Position Verification Methods?

[mr_electrician 9]

Hello everyone, I am looking into the best method for verifying nut location. A 4 axis manually placed arm will be used and will have a nut runner and torque monitor attached to the end of the arm. I was thinking that I could use a incremental encoder for each axis and feed the signals into some high speed inputs and use a compare instruction to verify location of the nut to be tightened. The operator must tighten the nuts in a specific order. That's why I need to see location and compare that to what nut they should be at in the sequence. Has anyone done this or have a more efficient/effective approach? Cheers

[TimWilborne 107]

Do the parts vary or is it always the same part and pattern? I think your encoder approach would be a good one. You could also do an analog measurement of the X/Y position or if there are only a few bolts simple switches in the proper position.

[mr_electrician 9]

I do not have the full details yet but I am sure the pattern may differ from part to part. I was wanting encoders so there is the option to teach position on future jobs.

[JRoss 84]

Encoders would work fine. If you use incremental encoders, you'll need to perform a homing routine every time the machine loses power. Absolute encoders would only need to be homed once, unless they are physically disconnected from the frame. However, absolute encoders require either multiple digital inputs or some sort of interface such as SSI or a fieldbus like Ethernet/IP.

[mr_electrician 9]

Thanks for the input JRoss. Teaching the encoder on power failure may not be an issue. This is a manually operated device and I could incorperate a re home step every 1000 parts or so? Can you use absolute encoders on a micro1200 or 1400? Also what is SSI?

[drforsythe 15]

I remember seeing a device just as you describe. It was a "packaged solution" for verifying that screws or nuts were tightened in the correct sequence. It was recipe driven so doing multiple parts was not an issue. Like I said, it was several years ago, and I cannot remember the name of the manufacturer. I did, however find a similar unit. I have no experience with this one, so I am not endorsing it. I am just sending you to their site for a reference in case a purchased device would be feasible. the website is www[dot]mountztorque[dot]com. I am not sure if you are quoting a machine for a customer or if it is for your own use, but a purchased solution may save you time, development costs and ultimately keep you more profitable. Let us know how you do.

[JRoss 84]

Even with incremental encoders, you should only need to re-home on power-cycle. The PLC would keep position until then. Yes, you can use absolute encoders with the Micrologix, you just have to decide on the electrical interface. The most basic is just a series of digital signals that make up a unique binary pattern for each position of the encoder wheel. These are usually only single-turn and low resolution, since you use up a lot of I/O. For example, a 256 pulse per revolution encoder would use 8 signals, a 512 would use 16, and so on. SSI stands for Synchronous Serial Interface, and is a fairly common interface for absolute encoders. It's just a serial signal that reports the position of the encoder on a periodic basic. The wiring is simple, but you'd need a special card for the PLC to interpret the data. A quick search turned up one from AMCI that works with the Micrologix. http://www.amci.com/specialty-io/plcmodules-1762-micrologix-ssi-interface.asp

[MrAutomation 20]

Depending on the physical design of the arm, a linear position sensor might make more sense than the encoder. This is a sensor installed inside a cylinder (ie. hydraulic/pneumatic) and uses a magnet to determine the exact position that the cylinder is currently in. It requires no teaching as the feedback you are given is the actual position (scaling required of course). SSI and analog inputs are common for these types of sensors. MTS Sensors makes a Temposonic R-Series which is available in an Ethernet/IP option as well. I haven't used the Ethernet/IP option, but plan on using it in an upcoming project. I can't say for sure if this is the best solution for your arm, but it's worth looking in to. The Temposonic RP or EP models don't need to be installed inside a cylinder, but can be surface mounted outside your machine, so this may be on option as well. (R series is the flagship line, while the E series is more basic, but may suit your needs if you don't want/need Ethernet/IP or SSI, among other features). Check it out. http://www.mtssensors.com/products/industrial-sensors/index.html Edited 5 Nov 2014 by MrAutomation

[gleblanc 1]

Is there a reason you need to do this using your own custom hardware? Have you already selected vendors for the nutrunner and/or 4-axis arm? I was at the Atlas-Copco facility in Auburn Hills a couple of weeks ago, and they have 2 and 3 axis arms on the shelf, with or without encoders for position, and software included with their controllers that can handle all of the positioning, so that you can avoid all of the heavy lifting. I'm sure that Apex/Cleco, Aimco, and Ingersoll-Rand also have similar setups, relatively off-the-shelf. I'm not in the London area, but we have a sister plant in Woodstock who probably has some local contacts for vendors. The other system option that comes to mind is from PinPoint (http://pinpointinfo.com/). They are more of an integrator than a vendor of a particular solution, but I've seen demos of their equipment with arms and position feedback on an operator screen). Greg