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Do I Really Need That Control Valve?

05/08/2014 10:13 AM

Hi, I am wondering if any one of you have ever try to control the motion of a hydraulic cylinder by varying the speed and or reversing the rotation of the pump? I mean having the pump or probably even better an hydraulic motor directly connected to the cylinder. Let use as a simple example a power steering cylinder, to eliminate the issue with unequal volume, with both ports directly connected to the motor ports. I know there is some safety factor to consider but a well thought computerized motor controller can take care of most of them. And with such a controller, positioning and speed control will be easily achieve. At last, such an "hydraulic system" should require less energy than the conventional one because it will displace only the fluid require to accomplish its motion.

Thank you for reading, I am looking forward read your comments.

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#1

Re: Do I really need that control valve?

05/08/2014 10:26 AM

yes you need it

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#2

Re: Do I really need that control valve?

05/08/2014 10:29 AM

When using a volumetric rotating disc multi piston pump, as a lot of steering pumps are, you can connect directly. However, you will need a max. pressure (safety) valve and/or a cylinder position sensor (at least stop valves at both ends), because the pump has no intelligence and does not know where the piston is positioned.

Giving us a more specific application detail, could save us both time and money

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#4
In reply to #2

Re: Do I really need that control valve?

05/08/2014 11:40 AM

Good points.

Also note that there will need to be a means of relieving fluid expansion due to thermal reasons unless this system will operate at very low stress; connecting the pump and cylinder ports directly in a closed loop may cause a problem here, although the safety valve might take care of it.

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#7
In reply to #2

Re: Do I really need that control valve?

05/08/2014 2:36 PM

Thank you for your reply, I had in mind a gear pump or a basic hydraulic. motor but I will look at the type of pump you mentioned. Also it is obvious that position feed back signal will have to be fit as well as a overload protection on the electrical motor for max pressure protection. Any way this is just a discussion that I like to share with knowledgeable people. Please do not spend any more money on it, it is not worthy...

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#3

Re: Do I really need that control valve?

05/08/2014 11:30 AM

This type of idea has been used on newer injection molding equipment, with servo driven pumps. The reason here being that you can not only control speed and direction without additional control valves that reduce efficiency, but also that it doesn't have to run at all in between cycles when not needed.

Note however, that this is on machines that can cost hundreds of thousands of dollars and use large amounts of electricity, and are designed to run multiple shifts where the return on investment makes it worth the additional capital outlay. Advances in technology are making this type of thing more common all the time though.

Ultimately, does the additional cost, complexity (including reliability) and energy loss of the controller, as well as minor things such as operator preferences, additional starting current requirements of the motor (which may be taken care of by the controller), response times when switching direction and efficiency loss of the motor when not running at rated speed offset the need for control valves? Only you or your potential clients can tell. So if this is for a backyard type of project, why not find out?

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#5

Re: Do I really need that control valve?

05/08/2014 1:18 PM

Power steering is driven off the engine. Since you have a problem reversing it. You would have to have some other device to reverse it. The fluid in the steering system is not under continuous pressure. It's an aid to the mechanical linkage. Only when there is resistance in turning does it divert fluid to the cylinder to turn the wheels. So most the time it robs very little energy from the engine.

All these extra devices will need energy to run. Your controller will have to micro manage all minor movements of the steering wheel in anticipating the need of pressure on the steering cylinder. Possibly shifting out rotation to the pump to the point of using more energy electrically. Bigger alternator more energy consumed to run.

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#6

Re: Do I really need that control valve?

05/08/2014 1:23 PM

The idea is not NEW already over 20 years ago such systems have been designed and used.

The most economical solution is to control not the pump as displacement but its rpm via the E-motor rpm. The use of a variable pump has a lower time constant but is more expensive since in the first solution you can use a simple gear pump for instance.

The system is not so expensive to be used only in huge machines, it is used more for them due to important energy savings.

I designed and build small "servo-integrated units" consisting of a double acting cylinder, a gear pump and an DC motor for rams 1 to 2" diameter for special units with distributed control where it was necessary to avoid the forest of hoses. The units were servo-systems with stroke and speed control. it should be mentioned that hydraulic pumps are also metering units so that there is, within some limits of course, a direct relationship between stroke and number of rotations. Errors can be generated by variable leaks at variable loads and can be compensated by driving from time to time or at every cycle start the piston to a reference position and putting the counter at zero again. If the precision is not too high it works very well.

Although efficiency is high a cooling is still in some operations necessary (especially is high speed due to friction of seals, rod guides and piston rings. The problem is also the oil reserve which has to be tight and which should be pre-compressed in order to avoid cavitation by switching directions. In such a system a shuttle valve and a pressure limiter have to be provided and of course a stroke control as in all open loop systems if it is an open loop.

It is much more to say but it is enough for a first taste.

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#8
In reply to #6

Re: Do I really need that control valve?

05/09/2014 4:29 AM

Hi! Nick name.

Firstly, thank for your interesting reply. I am glad to hear that this type of hydraulic system has been used in the past.

You are spot on, I guess my explanation was not as clear as I thought it would be, the power steering example confused some previous commenter's. Any way back to business, I gave some thought about the cooling (not educated enough to make the calculation), I will surrounded the cylinder with heat sink like found in power electronic component it will be a long thin cylinder meaning a lot of surface... I hope.

I had also though, a little, about pre-compression and thermal expansion, I am not quite 100% sure how I will manage that but there will be a solution...? Finally, the e-motor current limit should take care of the pressure limit but I still need to figure out what to do with the shuttle valve and the last part of this paragraph, the open loop thing.

Like you have said, and I am aware there is a lot more in to it but still a nice project.

Thank again, I am looking forward for the next episode.

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#9
In reply to #8

Re: Do I really need that control valve?

05/09/2014 1:34 PM

I can give you more hints when I shall know what you want to do. If it is better for you use the private channel for communication.

The shuttle valve has as scope the connection of the pressure limiter to the high pressure side whichever it is. The cooling is to be provided at the reservoir level and it is needed if the product p*v is high (system working pressure x piston velocity).

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