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Conveyor Safety Integration Problem

20 posts in this topic

Well here is my problem We have just installed a new 3 belt oven with fuji vfd drives. The conveyor is a large metal belt costing about $75,000. The problem is that belt got jammed, stopped and the drive kept trying to pull it breaking a link and damaging part of the belt. We have fixed the belt and i am trying to figure out a way to protect the belt by monitoring the movement of the belt. If the belt doesnt move it shuts off the drive. I was thinking of either a prox switch which looks for an on and off cycle. I don't think this will work as the drive runs very slow and the time between the links is more than 30 seconds. If this drive pulls the belt for the 30 seconds when it is jammed it will definately tear it up again. I have browsed this site for months and belive i can get the answer here. Thanks in advance Chris Marinello

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If the belt is stopped, the only method of detecting it is waiting for soemthing to happen (which wont) but as you say this could be 30seconds. The other option I would explore is measuring the current the drive motor draws, if the belt stops it should increase significantly for no reason.

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Check the VFD for a torque limit. This will fault out the drive in case of a jam up. You could also use an encoder and count the pulses. You sould not need a high speed input if the conveyor speed is slow. Good Luck

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Thanks for the input, i spoke to GE/Fuji techs about the torque limit and they say it can't be done with thier unit.

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if I understand your application: instead of just one sensor, how about a closely-spaced row of sensors (for example 10 of them) located along the belt path? ... that way instead of the belt having to travel a full 30 seconds to bring the next link up to single sensor, the movement of each link could be monitored every 3 seconds (30 seconds divided by 10 sensors) ... any respectable PLC could monitor the 10 input data points for changes ... no change of inputs for more than 3 seconds equals “jam alarm” situation ... best regards, Ron

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I know this is not a very automated answer, but what about a clutch? This would be an inexpensive answer. There was another post about metering current. You could put a current limiting relay in that drops the contactor out? May or may not be the approach that you want to take.

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I would fit a jockey wheel that freewheels on the belt, this jockey wheel drives an encoder. Then just monitor the encoder looking for the correct belt movement.

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Thanks to all that have provided input into this problem. I have decided to mount a zero speed indicator on the shaft of each conveyor(434.00) which will monitor the movement. I purchased them from maxigard.com if anyone ever needs a solution like this. Thanks again to everyone

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What is the rpm of the motor? You may be able to use a prox switch on the shaft of the motor connected to a standard input (not high speed). I use this method on some slower applications (rpm<100) to monitor broken couplings.

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Is that 46 or 0.46? I'll assume it's 0.46 rpm. You can have a split two piece gear attached to the motor shaft , with say 60 teeth. Then mount a prox switch to "read" the gear teeth. 1 rpm would be 60 pulses per minute or 1 Hz. Should be able to read that in a standard input card. 0.46 x 60 = 27.6 pulses per minute for your application. Should get a pulse every 2.17 seconds. You could reset a timer with every pulse, and use the done bit to stop the system. By adding more teeth to the gear you could get less time between pulses. Just one idea, others may have something more elegant. Ken

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I would probably go along with an analogue card reading the current of the motor through a CT. Another option if its available to you is to adjust the current limit settings on the drive or go along with the encoder solution which I think would also work fine. Let me know how you get on

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just my opinion...change the drive to one that limits tourque? obviously i dont know the size of motor you are turning but if its less than 10 hp then a drive is cheap....

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maybe they think that "torque limit" is something special. use term "current limit" when talking to sales guys...

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If it is possible on your inverters you can output the drive current via an analogue output from the drive to an analogue input in the plc. The value returned can then be monitored and you can see what normal running current is. Then by applying an upper tolerance you can cut out the drive and prevent mechanical damage. You may need to apply a filter timer to cope with starting currents/ short peaks in the current. You'll probably have to monitor the situation for a while to get values that don't cause nuisance stoppages.

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Just tweaking Ron’s idea, if is possible place equally spaced holes, metal pieces or something along side the belt and use one appropriate sensor, this way you know for sure the belt is moving. Then match the speed reading with the vfd speed, compare them, add a tolerance for the amount of slip allowd to make the decision. Load cells on different points also may be used. Just another view Edited by Eugen

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I know this may sound dumb, but. All of the ovens I have been around that use the chain belt to move product, Have spring loaded idlers on the bottom of the chain to allow when product is added or to allow some flex for the chain. Why not do as some have done and add limits to idlers. When chain tightens up springs flex and limit makes. Regards, Tom To tweek it a little if you don't have spring idlers,add prox switches to slack area of chain and when slack is gone you know chain is getting to tight. Edited by twbtxnca

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Why couldn't you have just set a current limit on the drive....surely the drives can do this simple application....If that doesn't work then considering the cost of the belt 75000 G mabey the VSD should have also been spec'd up a bit to one with a bit of capability... well thats my 2cents sorry i came in late but i am glad you have sorted it out...

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How about a din mount overload between the inverter and motor It would trip saving lots of headache if you wanted to get crazy the N.O. could turn on a light marked "jam" Chicken

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This is probably the most appropriate solution - install an overload clutch that will dis-engage on high torque. Given the slow speed, it is most likely that there is a high-ratio gearbox between the motor and the drive sprocket. Note that with speed reduction comes torque amplification. It is highly probable that the difference between normal running torque or current and jam torque or current as seen by the motor is very small. So trying to set a current limit would most likely result in lots of nuisance trips or a broken chain.

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