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Servo motor recommendation sought

Started by D.M. Procida November 7, 2019
For my plotter project <https://www.brachiograph.art/> I have
experimented with a variety of different servo motors.

Very cheap analog micro servos work very well (I have been using Tower
Pro SG90s), as you can see from the images on the site, but digital
versions do not, because they are too twitchy.

I'd like to try with some different digital (more expensive) servo
motors to see if they are any better in this particular application, but
would be glad of some advice about what would be worth trying next. 

What would you recommend for a next experiment?

Thanks,

Daniele

On Thu, 7 Nov 2019 18:47:26 +0000,
real-not-anti-spam-address@apple-juice.co.uk (D.M. Procida) wrote:

>For my plotter project <https://www.brachiograph.art/> I have >experimented with a variety of different servo motors. > >Very cheap analog micro servos work very well (I have been using Tower >Pro SG90s), as you can see from the images on the site, but digital >versions do not, because they are too twitchy. > >I'd like to try with some different digital (more expensive) servo >motors to see if they are any better in this particular application, but >would be glad of some advice about what would be worth trying next. > >What would you recommend for a next experiment? > >Thanks, > >Daniele
Well, for cheap I would say to either use brushless DC (BLDC) motors made for RC cars or just any cheap brushed DC motors. I know there is code for driving the hobby BLDC motors through an ESC. The ESC is the Electronic Speed Control and accepts the same signals as the analog servos you are using now. So you already have some of the code. You will need encoders too for position feedback. That's what makes the DC motor a servo. The servos you are using now just use a potentiometer for position feedback. CUI makes some pretty good and inexpensive encoders. Just search for the CUI AMT20 series encoders at www.cui.com These encoders have switchable resolutions, which is plenty handy. I have used these encoders on machine tools and they worked just fine. Eric
On 07/11/2019 18:47, D.M. Procida wrote:
> For my plotter project <https://www.brachiograph.art/> I have > experimented with a variety of different servo motors. > > Very cheap analog micro servos work very well (I have been using Tower > Pro SG90s), as you can see from the images on the site, but digital > versions do not, because they are too twitchy. > > I'd like to try with some different digital (more expensive) servo > motors to see if they are any better in this particular application, but > would be glad of some advice about what would be worth trying next. > > What would you recommend for a next experiment? > > Thanks, > > Daniele >
The problem with the cheaper analogue type servos is that the actuator position is determined by an analogue pot. connected to the output shaft. For a variety of reasons hysteresis has to be provided to prevent the actuator output 'hunting', with a result that in your type of application you will get quite large positional errors, and a low number of actual possible actuator output positions. Have a look at the Dynamixel MX series - these are quite expensive - about &pound;40 upwards, but are serial controlled - up to 255 servos can sit on one control bus. Actuator positional information is derived from a high resolution magnetic angle sensor giving 10 or even 12 bits resolution over 360 degrees. They are also solid, repeatable positions not subject to analogue drift due to time, temperature and ageing parameters.