This interacting pressure control system produces different pump responses. (Click image to enlarge.)
George: A notable pressure limit cycle (~35% of scale at about a 30 second period) developed in the two pressures causing periodic process shutdowns or poor downstream product quality. Attempts at tuning (on-line and via simulation) and minimizing loop dead time by speeding up control logic execution yielded little improvement. The waveform appearance is similar to that seen with a “sticky” control valve - but this process does not use control valves, it utilizes DC variable speed drives and speed reducing gearboxes between the drive and gear pump. Drive tuning and performance checked and found OK by corporate expert. Acoustical analysis of gearboxes did not indicate defects. After months of living with the problem a pump motor developed a bad winding and was replaced. Pressure variability was greatly reduced to normal levels when the system was restarted with the new motor!
Greg: A positive displacement pump is a constant flow device, which means the pump curve is steep and process gain is high (small variation in flow causes a large variation in pressure). There is a limit cycle whose amplitude is proportional to the resolution limit of the drive (e.g. 0.2%) multiplied by the process gain. This combined with the interaction between pumps in series and parallel could lead to bigger oscillations. Beyond checking the A/D resolution of the input cards I am a bit out of my element here (final element humor) so I asked Bob Heider, who has much more experience in the ins and outs of motors, to take a look at the application.
Bob: I simulated the pump response for three different windings of DC motors to a step change in viscosity in a non Newtonian fluid. For “separated excited” winding, the peak change in flow was only 0.1 gpm versus 0.8 gpm for a “shunt” winding and 1.8 gpm for a “series” winding.
Greg: Windings of the wrong type or in bad condition could increase the effect of a process disturbance or load change to the point where it triggers sustained oscillations from resolution limits and interactions. Before we get all wound up let’s break for a twist and a turn by Stan.
Stan: We have been criticized for unintentionally, neglecting the mechanical aspects of control. This is probably because of our backgrounds and experience. However, we have been approached to consult with the Work 4 Company. They are in the process of developing sensors and controllers to properly tighten tops or lids on bottles, jars and other containers.
The question/problem is, “How Tight Should Tops Be Tightened (HTSTBT)?”
If lids are too tight, senior citizens, handicapped, people with arthritis and others with weak fingers find it extremely difficult or impossible to open a container. This then requires new appliances and gadgets to assist them. Some people will say, “Don’t tighten them so tight”. The manufacturers will then respond by saying, “If not tight enough, there will be leakage and spoilage by air getting into the container. Also, we need to prevent children from getting into the bottles and jars”.
We would like to help Work 4 Company design and build a Variable Tightness Top Machine or a VTTM. This is our initial design concept. A customer who enters a market goes up to a panel on the wall, enters their secret code, and turns a large dial that would look like a typical container top. This Variable Torque Dial or VTD would measure their hand strength and become the set point for the VTT Controller or VTTC. When their selection of items in the store is complete, the customer will go to the checkout counter and identify themselves by either entering their secret code or inserting a credit card. The cashier will scan the items for pricing as usual. Then the cashier, bagger, or customer will pass the jars and bottles through a Variable Torque Torguer or VTTR. The VTTR in conjunction with the VTD and VTTC will provide a VTT.
Another Problem: After the VTT is set, the sack packer or bagger cannot pick up the container by the top, which they have a tendency to do. Picking up the item by the top may negate the setting on the VTTR.
PRESSURES BEFORE MOTOR REPLACEMENT
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PRESSURES AFTER MOTOR REPLACEMENT
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