Controlling A Hydraulic Solenoid valve With PWM

[npalen]

Machine Control System, Laser Land Leveling System | eBay

Products | CANAMEK

This company, Canamek, in Canada sells a relatively low cost laser controlled grading system that interfaces with a simple 12 VDC off-the-shelf on/off switching solenoid valve to automatically control the grader blade elevation for under $2K. (Not including the grader)

All other similar controls that I'm familiar with use a servo or proportional solenoid valve(s) where a complete system including a box blade, for example, will run $10K to $15K range.

A PWM (Pulse Width Modulation) control uses pulsed DC varying in amplitude and frequency to control things such as solenoids and DC motor speed. My question, if anyone is familiar with PWM control of an on/off solenoid, is how well the system performs compared to a proportional solenoid valve system.

I'm wondering, for example, about fluid heat buildup in an open center system with this constant throttling of the solenoid valve.

Doing a little reading on the subject brings up terms such as spool friction, stiction, hysteresis and dither with dither being used to control the first three. In other words, they are using electronic wizardry to proportionally control something that is normally controlled with simple on/off DC voltage. One article mentioned setting up a slight vibration in the spool to prevent stiction which sounds a little bit like an athlete constantly moving the feet to be ready to go either direction.

Would like to hear some feedback.

Some previous threads on laser grading:
https://www.tractorbynet.com/forums...-machine-grading-inexpensive-alternative.html

 

[Industrial Toys]

I can see PWM to control a rotating motor. For parameters such as torque. I am curious what the advantages are on a solenoid valve. The relatively slow response of a hydraulic circuit in my mind would make this a moot point. But I am not pretending to know. POSSIBLY, the energy to run a solenoid valve using PCM in demanding applications might be more efficient with less heat and requiring less in the way of heavy straight DC current requirements. That's Just a wild guess though.

 

[npalen]

I can see PWM to control a rotating motor. For parameters such as torque. I am curious what the advantages are on a solenoid valve. The relatively slow response of a hydraulic circuit in my mind would make this a moot point. But I am not pretending to know. POSSIBLY, the energy to run a solenoid valve using PCM in demanding applications might be more efficient with less heat and requiring less in the way of heavy straight DC current requirements. That's Just a wild guess though.

I guess the idea is to be able to fine control the valve spool position to provide variable response depending on how far and how fast the blade needs to move to get back to "laser zero". This would be similar to controlling the same valve manually and feathering except the PWM signal does this automatically, more precisely and faster than could be done manually.

 

[Industrial Toys]

The rates at which PWM can operate are meaningless to the operation of a solenoid valve, and oil flow, in my opinion. I could be wrong, but it seems like the so common trick of bamboozling people with terminology that seems to make sense but is really not applicable. Kind of like saying your computer utilizes "turbo" technology. What the F#%% does that mean?

 

[BukitCase]

The PWM I'm familiar with (somewhat) has a FIXED voltage; hence the name Pulse WIDTH Modulation - a few of the hundreds of controllers in the rare metals plants I worked instrumentation/control in for the last 35 years before retirement had started using PWM outputs, but most still had an analog output of 4-20 milliamps - most higher powered devices in that arena can take a milliamp signal and drive hundreds of horsepower devices. A few of our hydraulic systems (presses) had as many as 3 200 horse hydraulic pumps at 10,000 psi - forge presses, extrusion presses, etc -

I found a useful link (with waveforms that should help explain the effective outcome of PWM control

Introduction to Pulse Width Modulation - Embedded.com

I would guess that most controllers that output a PWM signal still run a digital PID loop, but output the PWM signal INSTEAD of a milliamp signal (a lot of the ones we used had options for outputs AND inputs, makes 'em really versatile - inputs of milliamps or voltage, plus several types of thermocouples, tach inputs, etc - by the time I left, even the industrial versions of such controllers were down in the $150-$300 range.

Because of all the on-off switching it takes to turn an on-off valve into a "variable" valve, I would expect a shorter lifetime for the valve (and probably its plumbing) than when it's used for its ORIGINAL function (see the "effective" waveforms in this link from wikipedia)

what is PWM control - Google Search

The RED graph in the pics represents the "effective" (sorta) sine wave of the PWM output when controlling an on-off device proportionally.

