Servo puzzle

Problems, experiences or just something to say about RC gear? Say it here.

Servo puzzle

Postby 7sp » Sat Aug 06, 2011 8:00 am

Poser for the servo experts out here...

I have a Servo speed regulator with reversing capability.

1) When I connect two servos up to the regulator and reverse one channel the two servo operate perfectly in sync with each other step for step.

Wanting to use a simple y-connector because have no need for the speed regulation in this application.

2) I take one servo and reverse both the motor wires and the potentiometer wires so the the servo is now reversed.

3) I connect up both servos to a simple y-connector and they both travel the same direction as wanted.

Problem: It is now apparent is that they are no longer perfectly in step with each other and the end points have changed.

I'm thinking this is due to the pot's not really being linear. I have returned the wiring to normal on the same servo and it again returns to being in sync with the non-reversed servo when run though the regulator. Just to make sure that it was not a fluke.

I have also tried this with several servos to make sure it was not just one mismatched servo. In all cases when reversed at the motor and pot they are no longer in sync. I went one step further and connected up one of the servos that I manually reversed to the speed regulator and reversed it back(in the regulator) they are again no longer in sync. It would seem it is dependent on the direction that is used threw the motor or pot.

Any suggestions on how to manually tune a reversed servo?

Thanks, 7SP
User avatar
7sp
 
Posts: 115
Joined: Sun Jan 09, 2011 9:50 am

Re: Servo puzzle

Postby ergocentric » Sun Aug 07, 2011 4:58 am

7sp wrote:I'm thinking this is due to the pot's not really being linear.


did the servo centering change when you moved the wires?
just thinking you could turn the pot
unlikely that it is a nonlinear pot
unconcerned but not indifferent - MAN RAY
User avatar
ergocentric
 
Posts: 350
Joined: Sat Jun 05, 2010 9:45 pm
Location: small town, Ontario, Canada

Re: Servo puzzle

Postby 7sp » Wed Aug 10, 2011 6:16 am

ergocentric wrote:
7sp wrote:I'm thinking this is due to the pot's not really being linear.


did the servo centering change when you moved the wires?
just thinking you could turn the pot
unlikely that it is a nonlinear pot


I did not notice that the center changed, but that would account for the change in end points. What appears to have changed strangely is the stepping rate they no longer are in lock step with each other and one eventually over takes the other. Noting that this occurs before they both reach the end point.

7SP
User avatar
7sp
 
Posts: 115
Joined: Sun Jan 09, 2011 9:50 am

Re: Servo puzzle

Postby ergocentric » Wed Aug 10, 2011 2:59 pm

7sp wrote:
ergocentric wrote:
7sp wrote:I'm thinking this is due to the pot's not really being linear.


did the servo centering change when you moved the wires?
just thinking you could turn the pot
unlikely that it is a nonlinear pot


I did not notice that the center changed, but that would account for the change in end points. What appears to have changed strangely is the stepping rate they no longer are in lock step with each other and one eventually over takes the other. Noting that this occurs before they both reach the end point.

7SP


could that be and artifact of a change in centering converted to linear motion or are you using a protractor to measure the servo angle?

http://www.ossmann.com/protractor/
unconcerned but not indifferent - MAN RAY
User avatar
ergocentric
 
Posts: 350
Joined: Sat Jun 05, 2010 9:45 pm
Location: small town, Ontario, Canada

Re: Servo puzzle

Postby ergocentric » Wed Aug 10, 2011 3:01 pm

another thought occurs, these servos are on a Y harness, right?
unconcerned but not indifferent - MAN RAY
User avatar
ergocentric
 
Posts: 350
Joined: Sat Jun 05, 2010 9:45 pm
Location: small town, Ontario, Canada

Re: Servo puzzle

Postby 7sp » Wed Aug 10, 2011 10:27 pm

ergocentric wrote:another thought occurs, these servos are on a Y harness, right?


Yes the problem is seen when I have one reversed servo and one stock servo on a Y-harness.

Thanks for the protractor links. Ran testing again and yes center did change (I had used another servo arm with slightly different teeth position offset so did not notice it at first.

The goal was just to have two servos in step with each other not necessarily measure so had not used a protractor. I used the direct approach with two servos mounted on the temporary jig apposing each other. With extended servo arms just touching each other. Making it easy to see and track the arc and if they are in sync with each other.

Again if un-reversed (move wires back) and run them both though a electronic "Servo speed regulator with reversing capability" they are perfectly in sync with each other. The problem only occurs when swamping the pot and motor wires regardless if on the y-harness or thew the regulator. It would seem that it is some how using the total resistance on each 1/2 of the pot to adjust the steps and rate causing them to become out of step. This was the only logic I could apply to why when reverses one has more steps and can even overtake the other when starting from the exact same position.

7SP
User avatar
7sp
 
Posts: 115
Joined: Sun Jan 09, 2011 9:50 am

Re: Servo puzzle

Postby ergocentric » Thu Aug 11, 2011 3:32 pm

best bet is to mount the servo up-side-down on cardboard or something and elastic band or tape a skewer to the servo arm to make it as long as the protractor radius, measure before and after and compare the two to try to reset the center position.

I still can't imagine the pot could be anything but linear, maybe there is a resistor in series or parallel with the pot?

parallel would make it non-linear, series would just change the center position
unconcerned but not indifferent - MAN RAY
User avatar
ergocentric
 
Posts: 350
Joined: Sat Jun 05, 2010 9:45 pm
Location: small town, Ontario, Canada


Return to Radios and Servos

Who is online

Users browsing this forum: No registered users and 13 guests