Hi Folks, I have the same need. People here are trying to be helpful, but not really answering the actual question when suggesting using a different motor or adding a tach or another motor as a tach for example. It would be great if we could do this, but it isn't possible.

I'll try to give some background (at least on my need). Many older automated machines out there simply do not have the ability, moslty due to outdated controls and software, to be converted to newer drives and motors. furthermore, space and cost constraints force us to try to retrofit newer parts to what is already there. There are commercially available conversions, but they are not cost effective. We cannot afford to spend $1,200 on such a package to replace a $300 motor six times over on a dozen machines.

The problem is that the OP seems to (and I) have older systems that use tachomotors for feedback. The replacement motors usually have encoders, and in my case 500 CPR quadrature (motors we are using on newer versions of the same machine). What we need is a simple D-A circuit that can be wired in-between the NEW motor and the OLD drive so that when the motor spins, the encoder pulse counts are converted to a voltage (in my case 0-12V). The converter needs to have some sort of Pot to adjust the voltage so that the encoder converts to the right voltage, and the circuit needs to be linear so that as the encoder pulses increase, the voltage increases.

I realize I can probably just use one phase of the encoder to get this done, but nothing I have tried seems to work. Being able to do this will allow us to fool the machine into thinking it has a tachmotor when really it has a newer encoder motor so it works properly. I have dozens of machines I need to retrofit at a low cost, and making a simple A-D converter to put in-line with the encoder and tach inputs would do the trick.

Can anyone offer a solution that just does this and doesn't require replacing the motor, tach or adding a tach?

Much appreciated.



Moderator's Note:
Please don't hijack other member's thread, now you have your own.
This thread was split from --
help to replace tacho generator
https://forum.allaboutcircuits.com/threads/help-to-replace-tacho-generator.116948/

 

A frequency to voltage converter IC such as these should do what you want. You would need to select component values to match the speed of the motor and the number of pulses per rev from the encoder to give your 0 to 12 volt signal. The output would be quite linear.

Les.

 

You don't say what kind of system you are referring to? Simple motion control? CNC etc?
It is difficult to recommend when details are with held!
Is this the common ±10vdc analogue control signal?
Are the Tach's simple DC type?
Velocity loop controls have been replaced now by torque mode amplifiers/drives.
This has meant that servo motors rarely come with tachometers, as you have found out.
If more details of the system was known then options may be out there that can be suggested.
Max.

 

Are you trying to measure angle or speed?

ak

 

I am guessing because he needs to convert the replacement digital encoders to an analogue signal it is velocity?
Max.

 

If you are just looking at pulses with a frequency proportional to motor RPM I would think about using a F/V (Frequency to Voltage) converter as was mentioned. Rather than build or roll your own circuit there are modules out there that can be bought and setup for what you have. Red Lion modules come to mind and there must be a half dozen other manufacturers out there. Many come ready to connect to an 8 pin or 11 pin socket allowing a wide range of inputs to scale to a voltage output. Besides Red Lion another name I recall using was Action Pak. A Google of Frequency to Voltage Signal Conditioners should bring up some turn key solutions. There are also inexpensive units coming off the boat from China.

Anyway is this what you are looking for? My take is you have a tachometer output in pulses you want to convert to a voltage. Would that be a good assumption?

Ron

 

Many older machines that use DC tachometers, these output a DC voltage proportional to RPM, they are fed back to the drive where they are summed with the analogue input signal and look for coincidence when the desired RPM is reached.
As post #2 LM2907 is a digital to analogue convertor.
Max.

 

motors usually have encoders, and in my case 500 CPR quadrature

I'm reading that as giving you 500 pulses per revolution? What is the maximum rpm and accuracy you are interested in?
In principle you could use each pulse to trigger a monostable and integrate the monostable output. Dedicated F-to-V circuits operate this way, but you could also do it with simple logic circuits.

 

There is also the issue of whether these work as common DC tach's do which reverse polarity on change of direction?
Max.

 

Hi Max,
That's a good point that I had not considered. As the new motors are fitted with quadrature encoders a circuit could be added to reverse the polarity of the F to V converter output for one direction of rotation.

Les.

 

You don't say what kind of system you are referring to? Simple motion control? CNC etc?
It is difficult to recommend when details are with held!
Is this the common ±10vdc analogue control signal?
Are the Tach's simple DC type?
Velocity loop controls have been replaced now by torque mode amplifiers/drives.
This has meant that servo motors rarely come with tachometers, as you have found out.
If more details of the system was known then options may be out there that can be suggested.
Max.

