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Variable Rate Fertilizer Spreader Contolled with FMX (pics)
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Posted 1/11/2018 10:09 (#6496122 - in reply to #6494314)
Subject: RE: Variable Rate Fertilizer Spreader Contolled with FMX (pics)

Southern Minnesota Between Freeborn & Wells
RF1 - 1/10/2018 02:22

Is fast close valve the same as a servo ?

Fast close is a type of Servo. Raven has fast close and standard valves. The words fast and close mean that the motor is not geared down so the valve moves fast and that it can be used to close the valve completely off. A standard valve uses gears to reduce the speed greatly and is not used to close the valve to stop application. Another valve would be needed to stop application.

The main reason to use a fast close valve is that only one valve is needed for a system thus lowering the cost. One valve handles both the rate control and also the ON/OFF function. This can work but has some limitations that make it less desirable in certain situations. Since the motor speed is not geared down, the controller must be configured properly so that it gives extremely short shots of electricity to change the valve position for control but yet is able to quickly shut the valve off when application is not desired. With a Raven system, if the system detects that the application rate is a bit low, but close, it will send two shots with the correct polarity to open the valve followed by one shot of the opposite polarity in an attempt to prevent overshoot. Depending on the situation, it may be difficult for the system to find the correct spot and oscillation (going over and then under the target rate) can occur.

Contrast this with a Standard Valve. Here the control and ON/OFF functions are handled by two separate valves. The control valve is geared down considerably and takes about 8 seconds to go from full open to closed. The On/OFF valve is not geared down and quickly goes from fully open to closed very quickly. Again the controller must be configured correctly for this type of arrangement. Now if the system detects that the rate is close but a bit low, it can send a fairly healthy shot of electricity to open the valve without fear of overshoot. Since the standard control valve is not responsible for stopping the flow it does not need to respond so quickly. When application should stop, the ON/OFF valve closes and stops the application. The control valve does not move during this time so when application resumes the control valve is in the correct position.

A third type of valve that can be used in hydraulic motor control situations is the PWM valve. This single valve is used for both control and ON/OFF. It consists of a hydraulic valve that has a plunger arrangement internally. With not power to it, the plunger is closed and no oil flows tot he orbit motor. The system can send out short shots of 12V followed by a gap which can hold the plunger part way open. Thus oil flows and the orbit motor turns. If the system determines that the applied rate is too low, it will change the ratio of the 12V shots to the gaps so that the average voltage is raised, the plunger opens more, more oil flows and the orbit motor turns faster. It does this in digital steps (like maybe 128 different steps). The system can remember which step it was using last so it can return to that value when application resumes. This means that the system has a good chance of starting at the correct flow. With the fast valve above, the system must increase the servo setting until the applied rate exceeds the target rate and then back off.
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