 360-degree horizontal rotation capability is desirable. The shutter, zoom and vertical tilt functions are well served by the other three servos.
I guess I'm still stuck.
Birdesye
You could always hack the servo and control the motor directly. If you are careful you may even be able to use the h-bridge on the control board inside the servo.
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"SX-Video·Module"·available from Parallax for only $28.95 http://www.parallax.com/detail.asp?product_id=30012
"SX-Video OSD module"·available from Parallax for only·$49.95 http://www.parallax.com/detail.asp?product_id=30015
Product web site: www.sxvm.com
Available now! Cheap 4-digit LED display with driver IC·www.hc4led.com
"I reject your reality, and substitute my own." Mythbusters
·
Either way, you don't have to redo your mechanical setup.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
First, load your control program *before* you turn on the power to the servos. Your control program should start out (i.e., first and subsequent frames) by setting the "offset" to the servo's·more-or-less deadzone position. Load the program to the controller, and then power up the servos. If the c-r servo is properly centered (using the pot via the adjustment hole in the servo), and then the "offset" is properly set, your c-r servo may move a little, but it should "center" and stop after that brief movement to center.
Second, or really first, you should verify that your c-r servo is centered (via the pot) before you attempt to operate it with the controller. It seems like yours may not be centered, based on your comment that you have to set the "offset" of the controller to one extreme to get the servo to stop.
If I pay due attention to the above points, my c-r servos move very little distance, or not at·all,·when powered up.
As others have noted, external conditions (e.g, temp)·may affect the centering/offset nulling you performed previously. So, in the end, if your application can not tolerate even small movements, then you probably have to go to another kind of actuator or circuit.
PAR
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Chris Savage
Parallax Tech Support
csavage@parallax.com
Many thanks for your input. The situation now is much better. The way I handle the control program may seem strange but it is working! Here is what I did:
Frame 0 - The servo for the shutter mechanism is assigned to Channel 0 in the PSC-USB. Set position, servo rate, duration of pause and Go to frame 1. Click 'record'.
Frame 1 - The servo to operate the zoom gearwheel is assigned to Ch 1. Input settings for Ch 1 and Go to frame 2. The data for Ch 0 are repeated. Click 'record'.
Frame 2 - The servo for the vertical tilt movement is assigned to Ch 2. Input settings for Ch 2 and Go to frame 3. The data for channels 0 and 1 are repeated. Click 'record'.
Frame 3 - The CR servo for the 360 -deg horizontal pan is asigned to Ch 3. Input settings for Ch 3 and Go to frame 3 to prevent looping. The data for channels 0, 1 and 2 are repeated. The "offset" slider is left in the middle position. Click 'record' , save file and then close the program.
Restart program, open saved file and the interface will show Frame 0 and Ch 0 settings. All other channels are at the default positions of 1500. If the power is turned on now, the CR servo begins to spin. To prevent this, I advance the frames one at a time so that the settings of all four channels are visible. At this point the power is switched on and the CR servo moves slowly. Moving the "offset" slider a bit to the right (not extreme right as previously) stops the rotational movement.
I'll test it again and again over the next few days to be sure I have a manageable and reliable solution.
Birdesye