Conveyor Winch Control

Hi there

I have been programming PLCs for the past 3 years. I am busy with software for a automatic tensioning system on a conveyor using a winch with disc brakes.

I should tension to 40kN before startup and slackand control on 28kN after run up complete. I have written it as follows.

I reckoned I need a bandwidth otherwise it is very difficult. Before startup if tension to low program tension until in middle of bandwidth 38 - 42 kN. At 40 kN I stop the winch and activate the brake. Now I monitor to be within band. Six times it starts up perfectly and the next time I am just outside of my bandwidth.

Has this anything to do with the speed of the winch, PLC scan speed, brake reaction time? Is there maybe someone who has an idea to do this in a better way. I have stopped the winch on different tensions within the band, but seem to find a difference in control when belt is full and empty.

Regards

2engswt1

2engswt1

I have a couple of questions for you:-

1) What are you using to control the winch motor, are you using a VFD or is the PLC switching a contactor in to start the motor direct?

2) Are you keeping the power applied to the motor until the brakes are set? Have you some sort of feedback as to the brake position i.e microswitches?

3) How are yoo 'measuring' the tension, what form of feedback are you using?

4) What PLC are you using?

5) Can you post some more information on exactly how you would like it to work?

We might be able to help you out on this, but, I am pretty sure that you can rule out PLC scan time as the problem, it likely points to the brakes, hence question 2.

Paul

It is hard to try to give alot of help without more info (PLC code, Input and output types, other component types, etc) but we will give it a try anyhow.

Assumptions:
1) You are using an analog input to measure your tension feedback.
2) You are using two digital outputs to turn on your winch - one for forward and one for reverse.
3) Your logic immediately engages the brake once the tension feedback measures within the bandwidth.

If these are all true, you might want to put in a short time delay to allow for the tension to feedback to stabilize after the winch stops and before the brake engages. I suspect you are fighting a mechanical issue that is causing this.

As far as your control technique choice, it sounds pretty reasonable to me.

Steve

Hi guys

Thanx for the help. Answers to Steve's questions:
Brake and winch controlled with contactors.
No feedback received from brakes.
When in tension band, winch stops then brakes set.
Loadcell used to measure tension with AD converter card, loadcell range probably contributing to problem (8 - 80kN).(Will be changed soon) The tension then converted to raw count of 0 - 10000.
Mitsubishi Q series PLC.

Do you think it will help by applying brake once tension is OK before stopping the winch motor. (Makes sense to me!) I am also not to sure about mechanical surges influencing the system. (I'm an electronics guy, but I reckon there must also be surges as in electrical equipment, for example transient currents when power is first switched on)

Regards

Okkie le Roux

If it is critical to have your tension be within the preset band while the brake is engaged rather than just close, I think you will likely need to build in a delay for applying the brake.

I imagine that what is happening is that the system is kinda spiking and really needs to stabilize before applying the brake.

Maybe you could modify the logic to say "with the winch stopped, the tension must be within the bandwidth for X number of seconds before applying the brake".

Steve

Since you're finding a difference in control depending on whether the conveyor is full or empty, which is probably due to inertia, why not modify your limits for the different conditions (i.e. full or empty). One other thing, you stated that 6 times it starts perfectly but the next time it's just outside the bandwidth. Which direction, high or low? You could possibly modify the bandwidth if it's always the same direction (high or low), or is the conveyor full 6 times then empty? Good luck to you.