Let us talk hydraulic motion control

[bornwild]

Hey all,

This my first year into plcs and programming. I am starting to learn how to control hydraulic cylinders. So basically a proportional valve does it. All I have to send analog voltage to valve to control the speed. Do i also need to control pump? Why do i need a hydraulic card? Why cant i use the plc. Just basic i know. If some one explain in laymans term it would be great.

 

[widelto]

Wait for Peter Natchwey, he´s the one to answer all your questions.
Meanwhile go to http://www.hydraulicspneumatics.com/200/eBooks/, they have good theory you can download.

 

[Peter Nachtwey]

This should keep you busy for a while

So basically a proportional valve does it.

The valve controls the flow but the PLC or motion controller control the valve

All I have to send analog voltage to valve to control the speed.

Yes.

Do i also need to control pump?

Not normally. Usually people use a pressure compensated pump that tries to maintain a pressure. Since the pumps don't react quickly a hydraulic accumulator should be used to keep the pressure more constant. This saves wear and tear on the pump and the valve sees a more constant pressure to use for control.

Think of an electric circuit board. There are capacitors ( electronic accumulators ) all over to keep the voltage as constant as possible.

Why do i need a hydraulic card? Why cant i use the plc.

Good question. Often a PLC can do the job if the positioning is not that critical. A good hydraulic motion controller will have scan times in the 0.5 to 1 ms range AND be deterministic. The I/O on a motion card will be MUCH faster and generally provide higher resolution digital to analog, and more importantly, analog to digital converters. A hydraulic motion card already has a lot of knowledge built in. You would have to rediscover this knowledge if you use a PLC. Then you would have to program the PLC to implement these features and PLC programming IDEs are not the best environment for doing motion control.

Here is an article about why a motion controller is used over a PLC.
http://www.controldesign.com/articles/2007/186.html

I can post many links to applications, technical articles and videos.

 

[bornwild]

Thanks Peter. But is the hydraulic card used to control the proportional valve or the swash plate on the pump? Could you you post some videos and links if you have them

 

[Doug_Adam]

Of late I have been using mobile controllers, such as the IFM 360 range, for controlling hydraulics. Effectively these are a combined PLC and hydraulics card all in one unit. Others also have these types of units such as Danfoss, Epic and some others.
Where I started using these was because a standard PLC will usually give you an output of 4-20mA or 0-10V for an analog output, but a proportional (solenoid) valve will need, depending on the type, around about 300 to 700mA to open the valve 0 to 100%. Generally this will require a PWM signal. You would not normally have any way of controlling the pump output directly.
Accumulators can smooth things out, but they are an energy store and you need to be careful as the the methods you use for emergency stop shutdown and functional safety. This is not a big problem, but I find it is ignored too many times.

Hope this gets you on the right path.

 

[Peter Nachtwey]

I will never pass up an opportunity to show what we do.

Thanks Peter. But is the hydraulic card used to control the proportional valve or the swash plate on the pump?

Normally the hydraulic card controls a servo valve because a servo valve responds MUCH faster than a hydraulic motors swash plate. A lot depends on the needs of the application. All the examples below use a servo valve to control the hydraulic actuator.

There have been some instances where our controlller controls a VFD/motor with a fixed displacement pump. In this case we were controlling the speed of the motor to control position and pressure and he hydraulic oil was just a means of getting the energy from one point to another.

Could you you post some videos and links if you have them

This video was made by one of our distributors to prove to a major aircraft manufacturer that hydraulic axes can be synchronized. This is simple.
http://deltamotion.com/peter/Videos/EM 0405-06 PrePaint.wmv

The next video is really a force control application. It is simulating forces on a tire but what is unique is that it controls the vertical and horizontal force on the tire.
http://deltamotion.com/peter/Videos/Innkeeper LLC2.flv

This is an example of controlling a 6DOF ( Stewart ) platform.
This Stewart platform is in Vancouver BC and used to film movies. Whole movie sets are placed on the platform. This platform was used in making the movie 2012.
http://deltamotion.com/peter/Videos/40x40test.flv

Curve or shaping sawing is one of our most common applications. The wood is scanned and an 'optimizer' computes the best way to cut the wood to get the most recovery. The wood is dried out later to straight out the curves. In this video a different curve or spline is downloaded for every piece of wood. One spline is downloaded while the previous spline is being executed.
http://deltamotion.com/peter/Videos/JAN-04 VSS_0001.wmv

Another wood application. This is how veneer is peeled. There are over 30 axes if hydraulic and motor control on this machine. The lathe spindles are motor controlled, the rest are hydraulic. What makes these machine a challenge is that there is little linear motion. Cubic splines are used to linearize the actuators. The blocks of wood are scanned and then the spindles are moved to present the largest right cylinders to the lathe.
http://deltamotion.com/peter/Videos/charger 4.mp4


This is a video I made in response to another forum member that was having problems getting his hydraulic system to do what he wanted. He was trying to make sine waves using an S7 PLC. It worked but only up to two Hz. Our controller scans much faster and actually has a sinewave motion generator build in since we do so many testing applications that require sinusoidal motion.
http://deltamotion.com/peter/Videos/SineWave.mp4
Another video I have made.
http://deltamotion.com/peter/Videos/PositionControl/PositionControl.mp4

More application examples can be found here.
http://deltamotion.com/applications/

More magazine articles
http://deltamotion.com/applications/magazine.php