Look-up tables w/AB uLogix 1200

I have to convert the output of a RVT output to measure a linear position. Using a different transducer is not an option. Does anyone know if you can implement look-up tables on the uLogix 1200? If so, how? If not, can the 1500 do this?

It's been awhile, but I remember that the Micrologix has a "Data Block" area that can be used to store data at the end of the ladder program. You have several commands that can manipulate this data.

Sorry, my memory is confusing different brands of PLCs. The MicroLogix 1200 has the ST (String) data type that can store any ASCII characters. If you are dealing with numbers only you can use the N7 (Integer) file and store any series of numbers. These may be accessed in the normal fashion, or by Indirect Addressing can be used as arrays.

By RVT you mean Rotary Variable Transformer, correct ?

How non-linear is the output of that device, and what span of voltage does it cover ?

If the output of the RVT is close to linear, you might get the best results from the SCP (Scale with Parameters) instruction, just to convert the voltage to engineering units. You could even use several of those instructions as a piecewise linear function if you needed to.

Indirect addressing for lookup tables is do-able, but it's memory and time intensive.

The transducer is a rotary variable transformer, it has it's own signal conditioning circuitry. The output is 0V @ 0 degrees, 2.5VDC @ 30 degrees, 5 VDC @ 60 degrees, 10 VDC @ 120 degrees. Relative to angular measurement, it's very linear, however, it is mounted to an actuator that, as the slide moves linearly, the RVT moves radially via a cam/follower arrangement. I want the actuator to linearly track the control input, therefore, I need to implement a look-up table to correct the RVT output. The only function the PLC will perform at this time is the PID control loop, do you think the memory and time demands will be too much? The FR of the actuator is low, probably < 5 Hz.

Why not get a motion controller?

tvlodek said:

I want the actuator to linearly track the control input, therefore, I need to implement a look-up table to correct the RVT output. The only function the PLC will perform at this time is the PID control loop, do you think the memory and time demands will be too much? The FR of the actuator is low, probably < 5 Hz.

Click to expand...

Many motion controllers can handle the trigonometry or at least a look up table with cubic interpolation between the points. A motion controller will cost more than an uLogix but then you will have better performance, diagnostics and setup tools that will make startup and maintenance easier. Time is money too.

tvlodek,

Why not just set aside 40 numbers (1 for each 0.25 volts of your 0-10 volt input) in the MicroLogix1200 F8 Data Table, (say F8:100 to F8:140, up to 256 32-bit numbers). Then go to that data table and type in your linear distance number for each 0.25 volt input change. Now set up 40 ladder rungs that do Comparisons: Is the 0-10 VDC input between F8:100-0.125 and F8:100+0.125? If so, Move F8:100 to your Linear Slide Position Indicator (F8:0 perhaps?). Do this for all 40 numbers, and you will have an adequate position indication, without a lot of fancy programming hassles. The 1200 can handle this with no problem.

Or you can do what Ken mentioned, break up your curve into linear steps, and use the Scale function for each step.

Pete,
I haven't investigated a motion controller because the people I'm working for are so cheap they squeak. If you could recommend one and give me an idea of the cost, it would be appreciated, maybe I can convince them it's more economical in the long run. In the meantime, I will look into it myself. I was planning to use a LUT and interpolating additional values or calculating the sine using a Taylor series approximation but am concerned with the execution time. I agree with your comment about the time aspect, the time I've wasted on this I'm sure could have bought a good motion controller and yielded a vastly superior solution.

Lancie,
That's exactly what I had in mind, I just didn't know if the 1200 supported it. If you read any of my earlier posts, I am a newbie to PLC's and didn't know if this one had that capability, the user manual has over 500 pages and a search for "look-up tables" didn't yield any matches. Anyway, thanks for giving me some ideas, now I've got to crunch some numbers to see if I can hold +/- 0.010" positional accuracy.