A wheel with a diameter of 139.802mm is mounted on the shaft of a servo motor which has an encoder resolution of 8000 counts and a gear box of 12:1 (therefore, a resolution of 96000counts/rev).

This wheel is used to drive a belt (stretched at the other side with a free wheel of similar dimensions) at increments of 10.7mm at per step. Can anyone tell me the proper method of calculating the required counts to perform this step of 10.7mm? To me, at the moment, it does not seem possible...

regards,
eugene

 

Depends on the tolerances of your system. Real life is not absolute numbers, but numbers +/- a certain tolerance each.

Meir

 

The number of counts for a 10.7mm step is:
(10.7x96,000)/139.802pi
It comes out to 2,338.792829.... counts for 10.7 mm. Each 10.7 mm increment will be off by 0.21 counts or about 0.01% and it is cumulative, of course, if the motion is continuous in one direction. If this is not acceptable you need to introduce a correction factor.

Good luck.
Tom
Thomas B. Bullock, President
Industrial Controls Consulting Div.
Bull's Eye Marketing, Inc.
104 S. Main St., Suite 320
Fond du Lac, WI 54935
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Am I missing something here? As you state the problem, it is very simple. If the wheel has a diameter of 139.802mm, it has a circumference of 439.201mm, corresponding to 96,000 counts. A distance of 10.7mm would then correspond to 2338.79 counts. (These calculations say nothing about the actual accuracy. They assume no slipping or dynamic stretching. They also assume the diameter correctly states the point of contact between the wheel and the belt.)

Curt Wilson
Delta Tau Data Systems

 

How would you do this? Currently, I implemented something like a three level hysterisis where I have 3 difference sized steps I set depending on feedback from an external sensor which measures the belt.

 

Let me put it in another way. I am on the software and electronic side. So, since we know that the distance of 10.7mm does not correspond to a round count number and involves decimal places (0.79 counts!!). What should I say to the mechanical designer? Should I:

1) Ask if he could use a wheel with a diameter, which fits the pitch of 10.7mm. I calculated if we made the degree of motion to be 9 degrees (i.e. 40 (x10.7mm) per revolution. I could just use 2400 counts per step. I understand that there is a lot of inaccuracies in the mechanical world, so I am not sure if this is possible.

2) Or should I replace the motor with one which has a higher resolution of 2^16 and hope that it has a better "match" to the pitch of 10.7mm. This is my first time dealing with servor motors and I am not sure of the "design" procedure that is required for it. For stepper motors it was easier, for 1.8 degree motors, just make sure the pitch correspond to a degree of motion which is divisible by 1.8 and we are good to go..

I hope this explains the difficulty...

regards,
eugene

 

His problem may be that the number of counts is not a whole number, therefore the belt will slowly drift out of position. A possible solution is this case may be to register the belt with a mark periodically. Maybe he could use the encoder index pulse or a flag on one of the pulleys.

Bill Sturm

 

Need more info:

What resolution do you have? Are there any optional resolutions? Most encoders are either base two (1024, 2048...) or base 10 (1000, 2000...)

What kind of drive and/or control? Does it have a high speed capture or registration input? This is the best help, because mechanical systems are never perfect enough to have a resolution with no rounding errors.

Bill Sturm