Tacho meter

speeder · Jan 18, 2005

hi can any one please suggest a heavy duty tacho meter that can be connected to plc's analog input, I need it to be similar to a rotary encoder in shape...and it must have a 0-10vdc output, this will be used for a rewinder application, the max spped is below 5000 RPM.

please help and if you can suggest a link, that would be gr8.

thanks in advance.

george oramel.

PLucas · Jan 18, 2005

Speeder

There are many DC tachometers availiable out there, you might have a problem getting one that gives you 10 volts at 5000rpm.

Servo-tec make tacho's that give 10v/1000rpm, 24v/1000rpm, plus others including a 3v/1000rpm, but not a 2v/1000rpm (10v @ 5000rpm)

The 10v/1000rpm tacho might be of use, if this is coupled via a 5:1 gearbox, you can have your 5000rpm input to the gearbox, the tacho will spin at a maximum of 1000rpm and your voltage input will be 0 - 10v like you wanted.

Google the keywords 'DC tachometers' and you should get a few hits.

Paul

speeder · Jan 18, 2005

hi Paul,

thanks alot ...I'll goooooogle

any other links inputs are really apreciated

regards.

george oramel.

rsdoran · Jan 18, 2005

I think what you are looking for is a speed transducer:
http://zone.ni.com/devzone/conceptd.nsf/webmain/B37D91BAC327BF2C8625684F00752646?opendocument

elevmike · Jan 18, 2005

DC Generator Tachs are frequently use on older closed loop generator field control systems. Aka ward-lenord.. 10 volts @ 5000 rpm sounds kina different. Tachs are usually rated in volts per 1000 rpm. So you might find a tach rated at 2 volts per 1000 rpm, with a max speed rating of 5000+ rpm (bearing wise).

I have expierance with both...go digital if you can.

http://www.servotek.com/Catalog RC700 Contig.pdf

rsdoran · Jan 18, 2005

I was in a rush this morning, was going to metion servotek this time.

May be easier and cheaper in the long run to add an HSC module to the plc and use an encoder.

Tom Jenkins · Jan 18, 2005

It may not suit your needs, but I've done speeed monitoring on 3600 rom equipment using a proximity switch (photo or magnetic) and the Red Lion converters. You can get almost any range you need with the IFMA unit.

DickDV · Jan 18, 2005

Why not use any higher voltage output DC tach and use a voltage divider to get down to 10VDC at maximum speed.

I've done this occasionally on DC drives. All it takes is two resistors with enough wattage to avoid overheating.

Let's see here. A 50vdc/1000rpm tach at 5000 rpm is 250volts. You want 10 volts so, if we series up a 100 ohm resistor and a 24k ohm resistor, that will give us 10vdc at 250vdc. The 100 ohm wattage is 10 x 10/100 = 1 watt so a 2 watt unit would do. The 24k ohm is 240 x 240/24000 = 2.4 watts. Probably ought to use a 5 watt unit there.

If you are concerned about the 100 ohm opening and blowing up the tach input, just add a 12vdc zener in parallel with it. That will give you a little redundance.

Fred Floggle · Jan 19, 2005

I'm not sure that you mean "enough wattage to avoid overheating "
you could use 2 x 0.01 ohm 500W resisters , but i doubt that the poor little tacho gen would have much chance of driving them .

I would see the current that the tacho is capable of producing while retaining its linearality and work backward with suitable resistances.
The crowbar is a worthwhile addition

DickDV · Jan 19, 2005

Fred F, you are correct that the total resistance chosen must not load down the "poor little tacho generator" as you say!

In my example, the total resistance is 24100 ohms which at 5000 rpm and 250vdc output, calculates to 10ma load on the tach. It deserves checking but I suspect the tach will be ok at 10ma.

If lower loading is needed, simply double or triple the resistance values until the load falls into the acceptable range.

elevmike · Jan 19, 2005

My question is....

Since the tach is going to input to a PLC analog module..why bother with all this and just use an encoder and counter module.. the results will be the same..changing numbers in some PLC address..A lot simpler setup, and less expensive field device..

Fred Floggle · Jan 19, 2005

elevmike · Jan 19, 2005

The cost for the components is about the same either way...then you add the co$t of screwing around with it..my guess is the encoder way will be less in the long run..

Fred Floggle · Jan 19, 2005

I'd be surprised if you can purchase a high speed input card or dedicated encoder card with encoder for less than the cost of a tachogen .
The other consideration in this application is the feedback time - with a tachogenerator there is at least an instant direct relationship between volts and speed , with the encoder option the software has to be carefully written to ensure that there is a short timebase used in integrating pulses against time to ensure that there is not to much lag .
I remember using a super little omrom encoder some years ago , it had its own dedicated 200hs card . It wasn't the easiest thing to set up as an encoder , but I seem to recall that actual shaft speed and encoder position were written to DM'S .

elevmike · Jan 19, 2005

Fred Floggle said:
I'd be surprised if you can purchase a high speed input card or dedicated encoder card with encoder for less than the cost of a tachogen .

We dont know if the analog unit is already in place. If so..then you've got a point..We also dont know what PLC..so I'm guessing also..

Fred Floggle said:
The other consideration in this application is the feedback time - with a tachogenerator there is at least an instant direct relationship between volts and speed , with the encoder option the software has to be carefully written to ensure that there is a short timebase used in integrating pulses against time to ensure that there is not to much lag .

Good point also....less code (none really) will be needed to deternine speed....

Ok..so you win....

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