Servo motor recommendation sought

[D.M. Procida]

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

 

[Andy Bennet]

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 ÂŁ40 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.