Continuous Rotation Servo Speed Control

[duda123]

Ok! Thank you very much
I'll take a look on all the info you've provided me with and 'll come back later with a solid design to get help on the electronic stuff.

 

[Mr RB]

...
If one goes to Servo City (https://www.servocity.com/), you can find the dead band. For most analog servos (i.e., the ones likely to be converted to continuous rotation), you will see that the dead band is about 5 to 8 uS. The full range is +/- 500 uS for +/-60° or so. Using a dimensioned sketch and known gear ratios, one could calculate its positioning accuracy. 2uS is about as small a dead band as one can find in hobby servos.
...

I believe the "servo" suggested by his friend (re post #1) is not an "RC servo" but is a "DC servomotor" with encoder feedback. That is generally the pro alternative to a stepper motor and is smoother and more powerful/energy efficient for driving a shuttle at a constant linear speed.

Normally you would just buy a DC motor with shaft encoder fitted, and use a feedback circuit to drive the motor at a fixed RPM. Since he only needs speed accuracy and not positioning accuracy it can be simple to drive, using an analogue feedback system that adjusts motor PWM inversely proportional to encoder freq.

 

[jpanhalt]

Hello guys! I'm trying to build kind of a motorized small skate that moves front/back, with high precision movements (the speed should remain the exactly the same if not changed) and a speed controller.

Basically, it would need to be able to carry 4-5 kilograms on an horizontal surface, with a constant speed (that could be changed using a wired controller).

A friend recommended me to use a servo for the "high precision constant speed", but I don't know how to control a servo or how to make a speed controller for it. Also, I'm not sure if a servo can move a skate with 4-5 kilograms on top of it.

So, basically, I'd need to build a servo controller with on/off switch, invert direction switch and a speed "switch".

BTW, I'd really love if I could build that without any programming, since I don't have any hardware to write to chipsets.

I know I'm asking a lot of things, but it would be really awesome if you guys could help me with this project.

Thank you very much!

@MrRB

I did not read that sentence the way you did, nor did subsequent posts by the OP suggest that he was going to use an encoder and industrial servo. In fact, the controller he gave as an example is clearly designed for hobby-type servos and dd not have encoder feedback.

BTW, as I look into CNC'ing my Bridgeport mill, I find we may be in a transition period or post-transition period. I talked with a person (Mel White, Las Vegas, NV, USA)) who has done quite a few conversions and had a business that specialized in such conversions. His view was that a servo with encoder was important many years ago for precision, but that the holding power of modern, high-quality steppers has eclipsed that need. And, they are simpler to control.

John

 

[Mr RB]

Yep modern steppers are good, but many of the people are still going for servos as the top level of performance. I've got steppers on my small CNC router/mill, but people with larger CNC router setups that need more power and speed go for servos. Bridgeports are slow anyway and would be fine with a powerful stepper.

Re the OP, I'm quite sure his friend meant a DC servomotor even though the OP may have missed that. It would be the pro choice for smoothly moving a 5kg shuttle at a fixed speed. Steppers make too much vibration and waste power (camera shuttles are likely portable), and an RC servo sold with continuous rotation is normally a 40mm type and is so small and flimsy as to be a very poor suggestion.

Pololu.com have some nice $40 DC 12v gearmotors with encoders and 6mm output shafts, they would be ideal for something like this.

 

[jpanhalt]

1) I believe he was talking about two positions, perhaps more, but they were stationary. He was not capturing images while it moved, but maybe I misunderstood that. Capturing images while moving presents its own set of obstacles.
2) 5 kg is nothing for a modern servo; 1/2 scale models are being flown with them. A 1/4 scale B29 (33' wingspan) was flown with much smaller servos in the 1980's.
3) His original plan was to use a wheeled trolley like you see in so many hobby applications. That will not be a silky smooth ride, nor will positioning be very accurate. You can't make a silk purse out of a sow's ear.
4) I suggested he consider linear rails. If he goes in that direction, moving 5 kg will be almost effortless. Moreover, it is that type of application where a timing belt shines for cost effectiveness. Gosh, my HP printer scanner makes really nice images and uses a stepper + timing belt.

John

 

[duda123]

Ok! Thank you very much