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Integra pwm settings...
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tedbear
Posted 12/12/2017 09:34 (#6423633 - in reply to #6422805)
Subject: RE: Integra pwm settings...


Near Intersection of I-35 & I-90 Southern Mn.
I agree with the answers given above for your questions regarding three items with PWM systems.

For a quick review, in a PWM system, the system sends out "shots" of 12V followed by a gap of 0V several times per second. The times per second is the frequency. So if your valve suggests using 100 HZ that means that the manufacturer has determined that this frequency works best with their valve. Using 100 HZ as an example: This means that the system will consider a time slice as 1/100 of a second. Within that 1/100 of a second, the voltage could be 12V or 0 V.

In normal operation, this 1/100 of a second is split between 12V and 0V. The extremes would be 0 V or 12V for the entire slice of time. If the value were 0V for the entire time slice, the hydraulic valve would be closed. If the value were 12V for the entire time slice the hydraulic valve would be fully open. For normal application, it would be somewhere in between.

The Percent of the time the slice is ON compared to the total time slice is called the Duty Cycle. So if the system sent out 12V for 1/200 of a second followed by 0V for the next 1/200 of a second, the Duty cycle would be 50% (On half of the time).

This PWM output can be fed to a PWM hydrauic valve that is normally closed. If the valve were to receive 12V, it would go wide open and allow its maximum flow. Generally it will be receiving those shots of 12V followed by 0V. Since this happens very rapidly, the valve doesn't have time to go fully open and then back fully closed. This means that the valve tends to "hover" at a position some where in between.

The system compares the applied rate to the target rate that the operator has entered and changes the ratio of the time ON to time OFF to get the valve to hover in a position that results in the desired rate.

This same principle can be applied to 12V motors and allow the system to control their speed in a very similar fashion.

This brings up the idea of zero flow offset. Sending out 0 V at all times would result in the valve being completely closed which is desirable. When the system is commanded to start application, it needs to start sending 12V pulses to get the valve to open. This would work fine but may result in an undesirable delay before the system gets up to speed. This means a fertilizer applicator may not put out the correct amount right away. A planter backed into a corner might not start delivering seed as quickly as desired.

This is where the zero flow offset comes in. This is a low limit value for the PWM system so that it doesn't go to a constant 0 V when commanded OFF. The idea is that the system will start to apply more quickly when the system is again commanded ON. The adjustment for this setting amounts to finding the PWM value that is "on the edge" of starting to apply or plant. If the offset is too high, the system would not stop or shut off on the ends. If the offset is zero or too low, the system may be slow in getting going again.

The gain value is what I think of as "acceleration" in a sense. It relates to how quickly the system changes the mix of the 12V vs 0 V that the system sends out. Documentation suggests a gain value for different rigs which is a good starting value. If when changing ground speed or rates, the system responds but takes longer than you wish, enlarging the gain value should help. The danger is in getting the gain value too large. In that case the system may overshoot when it tries to react to a change in ground speed or rate. That is why the above reply suggests putting in two rates and observing what happens when you switch from one to the other. This can help you decide if the gain is acceptable or if changing it results in better performance.

PWM control can work very well with the correct settings. It is usually much quicker to respond than a servo type hydraulic valve since the PWM valve can pop to its previous setting without much delay. The Servo type control needs to ramp up to the correct target rate which can result in slow response when first turned ON.



Edited by tedbear 12/12/2017 09:38
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