OT: Tacho voltage convertor???

[timbo_uk]

Hi all,

Slightly off topic question.

I have a web postioning unit that moves a web, in a printing press, sideways (to 0.05mm accuracy) to correct register. This is moved by a DC servo motor controlled by a custom made PCB.

Basically the DC servo motor (20 years old and obsolete!)is in good condition, other than a slightly reduced torque output due to (I assume) reduced power in the permanent magnets. The problem is the DC Tacho, it has a low output.

The tacho is supposed to give 20mV/rpm, but it is actually producing 16.5mV. The (non adjustable) control pcb is therefore overspeeding the motor when it's trying to postion. The controller is looking for 20V when moving at full speed; this equates to 1000RPM. With the reduced output from the tacho the motor actually runs at 1240 rpm.

This is causing overshoot and then a correction. The register then never "locks on" to the correct position and constantly wanders back and forward.

The OEM has quoted us around £12,000 for a replacement motor and tacho; BUT they need to send an engineer with it to modify the frame to make it fit. Only another £3,000!

I had an idea that if we could "amplify" the voltage from the tacho to the correct level, then I could see if it cured the problem we are having. For example if I get 16.5V out from the tacho I want to scale it to 20.0V

I have googled around but cannot find anything. Has anyone come across something that could potentially do the job?

Or have I just found a product that many would buy if I made one!!!

Cheers

 

[TheStarr]

What you describe is called a "boost converter" or "boost regulator".

But I wonder why the voltage is low in the first place. (Solve the root, not the symptom)

Is the power supply to the encoder weak?

Also, the end of your post says "For example if I get 16.5V out from the tacho I want to scale it to 20.0V" You mean "mV" (milli-volts) not "V" (volts), big difference!

 

[jrwb4gbm]

Try this link, they may have a signal conditioner that will work for your application.
Link:
http://www.api-usa.com/signalconditioners_dc_input.php

 

[Gil47]

Timbo
just a few thoughts tumbling over here.

If you have a DC tacho it normally requires brushes that need maintenance
this could be producing high resistance in wiring leads .
also diodes and capacitors on the card input could be faulty and loading the tachodown

Some Tachos were AC then were rectified in order to produce DC ,
do you have a faulty bridge rectifier ,
or a faulty smoothing capacitor ,

Is the Tacho suppling a signal to two devices, say the card and a meter,
when its only capable of suppling only one of them .

You can see my guess is its being overloaded or is failing to supply neccessary load.

 

[timbo_uk]

What you describe is called a "boost converter" or "boost regulator".

But I wonder why the voltage is low in the first place. (Solve the root, not the symptom)

Is the power supply to the encoder weak?

Also, the end of your post says "For example if I get 16.5V out from the tacho I want to scale it to 20.0V" You mean "mV" (milli-volts) not "V" (volts), big difference!

Hi, there is no power supply, its a DC tacho. I also meant 16.5V to 20V. The tacho is supposed to give 20mV out for every RPM; ie 100 RPM = 2v out, 1000RPM = 20V out etc.

 

[timbo_uk]

Timbo
just a few thoughts tumbling over here.

If you have a DC tacho it normally requires brushes that need maintenance
this could be producing high resistance in wiring leads .
also diodes and capacitors on the card input could be faulty and loading the tachodown

Some Tachos were AC then were rectified in order to produce DC ,
do you have a faulty bridge rectifier ,
or a faulty smoothing capacitor ,

Is the Tacho suppling a signal to two devices, say the card and a meter,
when its only capable of suppling only one of them .

You can see my guess is its being overloaded or is failing to supply neccessary load.

Cheers for the pointers. It is a straight DC tacho, not components. Brushes been checked every 6 months for at least the last 5 years, comm in perfect condition, smooth output also.

The tacho is supplying only one PCB directly, good point about beeing pulled down, forgot about that. Although, I think one of my collegues has swapped the pcb anyway. Ill just check the spec of the tacho (max milliamps) and put a mA meter on it to be sure.

I think I have tracked down a replacement tacho, will find out in morning.

Cheers

 

[TheStarr]

Hi, there is no power supply, its a DC tacho. I also meant 16.5V to 20V. The tacho is supposed to give 20mV out for every RPM; ie 100 RPM = 2v out, 1000RPM = 20V out etc.

Sorry, my bad!

 

[DRS]

Something to look at would be lost motion in the drive system, if there is much more then .002" backlash the system could be hunting.


Tommy

 

[timbo_uk]

UPDATE;

During the day yesterday my collegue contacted the company I had tracked down for a 20mV/RPM Tacho, the unit they had was ok for our motor.

They taxi'd it up to us and it was fittied yesterday pm. This morning I have done some tests; the motor is not running at 1038 RPM

Phew. I was slightly worried that they might have jumped in a little early with the slight posibility of the output being pulled down.

Hopefully this cures the problems!

PS. The tacho was less than £1000 even allowing for the same day courier driving it to us. Big difference from the £15k it was going to cost us!

 

[timbo_uk]

Sorry, my bad!

LOL, we all missread sometimes!

 

[timbo_uk]

UPDATE;

They taxi'd it up to us and it was fittied yesterday pm. This morning I have done some tests; the motor is not running at 1038 RPM

Ooops, I meant NOW running at 1038 RPM!

 

[Gil47]

the motor is not running at 1038 RPM
LOL, we all missread sometimes!
Ooops, I meant NOW running at 1038 RPM!

Just as well that we read between the lines

 

[keithkyll]

Permanent magnets fade over time. They can be re-Gaussed. Consider this if you plan to rebuild the motor, and can afford the downtime while the magnets are at the shop.