scairn said:
MrQ
Why do you need to control the speed of the pump!
I most of you know I get into a lot of hydraulic motion control applications. Some of these require very high flow and response. For instance a customer wanted to extend 60 inches in .8 seconds, dwell for .2 second and wanted to retract 60 inches in .8 second and dwell for .2 seconds. That is really moving.
This means that a variable displacements pumps swash plate will be moving back and forth quite a bit.
Two of the key things in hydraulic motion control is the cylinder diameter and keeping the system pressure constant. The larger cylinder diameters make the system stiffer and increase the natural frequency. Keeping the system pressure constant keeps the motion controller gains constant. If the system pressure drops then the system gain drops. This means the motion controller gain must increase to keep the overall gain equal to 1. The problem is that the motion controller can't output more than +/- 10 volts to make up for falling system gains. The problem with pressure compensated variable displacement pumps is that they respond to pressure changes and can't supply full flow at the system pressure. Variable displacement pumps have a swash plate that causes the flow to vary. When the error between the desired system pressure and actual pressure small set point then the flow is very little if any. This keeps from wasting energy. As the actual pressure drops 200 to 300 psi the swash plate will move to full stroke, but this means the pressure compensated pumps can not output full flow at the system pressure which is bad. The problem is not only must the pressure drop but then we must wait for the swash plate to move so the pump will respond. Typically the swash plate control is a very crude spring adjustment that provides some sort of swash plate motion in proportion to the difference between the actual pressure and the desired system pressure. This just just control using a proportional band. This type of control is slow because one can't make the gain larger or the proportional band smaller with out risking instability. Pump controls lack the tricks I like to use like feed forward gains and derivative gains for added stabilty. Stable proportional only control is over damped control so it is slow and unacceptable. In my example application above the pump response will lag the application demands by many tenths of a second. Pressure compensate pumps with only proportional control can't see the rate of pressure change and use this derivative action to react faster. Finally the pump is always reacting to pressure changes and not the actual demand. Large accumulators will help reduced the drop in pressure but for some reason I find it hard to convince people that 50 gal accumulators are necessary to keep the pressure relatively constant. I get a little excited when a hydraulic guy looks at the pressure gage and tells me the pressure is only dropping 200 psi when in fact it is dropping three times that. Most hydraulic people don't think in terms of milliseconds yet.
Adding extra accumulators slows down the pressure changes so that the variable displacement pump has time to react, but at first all the extra accumulators do is slow down the pumps ability to react to the changes in demand because the pressure changes more slowly with changes in demand with more accumulators.
Eventually adding larger accumulators just means that the pressure is relatively constant but at the pressure where the pump is on nearly full stroke. This is why hydraulic axes need to be tuned under average load and with all the other axes moving at their normal rates.
If I can ramp up a VFD driven pump at the same time I am opening the valve I can get whatever flow I need without waiting for the system pressure to drop. This will keep the system gains constant and the control easier. The accumulator would be there only to cover for small mismatches between the valve flow and the VFD driven pump flow. The problem here is that the controller must be able to do this. I have had a few customers control VFD driven pumps they didn't uses them the way I would. The customers just wanted something to work and didn't want to make a research project out of it.
BTW, what difference does it make if the pump is a fixed or variable displacement pump in regards to pressure pulses? The only problem with a fixed displacement pump is that it is always at full stroke but the strokes would be slower when not at full speed so I can't see what difference the pump makes in regards to pressure pulses. I can see pressure pulses on variable displacement pumps too.
Why does the VFD driven pump heat the oil? You have control over the speed don't you?
Now the question is what is MR Q doing that would justify all this because it is cheaper to add more accumulators to keep the pressure constant.