Wrapper Stretched

Just had an interesting problem with a pallet wrapping machine at a local plastic bottle manufacturer. It is used to wrap clear plastic film around a pallet of plastic bottles with a sufficiently small tension so that they don’t get crushed. This one was of the type that leaves the pallet stationary and rotates the wrapping mechanism around it.

The concept took a little while to tweak to as the film drive mechanism used a small DC servo motor and servo drive which had only two control connections. That is the connection of an independent tacho on one of the rollers into the speed set-point of the drive, and a digital enable signal. There was a spring loaded roller that looked like it wanted to be a dancing arm, but it had no transducer connected to it and turned out to be just a very small accumulator.

I deduced that the intended method of operation was as follows.

  • Power for the removal of the film from its roll as well as the pre-stretch of that film is provided by the small DC motor and Control Techniques Maestro Drive.
  • The speed of the last roller that the film leaves before contacting the pallet needs to be near the same as the relative peripheral speed of the pallet.

The stretchy nature of the film means that the system would be able to cope with a non exact speed. The prestrecth of the film would ensure the film wrapped at a low tension and stayed in place.

The rotation of the rotating wrapping mechanism started off slowly, with sometimes a bit of a kick on the spring arm roller, then accelerate up to full speed in a few seconds. When it worked properly (which it did almost all of the time) the spring arm would make a few initial wobbles then settle into a steady position). It appears as though this position is not set by anything specific as it parked in several positions during my experiments.

When the unit didn’t work the arm would do a much bigger kick out and then the film would snap. The operator would then need to re-thread the film and start again.


Pallet Wrapper diagram

Layout of the pulleys involved in dispensing the film

The diagram above shows the layout of the pulleys involved in dispensing the film. On the left is the replaceable Film Roll which unrolls in response to the film being pulled out by the pallet after roller (i) and the small DC motor (not shown) drives the stretching rollers (c) and (e).

Rollers (b), (d) and (f) are the tensioned rollers that keep the film firmly in contact with the pre-stretch rollers to ensure that the film is stretched from the original length. The movable section holding (b), (d) and (f) rotates around pivot (p) when closed at setup time. This allows the contact angle and grip on the pre-stretching rollers (c) and (e) to be set.

Both (c) and (e) are geared to run at about the same rpm so you can see there is a fair bit of stretch applied to the film due to their different diameters. The second last free roller (h) has a rubber ring belt coupling to a DC tacho to measure the film speed at that point. This speed is intended to represent the relative speed of the pallet with respect to the film mechanism so it is a bit confusing to consider that this signal becomes the speed setpoint to the drive which is actually driving two of the rollers in this same assembly!

My take on it is that the film is sufficiently stretchy that the tacho gets most of its signal from the actual pallet movement and that changes in the speed caused by the DC motor are isolated from it de to the stretch and the small dancing arm looking accumulator. The manufacturer found it necessary to include a mechanical one way clutch on the tacho so it cant turn in reverse!

Due to the intermittent nature of the fault occurrence I wired an EDM up to it to allow easier diagnosis to be done at a more relaxed pace and with more information available.

I wired up the following signals to the EDM

  1. Tacho Volts (dcV)
  2. Enable (0 or 10vdc)
  3. Motor Arrmature (dcV)
  4. Motor Armature Current. Only had an AC tong but better than nothing
A diagnostic module wired up to the DC drive in an electrical enclosure

The EDM can be seen wired up to the DC drive of the palette wrapper

Finally along with the many ‘good’ recordings I managed to see one ‘problem’ activity. The following two charts show the normal and the problem start in terms of the four signals must mentioned.

Graphs showing a good and a faulty pallet wrapper run.

An example graph showing the Tacho, Enable, Motor Volts and Motor Current values for a good and a bad run of the pallet wrapper

Note that in the normal startup case the enable rises to start up the drive, then about a 22/50 = 440mS later (the time scale is in 1/50ths of a second) the tacho signal (which is used as the speed setpoint for the drive) starts to rise and there is a corresponding simultaneous increase in motor voltage that causes it. At the same time the current measurement also shows an increase.

The current signal needs some explanation as there are two things to take into account.

  1. It is inverted so it goes negative as the motor voltage goes positive. I could have reversed the leads, or post inverted the signal but it was easier to see the signals in the presence of the noisy motor voltage with them separated a bit on the chart.
  2. I used my Fluke 1000:1 AC current tong for the measurement. The motor is DC so I should have used a Hall effect current transducer but mine was on another job at the time. I could also post condition the current signal by passing it through a network with an integrator and a zero at about 8Hz for my tong, but quicker to just hold these thoughts in mind while interpreting the signals as they are.

Anyway the comparison of the normal and faulty charts shows that the faulty one shows higher currents and voltages, starting earlier after the enable (80mS), and starting without any appreciable speed setpoint. Incidentally that fault occurred after I got tired of waiting for it to appear a second time so I gave the drive a firm tapping with the handle end of my larger screwdriver. The fault occurred on the very next test!.

After replacing the drive with a spare (actually the original which had been repaired after its replacement) the unit ran again for about ten sets of wraps without a fault. Not conclusively fixed but with the recent evidence I give it a 95% chance.

After I had left site the onsite electronics technician, John Smith, found the source of the fault with the original drive. It was a broken solder joint on the Armature feedback resistor. This explains why the output was pulsing intermittently (as the solder joint made and broke contact with the PCB).

Customer Testimonial

Thanks for your help. This job was a great example of why a data logger/EDM is a great tool when trying to diagnose an intermittent fault that has multiple parameters/ variables involved. Great  outcome.

John Smith