Near Intersection of I-35 & I-90 Southern Mn. | BLUF- Bottom Line Up Front: Your meter cal may be incorrect. Your injection pump may need repair and is not pumping what is expected. There might be an obstruction in the inlet to the pump.
I worked as a Tech for a Raven dealer for several years. This was before retirement. My remarks are based on memory (sometimes not so good) and how the injection pumps operated at that time. I was fortunate to be able to be a Beta Tester in the early years of injection due to my position as a Tech and a farmer.
The Raven injection pumps came in two sizes. One was the High volume model and the other was the Ultra low volume model. The main difference between the two was that a slow down gearbox was used with the Ultra low volume pump. This allowed the motor to run within a desirable speed range but yet turn the pump much slower. The choice of whether to use a High or Ultra Low Volume pump was based on the flow range required for the situation. This depends on the desired rate, implement width and ground speed.
The flow requirements of an injection system on a large SP sprayer with a large boom, fast ground speed and a relatively high applied rate would be far greater than on NH3 bar being used for N-Serv with a smaller width, slower ground speed and rate. The Ultra low volume injection pump was generally used for N-Serv. Incidentally the early work done with N-Serv was done at the dealership where I worked.
The Raven injection pump consists of a two cylinder positive displacement pump that is run by a 12VDC motor. The speed of the motor is controlled using Pulse Width Modulation. Pulse Width Modulation is widely used in many situations. PWM uses the approach of making and breaking the 12VDC circuit very rapidly. This has the effect of being able to change the speed of the motor without "wasting" energy by adding resistance. Since the motor cannot instantly start and stop, the speed of the motor behaves as the "average" of the voltage received.
A common frequency for this is 100 Hz. This means each second of time is split up into 100 parts. Within each of those 1/100ths of a second, the circuit could either be OFF, ON or some of each. The term Pulse Width Modulation literally means that the "width" of the time that the circuit is completed is "modulated" or changed for varying needs.
The ratio of the "ON" time to the time slice is called the Duty Cycle. It can range from 0% (motor stopped) to 100% (motor runs full speed). Generally the Duty Cycle is somewhere in between.
The system needs a way to measure the output of the pump. On a sprayer this is usually done with a turbine based flow meter. Using a flow meter for the small quantities involved with the injection pump is not practical. Instead Raven decided to use a shaft speed sensor mounted on part of the drive train. The shaft speed sensor produces pulses which are directly related to the speed that the shaft is turning. In other words, the quantity being delivered by the pump is not directly related to the actual pump output but its speed. As long as the pump is working properly the correlation is fine. If the pump is not producing the expected flow for each revolution, the calculated flow will be incorrect.
The system uses the meter cal that the operator has entered to convert the pulses from the shaft sensor into familiar units such as ounces here in the United States. The display shows the calculated applied rate based on implement width, ground speed and flow rate. In Automatic, the system compares this value with the desired target rate entered by the operator and makes adjustments to the Duty Cycle to change the speed of the motor to bring the calculated applied rate to close agreement with the target rate.
This system can and generally does work very well. However if the pump is not pumping the expected quantity for each revolution, the indicated output is not correct. This can happen for several reasons such as bad check valves in the injection pump itself. If there are problems with the check valves, the pump will not output its expected quantity. Pump repair may be necessary. Other problems might be an obstruction on the inlet side. The Vac sensors usually warns the operator of this.
In a sprayer if your pump is getting weak and not putting out as much as it used to, the flow meter will show this reduction. With the pump speed sensing approach you would not be aware of this problem.
In sprayers with injection systems, we put a column flow monitor (RedBall or Wilger) in the injection line. This gave the operator a visual indication of the flow which could warn them of unusual situations. Due to the nature of injecting N-Serv into NH3, a flow column cannot be used.
Edited by tedbear 3/31/2024 09:13
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