Go Down

Topic: Slow the speed of a continuous rotation micro servo  (Read 3838 times) previous topic - next topic

vinceherman


vinceherman

My conclusion from all of the info on this thread is that using servos that have been modified to be continuous rotation motor/gearbox/controller combinations gives you good and bad:
  • Good packaging of the components.
  • Good direction control.
  • Bad predictability of which servos will give reasonable speed control.



usenetfan

I beg to keep asking that since both my analog and digital servos are unmodified, off the shelf components, and the only difference is that analog allows for perfect speed control but the digital does not,

What is the difference in writing control code for analog vs digital?

vinceherman

They are not unmodified.

They were both designed to be positionally aware devices with the intent to move to the commanded position as quickly as possible and hold that position.

That design was them altered by removing the pot that gives positional feedback.  Now you can use the device for a continuous rotation device.

This is *not* what the original design covered.
It is usable, but it *is* a modification from the original design.
So the behavior you get out of it is not guaranteed to meet the expectations of the end user.

The fact that the manufacturer built them pre-modified does not suddenly make them behave differently than the original design.
The manufacturer took out the pot and replaced it with 2 resistors.
The manufacturer did *not* change the control board or its programming.
That control board is still trying to act like it should for a positionally aware servo.

So you get directional control, the same way that you would get it with the original design.
And you get questionable speed control, because different servo control boards approach the positional program differently.

Your analog servo has a control in it that mostly gives variable speed control.
Your digital servo has a control in it that mostly runs at full speed.

Both of these are within the original design of a servo.

If you do not want servo behavior, do not use a servo that is modified from its original design.
As stated earlier, you can put in inexpensive electronics that give good motor control without the servo design getting in the way.

usenetfan

Thank you very much for this detailed and comprehensive explanation.

I would very much like the sellers of servos to include this as part of their item descriptions, as I was totally unaware of the incompatibility of digital servos vs analogs. I would have stuck with analogs and will buy such now.

Again, many thanks for instructing me so patiently.

Ps. Anybody want to buy three digital microservos?

vinceherman

:)
Just to comment, the difference is likely not just analog vs digital.
Any different manufacturer or even different model within the same manufacturer might have the control algorithm that leads to good or bad speed control.
Speed control is not part of the positional servo design.  Except that it might play a part in the PID logic.

And usually, digital servos have better behavior when it comes to positional control.  That is why we pay more for digital servos.

usenetfan

That's probably very true. This is so cool in Arduinos, you learn something new every day.

usenetfan

Thanks for this. I seem to remember trying this at some point to no avail but will retry. The serial traffic only writes the current microsecond value and that does not interfere with the analog one.

Johan_Ha

I have a continuous servo in this build (Youtube).

One normal servo for steering and one continuous servo for moving forward and reverse. The continuous servo works quite ok, with speed control, probably because it's based on a simple analog servo. And for this kind of project, I think it's quite ok to talk about continuous servos instead of dc motors with integrated motor board. We could agree that a continuous servo is not a servo. It's a continuous servo.

I'd still try to control the speed of the digital continuous servo. Perhaps it works in a very tiny sector. Map your 0 - 1023 pot reading to 1495 - 1505 µs output to the digital servo. If the servo reads its potentiometer digitally with 10 bit precision (0 - 1023) and the servo specs say it has a dead zone of 1 µs, I don't quite believe it uses full torque, if it is only 1 µs off the dead zone. It could have say five speeds from full reverse to full forward at 1498 µs - 1502 µs. Just a wild guess.
____________________
- One day my stepper motor driver works like a charm. No task is too big for it and I can do anything with it. Next day it refuses to work and even the tiniest motor blows its fuse. What's wrong with it?
- It's bipolar.

Johan_Ha

Sorry, I deleted my post about possible serial data transfer interference with your servo data pin.
____________________
- One day my stepper motor driver works like a charm. No task is too big for it and I can do anything with it. Next day it refuses to work and even the tiniest motor blows its fuse. What's wrong with it?
- It's bipolar.

TomGeorge

Hi,
Until we get any voltage measurements back about the servo supply under load, we can only guess at the problems.

What is the "buckdown" converter? link to specs/data?

Thanks.. Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

usenetfan

This is the Servo Shop reply, and the case can now be closed:

"I have just tested some of the Tower Pro MG90D 360' servos here, and they all operate in the same manner. They do not have a proportional speed, only a constant speed in either direction of rotation. There is a large "Deadband" area in the centre, where on some occasions you can get a jittering effect from the servos.


I hope this helps,


Thanks and best regards


Robin

Sales at SWM Ltd"


Johan_Ha

Conclusion:

Analog servo = nice little thing
Digital servo = nice little thing with good accuracy
Analog continuous servo  = nice little geared motor with integrated direction and speed control circuit
Digital continuous servo = stupid useless invention
____________________
- One day my stepper motor driver works like a charm. No task is too big for it and I can do anything with it. Next day it refuses to work and even the tiniest motor blows its fuse. What's wrong with it?
- It's bipolar.

usenetfan

Tom, you asked about buckdown regulators. I am using these:

https://www.ebay.co.uk/itm/LM2596-DC-DC-Switching-Adjustable-Step-Down-Voltage-Regulator-Converter-3V-40V-/172058668488

This is how I am using them:

http://www.sabulo.com/sb/3d-printing-2/introducing-8mm-scanner-v2-0/

thank you one and all for your help with the digital servo issue!

Johan_Ha

Conclusion:

Analog servo = nice little thing
Digital servo = nice little thing with good accuracy
Analog continuous servo  = nice little geared motor with integrated direction and speed control circuit
Digital continuous servo = stupid useless invention
I don't believe it! After participating in this thread, I checked what I had ordered to our school. And in the list there was a bunch of continuous servos, which I thought would be handy for small robot cars, similar to the lego like car I linked to earlier. But I seem to have ordered digital continuous servos. These stupid useless inventions. The more I think of it, the more it puzzles me, why there even is a thing like a digital continuous servo. It has no benefits whatsoever from being digital. Only drawbacks. Whereas a conventional servo probably has probably lots of benefits from being digital.
____________________
- One day my stepper motor driver works like a charm. No task is too big for it and I can do anything with it. Next day it refuses to work and even the tiniest motor blows its fuse. What's wrong with it?
- It's bipolar.

Go Up