Near Intersection of I-35 & I-90 Southern Mn. | Tazzerblue - 2/4/2021 06:29
(Pulse Width Modulation) Is this what its doing? I'm not familiar with these, how hard are they on the pump? or are they designed for it? can you post photo or MFG? Thanks.
There are various methods for controlling the rate on a sprayer or fertilizer system.
One approach is to run a PTO or hydraulic pump at a speed that will produce adequate flow for the most demanding situation and possibly a bit more for agitation. Then a butterfly or ball valve can be used in the product line to either return any excess back to the tank (By Pass Control) or to restrict the flow going to the ground directly (Inline control). The latter happens to be how I control the spray on my planter and previous sprayer.
Another approach is to vary the oil flow going to a hydraulic spray pump. With this method the pump speed is controlled which in turn affects the output. The flow of oil going to the pump can be done with either a Servo or a PWM hydraulic valve.
I believe the original post was asking for advice or opinions between these two hydraulic control valve choices. Possibly the question was whether to go with controlling the speed of the pump versus using a control valve in the product line. I would say either would be fine.
Hydraulic Servo valves can be thought of as a manual flow divider that has a motor on it so its position can be changed electrically with a switch or more commonly with an electronic rate controller.
A PWM hydraulic control valve varies the oil rate also but uses the idea of PWM (Pulse Width Modulation) to accomplish this. PWM means the controller sends out extremely short "shots" of electricity followed by a gap. It does this at a certain frequency say 100 Hz or times/second. That means within that 1/100 of a second, the circuit could be ON all of the time, OFF all of the time or ON some and OFF some. The ratio of time ON compared to the time ON and time OFF is called the duty cycle. If the duty cycle is 100% then the hydraulic valve would be fully open and the pump would run at maximum speed and flow. If the duty cycle is 0% the hydraulic valve would be fully closed and the pump would not run at all.
Under normal circumstances the duty cycle is somewhere in between. The valve consists of a plunger to control the oil flow. Electricity creates a magnetic field which opens this hydraulic plunger. With a duty cycle of less than 100%, the valve doesn't have time to open fully and then close fully so it "hovers" somewhere in between. If more flow is needed, the duty cycle is increased causing the valve to open a bit wider, allowing more oil to the pump and a bit more output.
Both of these hydraulic methods can work fine with a sprayer. With a sprayer the pump is usually turning even when the booms are shut off to provide agitation. PWM systems often have a limit on the PWM settings so that this minimum is always maintained.
The idea of using either a servo or PWM valve to directly control metering wheels or a conveyor for granular products is quite common. In those situations, the orbit motor on the metering device needs to stop when application is no longer desired. When application resumes, the valve opens to start the orbit motoring driving the conveyor.
This can be done with either a servo valve or a PWM valve. The PWM valve approach is often more desirable in these situations. With a servo valve, the system needs to open the servo valve to get the conveyor turning and needs to overshoot the rate for a bit until the control circuit detects over application and then it reduces the oil flow. This can result in an oscillation since the flow initially may be too high, then too low etc.
A PWM valve is often better in these situations as the system "remembers" what PWM value it was using at the end of the last pass and can return to that value as a starting point. That value is generally a great starting point as long as the ground speed and target rate haven't changed. That means the system pretty much jumps to its last speed for the conveyor and then makes adjustments from that point.
One reply above asks about being "hard" on the pump with PWM. That is not a concern as the pulsing is of electricity which controls the height of the plunger in the hydraulic valve. The oil flow to the pump is steady.
The first picture below shows a PWM valve from the Brand company. This can be controlled with most modern controllers such as Raven, Ag Leader, Deere etc. The second picture shows a manul controller that could be used if an automatic controller is not available for for a constant rate situation. The operator manually selects the PWM output that they wish. It does not make any auto adjustments based on rate or ground speed. The third picture shows a manual hydraulic valve. A servo type is this type of valve with an electric actuator on it rather than the manual adjustment handle.
Edited by tedbear 2/4/2021 07:43
 (Brand PWM hyd valve (full).jpg)
 (Brand PWM control (full).jpg)
 (Manual valve (full).jpg)
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 Brand PWM hyd valve (full).jpg (65KB - 156 downloads)
Brand PWM control (full).jpg (99KB - 142 downloads)
Manual valve (full).jpg (119KB - 139 downloads)
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Pwm pump or servo valve on liquid system on planter
