Servo Valve

[Nooryh]

Hi
I'm a new employee and I have a background in computer engineering but I'm working as automation engineer
so I'm confused in understanding control valves

Servo Valve is used to control hydraulic fluid and it needs 4-20 mA to work but I'm confused about the I/O type.... is it Digital Output or Analog Output and WHY?

 

[bernie_carlton]

Because it uses a range of values, not just two, it is an analog output device.

 

[sthompson]

To expand on Bernie's explanation, a Digital Signal can only be zero(0) or one(1). Anything that varies in its value over a range is Analog. i.e. 0-10VDC or 4-20mA etc.

Stu....

 

[ndzied1]

Coming from the computer science world, perhaps the best way to understand analog signals would be to think of color values for computer displays.

Many times color is represented by the RGB values, These 3 values vary from 0 to 255 and control the intensity of Red, Green, and Blue. If you could only turn them all on (255) or all off (0) you only get black and white. That would be a digital system. But, you can write any value from 0 to 255 in the RGB values and get most colors of the spectrum. That is what analog control is like.

In the same way, the signal to the hydraulic valve varies from 4 to 20 mAmp to vary the opening amount of the valve. In general, the more the valve is open, the more oil will flow through it. There is more to it but this get's the basic idea across.

Hope this helps

 

[leitmotif]

Norm the comparison to RGB in computer monitors should be preserved for posterity.

Dan Bentler

 

[Nooryh]

4-20 mA means servo valve has Analog Input Signal; however, servo valve system is capable of operating either in closed or open loop mode so I think it has Digital Output Signal

What do you think?

 

[sthompson]

4-20 mA means servo valve has Analog Input Signal; however, servo valve system is capable of operating either in closed or open loop mode so I think it has Digital Output Signal

What do you think?

Open Loop means there is no Position Feedback.

Closed Loop means there is Position Feedback.

The only thing that may be Digital is End of Travel Limit Switches.

Stu....

 

[ndzied1]

A little more on Analog / Digital and Open Loop / Closed Loop

I thought the RGB was good for a computer person but my very basic slide comparing digital control vs. analog control is this (I think it is pretty self-explanatory):

Next is my Open Loop flow control slide:

When you set the flow to brush your teeth, you have a general idea of the flow required. More than a drip, less than a blast. You set the hand valve and forget about it. If someone on the same circuit flushes the toilet or starts taking a shower, your flow will go down but you usually don't care and don't do anything to adjust your water flow. You live with how the rest of the plumbing circuit affects your application and for this use, that is perfectly acceptable. You assume your command (the valve position) puts you where you need to be and live with the fluctuations caused by other parts of the system. Note that in this case the water valve is proportional (more valve turn = more flow) and its output is proportional but it is not under closed loop control because there is no active part of the system trying to keep the flow at a specific value.

Then there is the closed loop slide:

For this slide, the person is the closed loop controller. They compare the command value (from the speed limit sign) with the feedback value (from the speedometer) and adjust the control output (the pedal position) to drive the system the right way to make the feedback match the command. When you go down a hill, gravity assists you so you may have to reduce the pedal position to keep in the speed limit and avoid a violation). The opposite occurs going up a hill or against a strong wind. The idea is that there is something active in the system (in this case your brain) continuously comparing the feedback to the command and continuously varying the control output (pedal position) to keep them the same.

 

[Tom Jenkins]

Norm, those are great slides. I may steal the concept, but not the exact graphics, for some of my classes. Would I have your permission?

 

[ndzied1]

Thanks Tom (and Dan earlier).

Please see my PM

 

[sthompson]

Very nicely done Norm. Bravo!

Stu....

 

[Nooryh]

Thank you so much for the clarification

 

[Manglemender]

Norm you're a star!

I like to use the car analogy to teach people about torque control as the pedal controls torque and not speed. In drive terms, the car driver becomes the speed loop issuing a command to the torque controller.

Nick