control speed by moisture and temp

[kcor 1]

Have a question that I hope someone may have a good answer to. Have a machine that is a dryer that fabric is feeding through at speeds from 0 to about 70 yards per minute. We have been brainstorming about would it be possible to have the fabric as it exits the dryer run across some type of sensor that would measure the moisture in the sensor and also have some type of infrared temperature sensor to detect the temp of the fabric as it exits. Based on the moisture and the temp. determine how fast to run the fabric. For example, you enter a set point to run 40 yards per minute and maintain a moisture of 4%. and use the signals from the sensor to control a pid that would be used to tune the setpoint, plus or minus a percentage of the speed setpoint to maintain the 4% moisture. Does this sound something that would be possible, and if so, any suggestions, ideas would be greatly appreciated. Thanks

[Michael Lloyd 85]

If you are using a Control Logix PLC you could use a FGEN (Function Generator) to calculate the setpoint for speed. If you're familiar with the slope of a line equation, y=Mx+b then you'll love the FGEN block. It will actually do two calcs. X1 Y1 and X2 Y2. Drop an FGEN into a structured text or function block routine and play with it. One thing that I think you'll find to be difficult to come up with is a moisture sensor that obtains an accurate reading quickly

[paulengr 44]

You are attempting to apply 2-variable control on a single output. Usually this does not work so well, unless you can come up with a formula to convert it into a single variable. What do you do if the moisture is high and the temperature is high for instance? (vs. moisture low, temperature high; or moisture high, temperature low). Usually the best strategy is to use one variable per output. Usually, if heat increases, you get more evaporation, which leads to more cooling, which causes the temperature to rise slowly until you reach the point where the amount of free water available goes to zero, in which case temperature rises dramatically while moisture no longer changes significantly. So depending on where on the curve (psychometric curve that is) you are operating, it is usually better to operate off of either temperature or moisture but not both. It is also possible to operate a cascaded control loop where the outer loop is say moisture and uses PID. The inner loop is just a "clamping" function, possibly with some hysteresis. So the inner loop does nothing until temperature gets too high. Then it rapidly adjusts to try to keep temperature done. The response of the inner loop doesn't need to be very "PID like" or even linear. It just needs to act when things get out of hand. In the manual for the drive that you have controlling speed, take a look at the flow chart diagrams that are usually in the back of the book for inspiration. Most of these systems implement a "speed control" or "torque control" system, with an inner "current control" loop that prevents motor damage if the outer loop drives to the edge of the cliff.