RC Servo help

You could have a noisy power supply problem for example if you are using the same 5V supply for both the servor motor supply and the PWM generator's supply.

-howy

Hi,

I don't know how you are driving the servo's PWM signal (CPU etc) but I had the same problem with my camera base. On my system I improved it by using two voltage regulators (One for ATMEL CPU/Network card and one for the Servo's).

In the end I stopped the servo's hunting but switching off the interrupt that drove the servo's 3 seconds after getting the last position change command. The servo's I'm using have enough friction and the camera is so light that the whole construction doesn't move.

Regards Ian Dobson

Home of the Atmel based UDP mobile web cam

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"HAS2BOKwithoutspam@H..O..T..M..A..I..L.....C..O..M" schrieb im Newsbeitrag news: snipped-for-privacy@z14g2000cwz.googlegroups.com... I'm building a camera pan & tilt mechanism to hold a webcam for motion detection. I've used 2 hitec hs-311 servo's attached together by homemade brackets. The system does basically work but I need some advice as it's my first experience with RC servo's (bought from ebay @ £4 sterling each).

The servo's move as commanded but after reaching their demand position they drift/hunt. I expected a little of this but not as much as is occuring. Even without the load of a webcam there is still obvious hunting.

Is this common to all cheap servos? Any way to decrease the error?

The only thing I can think of is to rebuild the pan & tilt using gears to introduce some backlash/play to compensate for the servos.

Cheers.

What about stepper motors?

Normally, good servos shouldn't be hunting too much, as long as the power and signals from the controller are stable. You might have excessive noise on the servo buss, and adding filter and bypass caps can help this. It also helps to keep the controller and servo power separate, else the motor noise can get onto the processor, and servo load changes can pull the buss voltage around.

Also, I have tons of servos, and some of the older ones have gotten rather unstable, and twitch a lot. I could sell them on e-bay, but don't think that would be fair to the buyer, but some people will sell off anything. Maybe you can borrow some good servos from someone else to see if bad servos is the problem, and not one of the other things.

The servo's move as commanded but after reaching their demand position they drift/hunt. I expected a little of this but not as much as is occuring. Even without the load of a webcam there is still obvious hunting.

Is this common to all cheap servos? Any way to decrease the error?

The only thing I can think of is to rebuild the pan & tilt using gears to introduce some backlash/play to compensate for the servos.

How are you controlling the servos? Jitter can easily be caused by programming errors that result in slight changes in pulse width.

Mark

Check for electrical noise on the power and jitter on the control signal.

Try putting a big electrolytic across the servo's power, for starters.

John Nagle

JGCASEY wrote:

In general, cheap servos will tend to "hunt". I would think aiming a camera is a task that is very sensitive to any servo jitter, and especially if you want to do motion detection where any movement of the camera would create problems. Perhaps servos are not the ideal answer. (though I am sure that high quality servos are better at reducing the jitter). Stepper motors would seem to be a better choice.

Joe Dunfee

You don't want to have the servos holding much force. Design your pan/tilt cam so the the servos position the cam, but don't have to strain to hold the cam in a position. If the servos you have have been used a lot in the past, then there may be some wear on the posts or the gearing. You may want to take them apart and relubercate the geating and use some spray contact cleaner on the pot. Others have mentioned the possible power supply problems.

Thanks for all the replys.

The servo's are being controlled by an AVR micro programmed with the excellent BASCOM language (demo version).

6V supply to servos then through a 5V regulator for the logic supply. A PC is running the motion detection program and connected to the servos via RS232.

The problem ended up being the amount of debugging info and other data I was sending to the PC via the serial port. I was rewriting the program to switch the servo updates on/off as suggested by Ian Dobson. The most basic servo program had no hunting with the servos still powered. After a little revision of the main program the pan&tilt platform is nice and steady and won't be sending my motion detection software crazy :)

My main problem was I didn't have any experience with RC servos so wasn't sure if the hunting was normal. Also I didn't make sure the basics worked correctly either :(

Oh well, the large capacitors across the servo power lines (thanks John) make it look as if it means business now.

Cheers all.

I use a diode setup with a cap to protect my servo chip from power dips when the servo motors start to move. Seems to work quite well with a walwart power supply (setup at the bottom of the below page).