Torque limiting with VFD

We have tasked with developing a crowd/backing gate for a dairy where cows need to be gently pushed from waiting area into the milking parlor.

The hanging pusher moves on rails and pushes the cows from behind.
The pusher will be moved by a 1hp, 4 pole, 1400 rpm motor with 1/80 reducer. So the movement of the pusher is slow and should be gentle in order not to stress or hurt the cows.
We need to find a way stop the pusher/backing gate if there is too resistance from the cows.

One possible solution could be a hardware torque limiter installed on the motor.
So the question is if it's possible to use the VFD which will drive the motor to stop the gate when certain level of resistance is reached and then after a small period of time to be able to continue moving forward?

Something like in the video below at 4:33.
https://www.youtube.com/watch?v=x0pwNhWzrS4

kallileo said:

We have tasked with developing a crowd/backing gate for a dairy where cows need to be gently pushed from waiting area into the milking parlor.

The hanging pusher moves on rails and pushes the cows from behind.
The pusher will be moved by a 1hp, 4 pole, 1400 rpm motor with 1/80 reducer. So the movement of the pusher is slow and should be gentle in order not to stress or hurt the cows.
We need to find a way stop the pusher/backing gate if there is too resistance from the cows.

One possible solution could be a hardware torque limiter installed on the motor.
So the question is if it's possible to use the VFD which will drive the motor to stop the gate when certain level of resistance is reached and then after a small period of time to be able to continue moving forward?

Something like in the video below at 4:33.
https://www.youtube.com/watch?v=x0pwNhWzrS4

Click to expand...

The video you linked seems to show a simpler solution: a spring loaded gate. When a high enough resistance is applied against the gate, a prox is broken & the motor stops.

That being said, this would be very easily achievable with a servo motor; what's the reason to go with a VFD? As far as I know, you'll need external hardware to get torque information.

A 'Vector' type VFD would satisfy your application: 'Speed control with torque reference'.

My preference for this type of motor control is the ABB ACS880(DTC- Direct Torque Controlled), however the Allen-Bradley PowerFlex 750 series VFDs would be another worthy candidate.

Originally posted by Vlad Romanov:

...what's the reason to go with a VFD? As far as I know, you'll need external hardware to get torque information.

Click to expand...

Depends on the VFD he uses and what mode he uses it in. Drives that operating in field oriented control can provide reasonably accurate torque feedback (roughly 5% accuracy).

However, I agree with Vlad Romanov that the spring loaded gate with a prox is a better detection method. With such a high reduction you will find it fairly difficult to separate out the torque needed simply to turn the drivetrain from the torque required because the gate is pushing against something too hard. You run the risk of either having too many nuisance trips or not being able to stop when you push too hard.

Keith

I was suspecting that it would be difficult differentiate the torque levels because of the high reduction.

Our gate will be different from the one in the video as we need a very simple solution at the moment and there is no time to develop something more complicated. Probably it will be just hanging on wire ropes that will be fixed on the carriage. So it will look like a pendulum and it be difficult to have a prox sensor on it. I guess that when the gate comes in contact with the cow it will swing backwards.

Another limitation is that the whole mechanism should be very cheap so servo motors and high end VFDs will be the last option.

I wouldn’t trust the torque limit on any VFD when you are dealing with animals to many ways things could go wrong. If things go bad and were to push ageist cows it may scare them and effect the milk production.
I think a better option would be use a good quality VFD to set the speed set torque limit to just what you need to move the gate. In addition I would install a light curtain across the front to stop the gate before it contacts the cow in addition I would look at a tape switch on the leading edge of the gate if either the light curtain or the tape switch contact the cow for more than a few seconds reverse the gate for a few seconds and then restart it.
I would think that if you keep filling the empty space left from the cows leaving the holding pen when the gate closes up they will move away from the gate without harming them.
You should be able to come up with a combination using either or both the tape switch and the light curtain that will get the job done. The last thing you want to do is to shack up the cows so they reduce production.

I hope the concrete pad mesh is bonded and earthed to the system as l have seen stray current upset the animals although we weren't able to feel it.

I have used the current limit function to good effect on a Delta vsd to control the raising and lowering of a rake. Using motor current inside a very small window.

duckman said:

I hope the concrete pad mesh is bonded and earthed to the system as l have seen stray current upset the animals although we weren't able to feel it.

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There are very explicit regulations for wiring within farming/livestock applications for this very reason.

Something that would barely effect a human being ~5mA will kill a cow or horse.

I've seen a decent sized heard of milking cows that I was asked to investigate drop dead due to this.

Rob... said:

There are very explicit regulations for wiring within farming/livestock applications for this very reason.

Something that would barely effect a human being ~5mA will kill a cow or horse.

I've seen a decent sized heard of milking cows that I was asked to investigate drop dead due to this.

Click to expand...

Seems the push is mechanical not a cattle prodder !

We've done several crowd gates, just as suggested using torque limit adjusting and speed limiting with 2 potentiometers. They work very reliably.

The original mechanical limiter was not as repeatable, and caused issues with either not moving, or upsetting the cows.

There are lots of varieties of mechanical torque limiting clutches. Here is an example:
https://www.bostongear.com/products/clutches

Rob... said:

There are very explicit regulations for wiring within farming/livestock applications for this very reason.

Something that would barely effect a human being ~5mA will kill a cow or horse.

I've seen a decent sized heard of milking cows that I was asked to investigate drop dead due to this.

Click to expand...

Where could I find these regulations or specs on how to ground for stray voltages in Dairies, like how to ground water troughs,steel corrals/fencing or rebar in concrete.



How does a person check for stray voltage?

Gene Bond said:

We've done several crowd gates, just as suggested using torque limit adjusting and speed limiting with 2 potentiometers. They work very reliably.

The original mechanical limiter was not as repeatable, and caused issues with either not moving, or upsetting the cows.

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I have never tried to limit the torque of a VFD before.
Which drive did you use?
Do I need a high end drive like ABB ACS880 with direct torque control as dmargineau suggested to have adjustable torque limit?

I guess that torque limit and torque control isn't the same thing.

@Levi G
Take a look the link below:
http://www.omafra.gov.on.ca/english/livestock/dairy/facts/strayvol.htm
http://www.dpi.net.nz/resources/stray-voltage.aspx

Again, you will need a 'Vector' type VFD since you'd want to run the motor in 'Speed control with torque reference'; the drive will try to keep the motor running at commanded speed and the torque within the user set limits. If the torque exceeds the MaxTorque setting the drive will 'back-off' the frequency enough to return the torque below set Max. Correspondingly, if you set a MinTorqe, the drive will exceed the commanded frequency in order for the motor produced torque to stay above set Min.

High end drives with motor speed feedback encoders will give you the best torque control. ABB ACS880s seem to (still) be the best out there; I'd say they are below 1% error; AB PowerFlex 750 series are not too far behind- probably slightly less than 3%.