Reading Speed Through Inverter

[eyad267]

Hi Guys
I've got some application where i need a very high accurate speed measurement for a line and the encoder cann't achieve such accuracy due to mechanical issues...

Can i measure the speed of line according to the VFD connected to the motors pulling the line or not? I've got a D/A module controlling the inverter i just need to know if it is possible? and more information about its accuracy?

Thanks

 

[robertmee]

I assume by 'line' you are referring to a web application?

You can use the drive speed for one of the pulling rolls, however, there is always slip involved that is likely not constant depending on your process. There are also process considerations...Does the web thickness increase/decrease due to product changes? If so, then you have the error of thickness to consider as the web surface speed will vary because the radius from the center of the roll changes. You should also select a roll that doesn't have any outerloop trim control (dancer, loadcell, etc) to eliminate those speed fluctuations. Still, you aren't likely to achieve 'high accuracy', but until you quantify the degree of accuracy it is hard to say.

I'm curious as to what kind of mechanical issues you are experiencing with an encoder setup? Do you have a capstan roll in contact with the sheet connected to an encoder? You could also look at non-contact speed sensing sensors (laser) if you want to avoid contact measurement.

 

[milldrone]

Hi Guys
I've got some application where i need a very high accurate speed measurement for a line

There isn't enough information here. What is "very high accuracy" to one person is just a rough estimate to another.

and the encoder cann't achieve such accuracy due to mechanical issues...

The only scenario that comes to my mind this early in the morning is that the encoder is driven by the product moving through the machine, and not driven by the machine itself. And you want to measure the speed of the line with no product.

Can i measure the speed of line according to the VFD connected to the motors pulling the line or not? I've got a D/A module controlling the inverter i just need to know if it is possible? and more information about its accuracy?

Thanks

Well maybe. Some of the things that are important are:

1. Type of feedback the inverter drive has from the motor. This ranges from none to sensorless vector to encoder (presuming we are not talking about the same encoder that has the "mechanical issues").

2. Is there any way the motor can have any slip between the motor and the machine? This would include clutches, V belts hydraulic drive ect.

3. Then there is the inverter itself you have not stated what inverter you are using. The finest inverter I have used for speed control is a ABB ACS800 with encoder feedback. The accuracy I'm able to achieve with the ACS800 is: motor at 1750 RPM = 650 FPM line speed, .125" error in 20 feet of line travel.

4. The D/A module can also be a source of error. The worst error I have ever seen is on a smart relay analog setup. The resolution was only 4bit (16 points) between 0 volts and 10 volts. In the application I mentioned previously all control was digital between drives and controller.

Edit: I see Robertme responded before I did and my typing must be very slow judging by the post times!

 

[DickDV]

Its going to be tough to improve on encoder accuracy. Certainly, there is no output on any inverter that will be more accurate.

You can increase the encoder accuracy by increasing the number of pulses per revolution. Clearly, as you move from 128 ppr thru 1024ppr to 4096ppr, the accuracy is going up.

As mentioned above, just how good do you have to be?