Programming Syntax for Digital Servos

PSU Mech Eng

Hey guys,

For the ASME national design competition my team and I are creating a window washing robot.· We originally had four high torque analog servos to do the job but found a conversion mistake for two of the servos that left us under torqued.· We replaced the serovs with much higher torque digital serovs.· We looked and found they are supposed to be more accurate and have a higher standing torque which was the main consideration over analog.· They were also supposed to be fully programmable.· However, we have run into problems using our basic stamp to run them.· We originally figured the same pulsout command structure would work.· We can run the motors with this command but are having trouble figuring out duration and most importantly direction.· We need to be able to run the servos in one direction for a time and then backwards for a time.· If it is relavant, our servos are Hitec HS-5955 TG Titanium Gear Coreless Digital Mulitpurpose Servos.· They came to use modified for continuous rotation.· Any thoughts on the differences between digital and analog servos as well as the command structures would be appreciated.

MSDTech

For these servos, as you shorten the pulse from 1.5 ms, they will rotate faster in one direction and as you lengthen it from 1.5 ms, they rotate faster in the other direction. If you were using non-continuous rotation servers, the pulse width determined the angle they rotated to.
The modification to make many servos continuous rotation is to disconnect a feedback potentiameter that is used to sense the position of the shaft. With this removed, the motor tries to move to a position, but it can never be reached. If you don't need more than 180 degrees of rotation, you might check to see if the feedback can be re-established, allowing you to control the position of the shaft.

PSU Mech Eng

Thanks for the hints. We may have figured out a way to get around using continuous rotation servos but were kinda stuck now with the ones we have. Along those lines though, would the hack associated with changing to continuous reduce the torque output of the servo? We did an admittedly fairly crude test of the torque and only found the serovs to be producing a small fraction of the rated stall torque. Any ideas?

allanlane5

If the servo's are mechanically able to produce the stall torque, then the problem is probably the power supply you have driving the servo's. At stall, a servo can pull 1 amp or so -- which is a lot for a small power supply.

PSU Mech Eng

That was exactly it!· We upgraded the power supply and they whip around.· The guy at radio shack said it would have been obvious if we were electrical instead of mechanical.· We also had the servos modified back to non continuous for the positional accuracy.· Everything is coming together except we cant use batteries because of the amp needs.· Cant win them all though.· Thanks for the insight

erco

You can't use alkaline batteries, but you can definitely find rechargeable batteries that can keep up with your current requirements. Electric helicopters are using LiPo batteries now that will put out 10 amps or more. You could·probably use·decent hobby-grade NiMH sub-C cells pack. The type with solder tabs, not the individual consumer-grade AA or C batteries. And now that you know how much current is flowing, change to heavier wire where necessary to minimize voltage drop.

BTW, ME's do very well in robotics.·Pity the·poor devil working at Radio Shack; he'll be there long after you've made millions selling your window-washing invention to iRobot!

Good luck, sounds like a cool project. Post some videos when you can.

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·"If you build it, they will come."