-
Welcome back Guest! Did you know you can mentor other members here at H-M? If not, please check out our Relaunch of Hobby Machinist Mentoring Program!
RPM from VFD to TouchDRO?
- Thread starter Greyhound
- Start date
- Joined
- Jan 11, 2023
- Messages
- 28
I don’t believe there are any analog inputs to TouchDRO. One could construct an external voltage to frequency converter (with a logic-level output) to feed TouchDRO. Note that this is different from the more commonly available V -> PWM conversion.
You didn’t mention your application but if your machine has any in-use selectable gearing (or belting) between the motor and whatever rotation to which you want the readout to correspond, those changes would not be reflected in the readout without a corresponding change in the pulses/revolution setting.
Cheers, Louis
I was afraid an analog to frequency convertor would be the only way to achieve the desired result. My application is my Enco mill and I don't intend to change any gearing. Speed changes would be accomplished solely by the VFD.
Found this...would this work?
Found this...would this work?
The interpretation of the 0-10volt level would yield the motor frequency not the RPM. This is what @Backporch was pointing out in the comment about the gearing. If you really want to know the RPM then you need to measure it after the gearing. This is commonly done via a Hall sensor and a counter.
I do not know about the 1-10 volt output accuracy of your VFD, but the potentiometer input 1-10volt has a very limited relationship between the voltage and the frequency. I put a small 0-12 volt voltage meter display on the pot output of my VFD control and then calibrated it in the VFD software by setting the max frequency the VFD puts out at 10 volts. Hence, a 6 volt voltmeter reading going into the VFD corresponds to about 60Hz. It is not bad, but not totally accurate. It is better than trying to read a potentiometer dial. The VFD tends to load the potentiometer resistance making it a little non-linear. I could have used the VFD output to measure, but wanted to know what the Pot was set at. Also, it would have required I run another line to the front panel and the cable was pretty full. If you want to understand exactly what I did see the the attached document called PM1440GT Part2 VFDD..... at the posting:
PS. As you will see from the link, I used 10 magnets on the spindle for my hall sensor. This yields more resolution at the counter. It is sort of a poor man's rotary encoder, but I use it to measure the number of 1/10th revolutions, not just the RPM.
I do not know about the 1-10 volt output accuracy of your VFD, but the potentiometer input 1-10volt has a very limited relationship between the voltage and the frequency. I put a small 0-12 volt voltage meter display on the pot output of my VFD control and then calibrated it in the VFD software by setting the max frequency the VFD puts out at 10 volts. Hence, a 6 volt voltmeter reading going into the VFD corresponds to about 60Hz. It is not bad, but not totally accurate. It is better than trying to read a potentiometer dial. The VFD tends to load the potentiometer resistance making it a little non-linear. I could have used the VFD output to measure, but wanted to know what the Pot was set at. Also, it would have required I run another line to the front panel and the cable was pretty full. If you want to understand exactly what I did see the the attached document called PM1440GT Part2 VFDD..... at the posting:
PS. As you will see from the link, I used 10 magnets on the spindle for my hall sensor. This yields more resolution at the counter. It is sort of a poor man's rotary encoder, but I use it to measure the number of 1/10th revolutions, not just the RPM.
I understand the point about 0-10V output corresponding to motor frequency, not RPM, and that this doesn’t account for gearing. However, in my application since I don't intend on changing gearing, I’m looking to use the VFD's 0-10V output as a proportional signal to drive a voltage-to-frequency converter. This should translate the voltage into a pulse train that could then be interpreted as a proxy for RPM. While it won't measure "true" RPM, it should give me a usable representation based on the motor’s frequency.
As you mentioned, this approach relies on the VFD's output accuracy and assumes the 0-10V signal scales linearly with frequency. For my needs, I think this level of precision would be sufficient. If I determine that exact RPM is necessary, using a Hall sensor on the pulley would be the way to go, as you mentioned. I'm in the process of wiring everything and setting everything up, so running the additional wires is no problem.
Maybe I've overthinking this, but I'd like to use the TouchDRO's RPM display rather than mounting an additional counter display somewhere on the mill.
I don't know anything about TouchDRO but a better way to access the output frequency of a VFD is through the serial communication port, just a matter of reading a data register. You could use the frequency to APPROXIMATE the rpm.
I certainly agree with your not having unnecessary extra meters. I wanted to do something similar on my DRO, but it did not have the facility.
Looking at the TouchDRO web site I see the device has an input for a tachometer reading. This would normally be something like the Hall sensor output after being made into a digital signal. But if you do not wish to add a hall system I understand. However, all you want is motor frequency. So in that regard you almost already have it. It is the AC line going directly to the motor. Although it is probably 220 volt sine wave it would easy to steal a bit of a signal, make a pulse from it and not have to go through the 0-10 volt read out and then to a voltage to frequency converter and then the calibration etc etc. Perhaps your VFD already supplies a logic level pulse corresponding to the motor power frequency? I do not believe that mine dis. Which VFD are you using? Anyway, since you have knowledge about ADC you probably have no problem figuring out how to stealing a signal from the motor power circuit and making it into a logic level signal. I doubt that TouchDRO input requirements are complex, but their web site does not make it easy to find the info about the tachometer input.
If you do not have the electronics back ground to convert from the 220 to the digital level let me know.
Looking at the TouchDRO web site I see the device has an input for a tachometer reading. This would normally be something like the Hall sensor output after being made into a digital signal. But if you do not wish to add a hall system I understand. However, all you want is motor frequency. So in that regard you almost already have it. It is the AC line going directly to the motor. Although it is probably 220 volt sine wave it would easy to steal a bit of a signal, make a pulse from it and not have to go through the 0-10 volt read out and then to a voltage to frequency converter and then the calibration etc etc. Perhaps your VFD already supplies a logic level pulse corresponding to the motor power frequency? I do not believe that mine dis. Which VFD are you using? Anyway, since you have knowledge about ADC you probably have no problem figuring out how to stealing a signal from the motor power circuit and making it into a logic level signal. I doubt that TouchDRO input requirements are complex, but their web site does not make it easy to find the info about the tachometer input.
If you do not have the electronics back ground to convert from the 220 to the digital level let me know.
- Joined
- Jun 12, 2014
- Messages
- 5,135
Analog output of VFDs is typically adjustable for offset, bias and gain and can be set based on a number of display parameters. There are also adjustments as to the RPM displayed as to the scaling ratio factor, there might be a slight difference for motor slip, but with sensor less vector control the RPM accuracy is around 0.1%. Many factory VFD mills and lathes use the VFD analog output to display spindle RPM, in those cases there is switch that changes the RPM range displayed depending on the gearing/output ratio. On my mill they use the 0-10V VFD output and the RPM display has dual inputs with the scaling done in the meter, but one could as easily do a voltage divider. I do not know if you could use the VFD pulse train output, depends on the VFD.
You just wanted an approximation of the rpm..... but, oh, yes the motor rpm will not exactly agree with the line frequency. Motor slip on inductive motors. A You purchase a 1720 rpm motor and run it at 60 Hz. 60c/s * 60s/min = 3600 cycles/min. Even if you divide by 2 you get 1800/min not 1720. This is the slip and it usually varies with the load.
There was this other posting where they were trying to connect the hall sensor to the TouchDRO tach input. I did not read it but you might find something useful there.
There was this other posting where they were trying to connect the hall sensor to the TouchDRO tach input. I did not read it but you might find something useful there.
Similar threads
