View Full Version : Servo's, Torque and gearing

11-08-2011, 12:51 PM

I have a 1m long arm that I'm trying to lift smoothly and with a fine resolution on a custom pan and tilt arrangement.

I thought the arm had been made as light as possible .. however once I get everything attached I discover perhaps I haven't been as efficient as I thought. The tilt mechanism is straining a little causing a slight tremor, I also hear a slight squeak from the gears making me think they're under a lot of strain. The servo is also constantly buzzing so it's either struggling to hit the target points, or it's straining against the weight.

I'm using the servo city gearing system and have a Hitec 5485 servo running through a 3:1 gearing system using nylon gears. I'm not using modified servos, (so with this gearing they're operating to 60deg down from 180deg)

Right now I think my options are as follows :

Add a second servo on other side of the tilt mechanism and run them in tandem,
Swap my Nylon gears for a metal, or metal pinion and Delrin hub,
Look into a stronger servo,
I could look into counterbalancing, but this could be very complicated as the arm has a variable length,
I'd be really appreciate any tips or advice anyone can offer. I'm based in the UK and whilst servo city have great parts and prices, the trail and error purchase process is getting a little expensive ;)

Thanks in advance,

11-08-2011, 04:39 PM
I'd recommend counterbalancing first. Pick a weight somewhere in the middle of your range - it should only help your situation. You can easily add and remove weight as well to adjust this. Regardless of what servo or setup you have, it would be recommended to do this.

11-09-2011, 04:12 AM
Cool thanks cire, it's a really tricky one as I'm constantly extending and retracting the arm so when the arm is at it's centre point everything is actually balanced already.

11-12-2011, 06:21 PM
Hello mrbencowell,
I'm on a similar projet.
I may have asked for help in the wrong category:
I would be curious to see the tilt pan arrangement you created to lift such an arm. ( if you don't mind to share)
I'm a real beginner and don't want to make any mistake in the choice of my servos.
Thanks a lot.

11-13-2011, 07:07 AM
Hi there,

I'm also fairly new to all this, it's been 6 months of link chasing and getting help on forums such as this one.

My pan and tilt is constructed using aluminium channels along with the excellent gearbox's offered by Servo City (check here: http://bit.ly/suY0f8). The gearboxes perform two tasks, they take the stress of actually holding the weight in place off the servo itself, and in my application they increase servo accuracy as I've not used an external potentiometer (this reduced the overall travel of the servo but for me this isn't an issue).

Lifting and holding a weight in place is the big issue, my pan is working fine .. however the tilt cannot be counterbalanced and so the servo is under some stress to lift and hold it in place. In an effort to fix this I've ordered metal gears from Servo City, and I'm going to upgrade the servo to something like a Hitec 7955TG which should have more torque than my current servo. I suspect this will work better than running two servo's in tandem as even a fractional difference in startup time would mean the servo's would end up fighting each other.

If your lamp is heavy then I'd suggest you create a structure that can support the weight before you add the servo's, lamps usually have springs to hold them in place. That way your servo's won't actually be holding the lamp up .. which'd cause the lamp to fall over when you turn off the power !

Here's a link to another project that uses Servo's in a pan and tilt arrangement, interestingly this guy has created a 'lock' to hold the gears in place : http://robin.waarts.eu/panobot/

B (http://robin.waarts.eu/panobot/)est of luck,

11-13-2011, 09:51 PM
Doing this properly can be somewhat involved.

As soon as you start adding a lot of inertia to a hobby servo (which are often designed for low inertia but high force applications such as actuating the control surfaces of a model plane) you start having issues with oscillation, overshoot and steady state error. This can actually be a major issue even between different brands of servos - I've had an issue once where a camera mount would oscillate uncontrollably with an HK servo but track fine with a hitec despite both servos having similar measured torque.

Some of the digital servos allow the PID values to be manually tuned using a programmer which should help.

My personal opinion is that with a cheap arm you are probably best using a worm gear as opposed to a hobby servo so that you don't need to deal with back driving and chatter.