Help me automatically slow a DC drive down

[Kfletch2010]

I have a 90VDC motor and gearbox running a spool at 1 RPM. Empty the spool is 4" in diameter. It has an aproximate linear speed of 12 inches per minute (pi X diameter). Full, the spool will be 16" in diameter (approximaely 48 inches per minute). So as the spool takes up more material its diameter will grow, and I need to slow the speed down gradually to maintain the 12" per minute. So 1 rpm empty and 1/4 rpm full. Any ideas would be much appriciated.

 

[lw1ecp]

Can't you just sense the linear speed of the material within a closed loop that controls the DC towards the motor?. Tell us about your material to see what kind of sensor could be used.

 

[Kfletch2010]

The material is cloth with a teflon type coating sprayed on to it. I have thought about running it through a set of pinch rollers to guage the speed, but am unsure what to use as a feedback. A tach would not put out any voltage at such low rpm unless we greatly geared it up.

 

[KMoffett]

Pinch roller with chopper-wheel type encoder. X pulses/rev. Pulses fed to a microcontroller or PLC. Microcontroller or PLC controlles motor speed.
or
Tension idler with position output to microcontroller or PLC to motor.

Ken

 

[lw1ecp]

A mad idea: place a sample of that cloth on a table, can you reliably use an optical mouse over it?. So it could double as speed sensor in your process as well. In every burst of bytes towards the computer or microcontroller, the mouse is telling how much it moved. An appropiate software would adjust motor speed to have an approximately constant value. 12''/minute... 1'' every 5 seconds... I am simulating it with my mouse... yes, it IS slow indeed, so your application should take decisions after having integrated several communications from the mouse.

 

[lw1ecp]

A not-so-mad idea: sense the instantaneous spool radius, e. g. a potentiometer with an arm touching the wound cloth, and using a lookup table adjust the motor voltage. Open loop, simple.