Post by thread:[PIC]:PIC12F675 Servo motor wobble problem

Hello All, I had written to the list long time back regarding the drive of servo motor using a potentiometer and a PIC12F675. I was successful in doing driving the same. I got back to working on this project and I am happen to face a new problem. When there is no load on the motor, it works perfect. When I load the motor, it keeps wobbling. I think the motor is not able to drive the load or there is a small slippage that is causing the problem. All I wanted to do was that the motor should rotate between 90 and 180 degrees when a switch is toggled. Should I just switch off the motor when it is near the desired position? It looks like the motor has reached 3 degrees and trying to correct the angle and slipping off to -3 degrees. It keeps repeating this cycle. I am posting a snippet of the code: if(GPIO & (1<<3)) { if(servoVal>=(1499)) { servoVal = servoVal-(quotient*10); } // do_servo(); } else if(!(GPIO & (1<<3))) { if(servoVal<=(2300)) { servoVal = servoVal+(quotient*10); } //do_servo(); } Sai

It does sound like you have a load problem. Can you reduce the load through any mechanical means? "Turning off" the motor is probably not a good solution- it'll lose all its holding power at that point and you'll likely be worse than you were. Mike H. On Tue, May 22, 2012 at 9:11 PM, yamanoor sairam <spam_OUTyamanoorsaiTakeThisOuTyahoo.com>wrote: {Quote hidden}> Hello All, > > I had written to the list long time back regarding the drive of servo > motor using a potentiometer and a PIC12F675. I was successful in doing > driving the same. I got back to working on this project and I am happen to > face a new problem. > > When there is no load on the motor, it works perfect. When I load the > motor, it keeps wobbling. I think the motor is not able to drive the load > or there is a small slippage that is causing the problem. All I wanted to > do was that the motor should rotate between 90 and 180 degrees when a > switch is toggled. > > Should I just switch off the motor when it is near the desired position? > It looks like the motor has reached 3 degrees and trying to correct the > angle and slipping off to -3 degrees. It keeps repeating this cycle. > > I am posting a snippet of the code: > > if(GPIO & (1<<3)) > { > if(servoVal>=(1499)) > { > servoVal = servoVal-(quotient*10); > } > // do_servo(); > } > else if(!(GPIO & (1<<3))) > { > if(servoVal<=(2300)) > { > servoVal = servoVal+(quotient*10); > } > //do_servo(); > } > > Sai > >

It sounds like the refresh rate is not fast enough. Best regards, Bob On Tue, May 22, 2012, at 08:11 PM, yamanoor sairam wrote: {Quote hidden}> Hello All, > > I had written to the list long time back regarding the drive of servo > motor using a potentiometer and a PIC12F675. I was successful in doing > driving the same. I got back to working on this project and I am happen > to face a new problem. Â > > When there is no load on the motor, it works perfect. When I load the > motor, it keeps wobbling. I think the motor is not able to drive the load > or there is a small slippage that is causing the problem. All I wanted to > do was that the motor should rotate between 90 and 180 degrees when a > switch is toggled. > > Should I just switch off the motor when it is near the desired position? > It looks like the motor has reached 3 degrees and trying to correct the > angle and slipping off to -3 degrees. It keeps repeating this cycle. > > Â I am posting a snippet of the code: > > Â if(GPIO & (1<<3)) > Â Â Â Â Â Â Â Â Â Â { > Â Â Â Â Â Â Â Â Â Â if(servoVal>=(1499)) > Â Â Â Â Â Â Â Â Â Â { > Â Â Â Â Â Â Â Â Â Â servoVal = servoVal-(quotient*10); > Â Â Â Â Â Â Â Â Â Â } > Â Â Â Â Â Â Â Â Â Â // do_servo(); > Â Â Â Â Â Â Â Â Â Â } > Â Â Â Â Â Â Â Â Â else Â Â Â Â if(!(GPIO & (1<<3))) > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â { > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â if(servoVal<=(2300)) > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â { > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â servoVal = servoVal+(quotient*10); > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â } > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â //do_servo(); > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â } > > Sai -- http://www.fastmail.fm - Faster than the air-speed velocity of an unladen european swallow

yamanoor sairam <yamanoorsai <at> yahoo.com> writes: > > Hello All, > > I had written to the list long time back regarding the drive of servo motor using a potentiometer and a > PIC12F675. I was successful in doing driving the same. I got back to working on this project and I am happen > to face a new problem. Â > > When there is no load on the motor, it works perfect. When I load the motor, it keeps wobbling. I think the > motor is not able to drive the load or there is a small slippage that is causing the problem. All I wanted to do > was that the motor should rotate between 90 and 180 degrees when a switch is toggled. It sounds like the generated pulse width is not stable, check the duration of the pulse. I had similar problem when an interrupt unrelated to pwm generation was firing at random time with respect to the pulse, which resulted in slightly variable pulse width. Sergey Dryga http://beaglerobotics.com

Servo control is not that simple, there a lot in control theory that you must deal of. Closed loop instability is common and you must address it with a more sophisticated algorithm. It seems that a PID controller may be better suited for your problem. Have you tried to add a dead-band to your logic (simply tolerate the 3 degree error. If the position is inside the 3 degree band, then stop trying to correct it). Isaac Em 23/5/2012 00:11, yamanoor sairam escreveu: {Quote hidden}> Hello All, > > I had written to the list long time back regarding the drive of servo motor using a potentiometer and a PIC12F675. I was successful in doing driving the same. I got back to working on this project and I am happen to face a new problem. > > When there is no load on the motor, it works perfect. When I load the motor, it keeps wobbling. I think the motor is not able to drive the load or there is a small slippage that is causing the problem. All I wanted to do was that the motor should rotate between 90 and 180 degrees when a switch is toggled. > > Should I just switch off the motor when it is near the desired position? It looks like the motor has reached 3 degrees and trying to correct the angle and slipping off to -3 degrees. It keeps repeating this cycle. > > I am posting a snippet of the code: > > if(GPIO & (1<<3)) > { > if(servoVal>=(1499)) > { > servoVal = servoVal-(quotient*10); > } > // do_servo(); > } > else if(!(GPIO & (1<<3))) > { > if(servoVal<=(2300)) > { > servoVal = servoVal+(quotient*10); > } > //do_servo(); > } > > Sai >