BTW, if you've ever tried to control a hydraulic valve with compressed air, you likely saw an EXTREME example of "stiction"... Steve

 

[mikester]

Machine Control System, Laser Land Leveling System | eBay

Products | CANAMEK

This company, Canamek, in Canada sells a relatively low cost laser controlled grading system that interfaces with a simple 12 VDC off-the-shelf on/off switching solenoid valve to automatically control the grader blade elevation for under $2K. (Not including the grader)

All other similar controls that I'm familiar with use a servo or proportional solenoid valve(s) where a complete system including a box blade, for example, will run $10K to $15K range.

A PWM (Pulse Width Modulation) control uses pulsed DC varying in amplitude and frequency to control things such as solenoids and DC motor speed. My question, if anyone is familiar with PWM control of an on/off solenoid, is how well the system performs compared to a proportional solenoid valve system.

I'm wondering, for example, about fluid heat buildup in an open center system with this constant throttling of the solenoid valve.

Doing a little reading on the subject brings up terms such as spool friction, stiction, hysteresis and dither with dither being used to control the first three. In other words, they are using electronic wizardry to proportionally control something that is normally controlled with simple on/off DC voltage. One article mentioned setting up a slight vibration in the spool to prevent stiction which sounds a little bit like an athlete constantly moving the feet to be ready to go either direction.

Would like to hear some feedback.

Some previous threads on laser grading:
https://www.tractorbynet.com/forums...-machine-grading-inexpensive-alternative.html

Thanks for the links, interesting hardware.

FYI DC PWM does not vary amplitude only duration and frequency. Unfortunately I only have some experience with PWM electric motor control and can't answer your questions on the hydraulic valves. I would suspect the valves are modulated when motion is required instead of constantly modulating like an LED dimmer circuit.

 

[workinonit]

Are we talking about using this on a 3 PH application? Or FEL?

 

[npalen]

Are we talking about using this on a 3 PH application? Or FEL?

This would be for controlling the hydraulic cylinder on a box blade for laser grading similar to this animation:
Laser Land Levelling animation (IRRI) - YouTube

 

[npalen]

The rates at which PWM can operate are meaningless to the operation of a solenoid valve, and oil flow, in my opinion. I could be wrong, but it seems like the so common trick of bamboozling people with terminology that seems to make sense but is really not applicable. Kind of like saying your computer utilizes "turbo" technology. What the F#%% does that mean?

I believe the solenoid only sees a "normal" DC voltage/current that is variably less than the normal 12 VDC (in this case). This controls the position of the valve spool between closed and open depending on the "currently" sent by the PWM. Yes, the PWM can operate at very high frequency but the solenoid doesn't see the fancy stuff, just the resulting voltage/current.

The laser receiver on the grading machine first outputs a PWM signal depending on where the laser beam is hitting it, this signal then goes to the machine controller on the tractor which, in turn, outputs the PWM control to the solenoid valve which in turn controls the hydraulic cylinder to level the ground.

 

[npalen]

Thanks for the links, interesting hardware.

FYI DC PWM does not vary amplitude only duration and frequency. Unfortunately I only have some experience with PWM electric motor control and can't answer your questions on the hydraulic valves. I would suspect the valves are modulated when motion is required instead of constantly modulating like an LED dimmer circuit.

I'm thinking that PWM does control amplitude as a result of duration and frequency. The solenoid would move the spool depending on how much voltage is applied wouldn't it? Or perhaps current moves the solenoid and is the result of duration and frequency? I sometimes have trouble visualizing the difference between voltage and current even considering the formulas such E=I/R etc.

Edit: I'm on the fence about pulling the trigger on the Canamek machine control mentioned in the first post. They are the only company, to my knowledge, that sells the system using a conventional on/off (switching) type solenoid valve while others use the much more expensive proportional or servo valves. The on/off solenoid setup has no feedback to indicate actual position of the valve spool and resulting flow other than the laser receiver will indicate when it is approaching and at level so this is feedback in some sense.

I don't know how sophisticated the Canamek control system is in regard to such things as providing a vibrating action to the spool to prevent stiction etc.