Using PWM to control Variable Force Solenoids

Thread Starter

Bryan Denmark

Joined Mar 29, 2019
3
Hello, I have a small tractor using 12V DC and a 120A alternator. Movement is controlled by a hydraulic motor that is controlled by variable force solenoids(VFS). There are two VFSs, one for forward, and one for reverse, controlled by two POTs. Originally, 5V DC fed into each POT and the variable output went to a computer that then controlled the VFSs with variable 0-12V DC. The computer has died and cost $1200 to replace.

My main question is, could I use a DC motor speed controller that uses pulse width modulation to control my VFSs, or would that just cause the solenoids to open and close rapidly, rather than a smooth gradual opening? I was considering trying two of the ones in the image below by connecting my POT instead of the one provided. I don't know exactly how much current the solenoids require, but I do know 20A is more than enough.

I am open to building a circuit if I must, I just don't want to spend $1200. I can still use the original 5V or a 12V input for my POTs. They just don't output enough current to open the solenoids. I have plenty of amps available from the battery/alternator, but need to be able to control it smoothly using the POTs as they are attached to forward/reverse foot pedals.
DCmotorCont..jpg
 

MisterBill2

Joined Jan 23, 2018
4,583
How well the control works depends a lot on the frequency of the PWM switching. So it is something that you will need to experiment with. The hydraulic servo packages that I have worked with used linear DC control currents, I think up to 100mA, but possibly it was less than that. A suggestion to help with the experimenting is to add a 12 volt light in parallel with the solenoid valve coil to show what is happening.You will also need some arrangement to switch between valves with the one controller module.
But it should work. I suggest adding a "kill switch" so that you can quickly stop the engine if things start to go wrong.
 

MaxHeadRoom

Joined Jul 18, 2013
19,466
Generally there are two types Proportional Open Loop and Servo valves that usually require a feedback to the control by some means.
The former may be possible to use a PWM signal, but the custom with the Servo valve usually a analoge signal is required.
You can get 12 high current PWM controller on line for a couple of $$'s to test the theory..
Max.
 

MisterBill2

Joined Jan 23, 2018
4,583
I have used a PWM powered solenoid actuator to control the throttle of engines under test and that device worked quite well. Hydraulic servo valves are usually more linear in their response, which makes the control easier. The final proof, of course, will be in the performance. Since it is a speed control in a tractor the driver will be providing the feedback and so the controls can be much less complicated.
 

DNA Robotics

Joined Jun 13, 2014
516
Speed control methods: potentiometer adjustment 0-5 v adjustment range or enter 0 to 5 v voltage control signal to adjust. 10-20 k potentiometer rotating button (potentiometers for three line connection: there are two is to control the end, a earth, two line end at top speed conditions such as motor voltage of: 5 v level voltage. 0 to 5 v control connection: 0 to 5 v control signal by PWM - 5 vinput among terminals
Can control signal to the governor GND connection
They use a 20K ohm pot. What are your originals?
 

Thread Starter

Bryan Denmark

Joined Mar 29, 2019
3
Thank you everybody for your helpful responses. I just finished installing the PWMs and they work perfectly. Machine operates like new. I used two separate PWMs for each direction so switching between them is not required. At $18 a peice it was certainly cheaper than a new computer. And technically the computer needed to be reprogrammed but that cost $1200 from John Deere or I could buy the $200 service manual and $600 programmer to do it myself. Instead I spent $39 and got the same results.
 

MisterBill2

Joined Jan 23, 2018
4,583
Could you please share the link to the source of those PWM controllers? It is handy to not have to look through the thousands of sources that claim to have a product that they don't even know what it is. And also, if they work for you that means that they are actual devices that function as described. The one that I designed into a test stand several years ago included the solenoid and cost a whole lot more. Of course it was intended to control the speed of medium sized diesel engines (400+HP), but it used exactly the same principle.
 

Thread Starter

Bryan Denmark

Joined Mar 29, 2019
3
Absolutely. I found those devices on Amazon and the sellers name was DROK. This a direct link - https://www.amazon.com/DROK-Controller-Sensitive-Frequency-Automotive/dp/B071XXP1F1/ref=sr_1_10?keywords=dc+motor+speed+controller&qid=1554956692&s=gateway&sr=8-10

Or copy and paste this into the amazon search bar - DROK DC 9-48V 16KHZ PWM Electric Motor Speed Controller, Small 20A DC Motor Controller with Sensitive Knob, Support 0-5V Frequency Converter Input Control for Car Automotive DC Electric Fan
 
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