Near Intersection of I-35 & I-90 Southern Mn. | Yes, the folks there are very knowledgeable. I used to work there before retirement.
The PWM valve concept uses the idea that the electricity sent to the valve consists of 12V shots followed by a gap of zero volts. The frequency that you see listed is the number of these time intervals per second. If the frequency of 100 is suggested, this means that the system splits up every second into 100 parts. During each of these 1/100 of a second, the electricity could be ON, could be OFF or split between On and OFF. The amount of the split between ON and OFF varies.
The relationship of the On time to the time interval is called the duty cycle. If for example the ON time and the OFF time were equal then the duty cycle would be 50%.
If the electricity were ON for several time intervals the valve would go fully open and you would have maximum flow. If the electricity were OFF for several time intervals the valve would go fully closed and you should see no flow. In normal operation, the system is producing the ON voltage for part of the time interval and OFF for the remainder. Since this happens very quickly the valve can not go from fully open to fully closed and back to fully open, so it tends to "hover" in a position somewhere between fully open and fully closed. This results in a flow rate somewhere between nothing and maximum. The system compares its calculated applied rate against the target rate and adjusts split between the ON and OFF times to get the valve in the correct position.
Here's where the zero flow offset comes in, With the zero flow offset set to zero, the PWM value will drop to zero so there is no oil flow when the system is OFF. That is fine but when the system needs to start back up, it will need to ramp up to the value necessary to get the applied rate to match the target rate.
This might take more time than one likes. Here's where using a zero flow offset comes in. The zero flow offset is a number that you determine by trial and error. It is a value where the system just about starts to turn. If the value is too high, the system will not shut down enough to stop. If the value is zero or too low, there may be an unacceptable delay before product flows.
This idea is used with hydraulic planter drives as well. This is very useful for backing into corners so the planter starts planting quickly after starting to move forward.
Edited by tedbear 11/7/2017 06:52
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