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Powering a PWM valve with 24V PWM

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Jamie Janke

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Hello new to the forum. Somewhat inexperienced with a alot of general electronics but i am a Mechanic and understand well 12V systems.

Please forgive me if i should have found my answer somewhere else.

My situtation: I have been asked by a customer to try run a John Deere GPS system on a new CAT D6T machine. I have made a way to provide 12V or 24V Diret voltage onto the Cat steer pump.

Problem: Cat currently steer the machine using a PWM 24V output from and ECM to the PWM valve on the steer pump. They strongly recommend not putting 24V Direct voltage onto these valves as it will drastically reduced the life.

Question: Is there a simply way of providing either 12V or 24V in a PWM current to this valve from either the 12v or 24V direct current.

I dont know the current requirement and or the Frequency that the valve would require to run at but can find out if required.

I would like to keep this system as simple as possibly as it will be on a Machine that will be in rural and remote areas and reliability is essential.

Thanks in advance for you assistance
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
I'm puzzled as to why you want another 24V PWM drive if the ECM already provides one, or why you need the 12V supply if the steering requires 24V PWM :confused:
Are you saying you want to replace an existing ECM system with a GPS system?
What are the signal/supply specs for the GPS system?
 

dr pepper

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Does gps mean global positioning system, or as I suspect an acronym for some kind of steering system?

So you have a power supply which can be 12v or 24v, and a solenoid valve, there are 2 pieces of info we dont have, what current does said solenoid require, and what is operating the solenoid, do you currently have a signal from a steering system or are you implementing your own?

You'll probably need to build some kind of amplifier circuit using a mosfet to do that, or if you can cope with a simple on/off maybe some way of dropping the voltage by using a power resistor, depends on the application.

I amongst other things maintain a small fleet of fork lifts, so I have some experience.
 

alec_t

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Have you checked with John Deere that the Cat valve/hydraulics system is compatible with their GPS steering kit?
 

Jamie Janke

New Member
Thanks for the Replies.
Alec_t: Currently the Cat steering system is electric over hydraulic. Steering arm gives the Cat ECM an input then the ECM give a 24V PWM output Signal to the Steering valve. I am trying to tap into this output circuit ( I will obviously need a diodes) using a feed from the John Deere GPS ( Auto steer System).

Dr Pepper: I am a former John deere Tech and currently run my own John deere machinery. I still have close relations within John deere but as none of their Steering systems are designed for a Dozer they arent interested in this small Market. Sources within CAT have assisted me with information to control this Pump solenoid but basically are not interested and or wont support a GPS/ Auto steer system on their dozers. To answer your first question I dont have the information at the moment what current would be required for that solenoid but it could be a 20W solenoid coil ( I have seen 20W coils used on similar applications).
 

dr pepper

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20W is just under an amp for 24v and just under 2 for 12v.
Ok so what are you wanting to drive this solenoid from, an existing output from a control module?, why is it not possible to connect this to the solenoid as is, is current an issue?
Increasing the current capacity isnt a problem, however if the ecm o/p is not pwm or uses something a completely different control technique then this makes things more complicated and will probably require something like a microcontroller.
 

alec_t

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Steering arm gives the Cat ECM an input then the ECM give a 24V PWM output Signal to the Steering valve.
Does the CAT ECM receive any positional or other feedback from the valve?
Does the JD GPS expect any such feedback?
I am trying to tap into this output circuit
I'm concerned that you'll end up with a slave having two masters. Should the JD GPS over-ride the steering arm, or vice versa? Or would a change-over switch be used so that at any time there will be only one master?
 

Jamie Janke

New Member
I dont believe the Cat ECM would receive any feed back nor the JD system any feed back

There is a possibility it could result in a 2 master 1 slave situatuion but i could use a change over relay and or diodes to help over come this.
 

alec_t

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Most Helpful Member
So, as per post #2, what are the signal/supply specs for the GPS system? Is JD putting out 24V PWM? It seems likely, but while the JD web-site extols the virtues of their GPS auto-steer stuff, unhelpfully it doesn't disclose the spec :rolleyes:.
I have no practical experience of proportional valves, but understand they are normally pulsed at a few hundred Hz (hence PWM) so as to overcome stiction.
 
