Thread: Digital Servo Power??

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This is my first year building animatronics, and I'm building some servo animated props. I went with digital servos because they had much higher torque and because the Animated Lighting Board that was available from the manufacturer of my lighting/music syncronization controllers was a digital controller.

I bought a 7.5V regulated power supply synchronized my first servo to my mp3 file and let her rip. I ran into problems in that the servo would start acting eratically, it would start moving slowly part way through the song. I do have a video of this on you tube if this would be helpful. Although I do have one answer for what the problem is, just haven't been able to fix it yet.

YouTube - servo problems with my skeleton hand

I contacted both the board manufacturer (animated Lighting) and the servo manufacturer (Hitech) and the servo folks told me it was likely due to my power supply being regulated.

"if your supply is a regulated supply, digital servos do not work well with them since the power is being switched on and off at a high rate to regulate the voltage"

That being the case I'm having some trouble finding a 6 or 7 volt non-regulated power supply.

Is anyone else having a similar problem?
What are you using to power your digital servos?
Any links for a 7V 3amp (or less) non-regulated power supply?

Thanks.

These look like nice linear (Not Switching) power supplies:

https://www.acopian.com/single-l-goldbox-m.html

I use battery power. One huge honkin' 6v deep cycle RV type battery. I've run over 40 servos from it for 4 hours with minimal voltage drop. But I use separate power supplies for the boards and controllers.

Never thought of an RV battery, since weight and size aren't an issue that should work great, and I can use it as a spare for the pop up trailer I have parked in our driveway right now....


Or, Linear regulated then means it doesn't turn the power on and off is that right?

Thanks for the help, at least I have a couple of options.

AC by it's very nature turns on and off, the goal of the power supply is to turn the sine wave that is AC into a smooth DC output.

Linear power supplies use a transformer to reduce the 110VAC from the wall to the lower level ( in this case 7 VAC) then the AC is turned into DC using Diodes, after that the regulation comes into play, where the ripple left over from the AC to DC conversion is removed and the DC level is held to 7VDC. Because of the transformer linear weigh more.

Switching power supplies use transistors to switch the AC Mains and control the duty cycle of the output DC power - therefore the power is sometimes considered "Dirty" - however switching power supplies have become the norm in computers and shouldn't be causing the problem you are having as long as you bought a nice one. Because switching power supplies don't have a transformer, you can usually get more current out of them - due to the fact that you are drawing directly from the household power.

The battery suggestion is ideal, as batteries are a true DC source and no conversion needs to take place. Batteries are used to power the phone companies networks for this very reason - nothing is cleaner then power from a battery.

Erratic servo behavior is commonly due to insufficient current supply. You said your power supply is 3A. Is the servo the only thing its powering? One servo alone can pull 1A. Is something else kicking in and pulling juice?

I'm not sure I'm buying the regulated/non-regulated power explination they gave you. The power to the servo motor is just that...power...the motor windings and magnet doesnt really care.

The power to the board is regulated no matter which type of supply is used...there is an onboard 5v regulator which powers the microprocessor and the signal between the microprocessor to servo chip.

Also, at 7.5V your probably over volting the servo. Most all R/C servos use 4.8v or 6v...

This Servo is rated up to 7V and the manufacturer assured me that at 7.5V it would be just fine. In the tests I have been running this is the only servo running and my power supply is actually a 10amp unit, the controller board on board power is limited to 3Amp.

I was told that this issue was unique to digital servos because the digital servos recieve their pulses about 15 times faster 300 per second vs. 20 per second.

http://www.futaba-rc.com/servos/digitalservos.pdf

"The second, is that a digital servo sends pulses to the motor at a
significantly higher frequency. This means that, as opposed to the motor
receiving 50 pulses/sec., it now receives 300. Although the length of the
pulses is reduced in a direct ratio to the higher frequency because the power
is being turned on/off to the motor more frequently, the motor has more
incentive to turn. This also means that not only does the servomotor respond
faster to the commands, but that increases or decreases in power for
acceleration/deceleration are able to be transmitted to the servomotor
far more frequently. This gives a digital servo an improved deadband, a
faster response, quicker and smoother acceleration/deceleration, and better
resolution and holding power."

I'm inclined to agree with this particular explanation as the reseller also warned me about using anything other than batteries to power the servos.

Thanks for the help though.

Sounds like you need to do some testing of battery versus the supply you have. You may also need to do some testing of the servo cables with respect to noise shielding. At higher data rates, the cables are susceptible to noise and crosstalk from adjacent cables.

7.2 volt RC batteries are common and inexpensive.

If it runs fine on a battery then you have choices:

Add some filtering (capacitors and or inductors) to the power supply output.

Use a non-switching regulated linear supply.

If it exhibits the same behavior on battery power:

Shield the servo cables ( loop the cable around a ferrite bead near the servo ).

It might not be a bad idea to check the supply voltage under a load either, to make sure it is what you expect and not too high or low.

www.jameco.com

wow...i overlooked a fundamental...hey Im on drugs...gimme a break...wait I already have 3...

On the surface I wouldnt have thought the pluse rate to the windings would approach or surpass the pulse rate of the power supply. But it looks like at some point the two signals come into conflict. It would be cool to see this on a oscilloscope. I did some reading and looks like to me that both regulated and linear supplies are too noisy to drive a digital servo and only the pure dc signal of a battery will work...I suppose this is why the tech offered the warning.

Cool project...looking forward to seeing video when its done.