Hi.
Does anybody know where I can find a worm gear assembly that is suitable for hobby servos or a hobby servo that does not backtrack (rotate) when the power is removed?
A self-locking worm-servo would be a neat idea, but I have never seen or heard of such a thing.
I don’t think you’re going to find a servo that you can’t force to backtrack when you disconnect the power (well, except for one of the really tiny ones, which will break before they turn!) but you might find one that has enough resistance to stay put in your application. In general, the higher the torque rating of the servo, the higher the gear ratio, and the harder it is going to be to force it to turn when the power is off.
What do you want to do with this servo, and what sort of torques might it need to resist when powered down?
-Adam
P.S. Another solution to your problem might be digital servos. They don’t advertise this as a feature, but every digital servo I’ve used (all Hitec brand) will hold it’s last commanded position while powered, even if the signal is shut off. Actually this has caused problems for me, since that is an easy safety shutoff for analog servos. I know you said you were going to disconnect their power, but if you could leave their power on and just needed to shut off or disconnect whatever was generating the signal, this could work too.
I don’t know of one, but I have to agree that’s a pretty cool idea.
The big huge womping gotcha with worm drives is they’re not horribly efficient. The whole idea of the involute gear tooth form is that the tooth-to-tooth interaction on two pinion gears is a rolling contact rather than a sliding contact. Wear and friction is minimized.
On a worm gear there’s always sliding contact. Friction goes up, wear goes up.
Not to say it can’t be done. If you gut a servo you’ll find a gear train with a motor at one end and a potentiometer at the other. You can always pull these out and stick them on a new gear train. (Just be sure to include some sort of stop so the motor can’t crank past the hard limit on the potentiometer! Remember after all that gear-down there’s a lot of torque available for trashing a pot.)
Actually, I take some of that back:
rcgroups.com/forums/showthread.php?t=481847
Apparently GWS has thought about this and can build custom servos (for a price, I’m sure.)
Tom
Hi Guys.
Thanks for the replies.
What I’m trying to do can be seen in this post (5th post down)
Nexisnet, we talked about this in the above mentioned post.
So a digital servo holds it’s place and does not backtrack when the signal is removed and the power is on. That’s great, no signal - no feedback= no chattering. I can leave the power on and stop the control signal.
Does a digital servo need a special type of controller or the same type as the analog servo?
Digital servos don’t act any differently when you stop sending them a signal then when you signal them to stay put, there is still a feedback loop internal to the servo. If the servo is chattering while you signal it to stay in one place, it will still chatter just the same when you disconnect the signal line. Digital servos do correct their positions more frequently, so they tend to be less noisy (and higher-pitched).
In general, all “hobby” servos will work with any “hobby” controller. The “Digital” refers to the electronics internal to the servo and how they measure and control their position. They still take the same pulse-width signal, and will work just fine with your Pololu servo controller.
You would only need additional electronics if you wanted to program speed and range characteristics internally in your digital servo, and even then I don’t know of any brands other than Hitec that offer that feature.
Disclaimer: As far as digital servos go I have only worked with the Hitec brand, and while they are nice, I’m not trying to be a Hitec salesman here. Can anyone confirm that other brands of digital servos? Are the “Joinmax Digital Servos” sold here actually digital servos, or are they analog servos from a manufacturer called “Joinmax Digital”?
Sorry to add more questions.
-Adam
Heyyy! I read that post!
Ok, I can toss out some ideas to try:
Many moons ago I built a drive train for something with similar requirements. The idea was to leave the system in one of two end positions. The way I designed it a TTL signal would act like a toggle and throw it to the opposite position. Pulse it again and it would come back. A switch at either end would let the controller know what position it was in. No signal meant an intermediate position, and by golly it had better be in motion.
The electronics were pretty simplistic. You’re driving this with a micro, so there’s no sense re-visiting them. But the design approach could work here and give you everything you need:
Have some sort of a switch to indicate the down position. When powered up, drive a DC motor until you hit the down position. Turn off the motor. You’re down.
Likewise have a switch indicate the up position. When commanded to go there, drive the DC motor the opposite direction until you hit the up position switch. Turn off the motor. You’re up.
And yeah, your idea of using a worm drive is a good idea. No way to back-drive it, so once it’s there it can power down and not draw current / produce heat / make noise.
There are scads of small worm drive DC gear motors out there. Here are a couple:
solarbotics.net/library/piec … _ucas.html
pololu.com/products/tamiya/0068/ (which I think you already saw)
pololu.com/products/tamiya/0069/
(likewise)
pololu.com/products/tamiya/0073/
(likewise)
Lots of choices out there. Just a matter of making it fit. So you’re back to the size restraint problem.
If you’re looking for switches that can’t get dirty or break down, hall effect switches and neodymium magnets make a great limit switch. They’re pretty repeatable, give decent accuracy, are cheap to buy, and don’t care a hoot about dirt, grease, oil, water, etc.
Hey, best of luck on that project. That looks cool!
Tom
Hey Guys
Thanks for the advice. I’ll see what I can do and post the results.
Thanks for your time