Ovality Gauge

[Paul_BerryPlast 0]

Hello Everyone, My name is Paul, I am a product development engineer with a plastics manufacturing company just outside of Chicago. I am looking into designing an ovality gauge for a round container that we produce. The gauge would rotate the container via a servomotor and would measure the ovality of the part by a CMOS laser gauge. Ideally, the data logging system will track the rotational position of the container as it spins, and will log that data in sequence with the measurment data, for later exportation and data cleaning. That way, a distribution of min/max ovality can be correlated to the circumference of the container. My concept is only in a preliminary stage as I have a large amount of research that I still must complete to fully understand the scope of what I may be building. At the moment, I only have a small amount of formal PLC program training from a single class in college a few years back. Outside of that, I am completely naive to both the fundamental elements that my gauge will require, as well as to what interface programming will be done once a build has been completed. If anyone cares to act as training wheels to my learning process, I would much appreciate any simple guidance on first, the required hardware for this project. My list thus far: (could be far off with some of these) Gauge Fixture Hardware Servo motor Servo Motor controller?? Servo motor PLC interface?? Data Acquisition system A concern of mine right now will be in the starting and stopping of the cup rotation and the starting of data logging. For the sake of simplicity, and reducing operator interaction as much as possible, I would like the entire process to be triggered by the single touch of a button or footswitch. This button would initiate the rotation of the servo motor and with the start of a constant rotational velocity, would trigger that start of the measurement and rotation position logging. After X amount of rotational counts in the servo motor, the system would stop, and the data would be exported. I would truly appreciate the insight from anyone willing to lend some help. Here are the conceptual drawings:

[drforsythe 15]

We do something similar measuring thickness and width of extruded material. Our system is kind of pricey because we bought a 'Cadillac' system to do our initial testing and then never refined it to trim out unnecessary costs. We could have used a less expensive plc (CompactLogix vs ControlLogix), but nonetheless, here is the hardware we use: Servo motor & Controller - A-B Ultra3000 with analog input (command from plc) PLC interface - ControlLogix rack with processor, analog input card (for reading laser gauge output), two-axis servo card (only one channel used) and one digital in and one digital out card Data Acquisition system - RSView32 running on industrial PC with touchscreen. The RSV32 application has screens that show thickness, thickness & width info as it relates to servo motor position. We also have the ability to log the data to our network server. Miscellaneous - safety circuit to remove power from servo motor and laser output upon e-stop Your system could be similar, but you would need to 'home' the motor or reset the postion to zero prior to the start of the cycle. Hopefully this is helpful.

[paulengr 44]

Did you consider the fact that the object may not happen to be centered exactly on the axis as expected? This makes ovality a bit of a trick to measure because you have to "recenter" the data. Fortunately if you think about how a circle appears when you "unwind" it linearly, it becomes a sinusoid. So if you run a discrete Fourier transform across the data with a period of just ONE, you can easily remove this from the data. The remainder is pretty easy to crunch because you can calculate highs, lows, etc. Also, position is important. Consider using an encoder on your turntable so that you can take readings at absolute positions. This exercise is not all that difficult from an electromechanical or even "controls" point of view. All the challenges are in the data crunching part.