Guys, thanks for all the help. Here are some answers.

One direction only.
Watching speed only.
I will look for some of those off the shelf items. It is what I was hoping to find, but couldn't.

More Detail:

Current motor is a DC gearmotor made by Pittman. The last few years, the tachs have been so bad, that after a few weeks, the signal is erratic. The tach feedback is 50% of the applied motor voltage. We're tempted to just use a voltage divider to feed that back to the drive card but then we would never get a speed alarm if the motor fails. I have 10-year old motors that still work, but the new ones die in less than a year. The tach section armatures are worn out. Maybe due to softer copper or harder brushes. Pittman does nothing to help. Other motors are not cost effective.

The drive cards are proprietary to the no longer in business machine maker. We plan to use the same gearmotor, but with a 500 CPR encoder (5v) that fits the machine and has the same drive voltage and gear ratios. We use them in newer machines. The gearboxes still suck, but at least they don't have the tach issue and the encoders seldom fail.

Two motor types. 0-30VDC and 0-24 VDC. IF I can make a simple cheap Pulse to voltage board this would solve a lot of headaches. We do not know the load of the drive card. I suppose I can measure the current on a running machine if needed.

The red lion ones are nearly what I need, but they have fixed output ranges (0-5, 0-10, etc) so they wouldn't give me the scaling I need. I have odd voltages here (0-12 and 0-15).

 

It is sounding as the control loop is closed back to the drive only and not the controller itself, as in a CNC system for e.g.?
Linear tachometer feedback are usually velocity only and not positional.
What is the actual nature or function of the machine?
If a uni-polar DC tach is sufficient then maybe the LM2907 solution could be pursued?
Max.

 

The red lion ones are nearly what I need, but they have fixed output ranges (0-5, 0-10, etc) so they wouldn't give me the scaling I need. I have odd voltages here (0-12 and 0-15).

Granted a little odd with the voltages. Depending on the normal motor speed you may be able to use a 0 to 10V version. For example 10 is 83% of 12. It could be scaled that way, as long as the normal speeds never exceed 10 volt feedback or the motor running greater than 83% of max speed.

Another option is call a company like Omega Engineering or Opto 22 and chat with one of their applications engineers. These guys can generally come up with solutions within a budget constraint.

There is always the roll your own option. If you will have enough demand you can design your own version and board. It would be practical if the demand is there to make a dozen boards allowing for spares. Build around the LM2907 as Max and Les suggested. Just design something universal enough that it can cover all your needs. The beauty of roll your own is you can really customize to your needs.

Ron

 

It is a silicon wafer scrubbing machine. No position control. Just spinning rollers and brushes. Feedback is to alert the system if the motor is not turning. Software has tolerance range, so the feedback has to be comparable to the tach.

 

So are you saying that the tachometer is NOT used for closed loop speed control of the motor ? If this is so then if you design your own PCBs all you need to do is fit different value resistors and possibly a different value capacitor to one place in the circuit.

Les.

 

No, it uses it for speed control. Sorry for the misinformation.

 

Analogue tachs are usually for velocity control, not positioning, Have you investigated the LM2907? It could be scaled accordingly.
Max.

 

Yeah, and I also found something online that is pretty cheap, (under $10) and I think I can just tweak it to work either with the pot or some changes to some of the resistors on the board. The IC labeling is scraped, but more than likely an LM2907.

http://www.ebay.com/itm/182369304895?_trksid=p2057872.m2749.l2648&ssPageName=STRK:MEBIDX:IT

 

Yeah, and I also found something online that is pretty cheap, (under $10) and I think I can just tweak it to work either with the pot or some changes to some of the resistors on the board. The IC labeling is scraped, but more than likely an LM2907.

http://www.ebay.com/itm/182369304895?_trksid=p2057872.m2749.l2648&ssPageName=STRK:MEBIDX:IT

At the price you have really nothing to lose. There is all sorts of stuff pouring off the boats from China. Much of it works.

Ron

 

At the price you have really nothing to lose. There is all sorts of stuff pouring off the boats from China. Much of it works.

Ron

Yeah, no kidding. ordered one, will wait a few weeks for it to arrive, will report my findings...