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Jamie Janke

New Member
No so what i currently can use from JD is a steering wheel kit ( as this is the only auto steer system that does not require a wheel angle sensor) and i plan to make up a stub shaft that basically could turn on a Lh and Rh Micro switches. This could either switch as constant 12V or 24V but as the PWM valve is designed for a PWM input i would like to know how i could turn a constand voltage to a constant Hz PWM signal
 

dr pepper

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There are various kinds of proportional and servo valves alec, the latter sometimes use pwm, but the former, all the ones I've come across do use pwm.
For a cylinder over 3" or so a dual stage valve would be required, this is usually accomplished via a smaller proportional valve, or i to p as they are referred to as, followed by a pilot controlled pressure regulating valve, the pilot pressure comming from the i to p, this then followed by a directional control valve to obviously control direction, parker do these all in one, and they cost £££'s.
 

alec_t

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Most Helpful Member
what i currently can use from JD is a steering wheel kit
Do you have a link to its spec?
i would like to know how i could turn a constant voltage to a constant Hz PWM signal
I'm not clear what you mean. A constant voltage provides no steering information and no modulation of pulse width. Converting a constant voltage to a pulse train is simple, but what will be changing the pulse width?
What is the 12V for?
@Dr.p Thanks for the info.
 
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Jamie Janke

New Member
Here is a link to the mentioned steering kit. What i have made though is a basic shaft that acts like a steering column and when wheel turns left it activates a micro switch which would in turn power the LH PWM valve. same when turned right.

I dont intend being able to change the Pulse width with this design but have a set pulse width that it could stay at that permanently (obviously would have to try set PW at an appropriate setting)

What is the 12v for? The CAT Dozer is 24V but the JD GPS is 12V so i need to make a 12V power supply for the JD GPS. So i Can either switch 12v or 24V with the above micro switches.

What do you recommend to make a Pulse Train
 

alec_t

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The link is missing :(
In post #5 you refer to a 'steering arm', but in post #13 you have a 'wheel'. Are these synonymous?
Reference to 'the LH PWM valve' suggests there are left-hand and right-hand valves . I thought there was only one valve?:confused:
I dont intend being able to change the Pulse width with this design but have a set pulse width that it could stay at that permanently
But with a fixed pulse width the valve will take up a fixed position, so how will you get steering control?
Having 12V available will be useful for any control circuitry needed, e.g. for driving a power transistor to generate a 24V pulses.
 
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alec_t

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Ah, now I understand better (I think :) ) what you aim to do.
But ...... I don't think using a fixed-width pulse train will be satisfactory. Both the present CAT system and the JD one provide smooth proportional control of the steering, whereas the use of microswitches to send fixed-width pulses to either the LH or RH solenoid will result in a bang-bang type of control, i.e. you will be rapidly slamming the CAT hard left or hard right. Surely that can't be good for reliability?

Edit:
What angular range does the JD kit turn the wheel through? More than 360°? ±180°? ..... ?
I ask because I reckon you are going to need a wheel angle sensor (they're available for various angular ranges) instead of switches as input to a 24V PWM generator circuit.
 
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Jamie Janke

New Member
Yes steering wheel can turn 360' +- ideally a sensor would be ok.
I understand that a bang bang situation isn't ideal but that's the only option I have at the moment. But hopefully you can help make a better system.
 

alec_t

Well-Known Member
Most Helpful Member
Ok.
Is the wheel constrained to 360° or does it have unlimited movement (this would dictate the type of wheel angle sensor I envision you needing)?
I'm working on a proportional (variable duty-cycle PWM) design but could also give you an interim fixed duty-cycle (bang-bang) design.
Could do with some more info re the 24V solenoids though.
Are they PWM-switched high-side or low-side?
What is their coil resistance, inductance and rated Wattage?
Are they fed 24VDC plus the PWM signal, or just the PWM signal?
Do they also receive an anti-stiction 'dither' signal superimposed on the PWM?
What is the maximum duty cycle they currently receive?
What are the left and right solenoid duty-cycles when the dozer is aimed straight ahead?
What is the PWM frequency?
 
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