Electro-Hydraulic Servo Control

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Thread Starter

Grishma

I was reading an application note on Electro-Hydraulic Control Systems... The author of the note wrote: "In general, applications with bandwidths of greater than 20Hz or control power greater than about 15kW, may be regarded as suitable for Servo Hydraulic Techniques"...

"Hydraulic system are mechanically "stiffer", resulting in higher machine frame resonant frequencies for a given power level"...

My questions are as follows:
1) What is the physical significance or meaning of 'bandwidth' in a Servo Hydraulic System?? I mean, in physical form what "reaction" of the Servo Hydraulic System will reflect bandwidth?? (I assume that a Servo Hydraulic System consists of a servo controller, a FCV, an actuator, a position feedback pot and a hyd power supply... Where is the 'bandwidth' associated in these components?)

2) What is the meaning of the word "stiffer" in the above paragraph??

Regards
Grishma
 
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Peter Nachtwey

> I was reading an application note on Electro-Hydraulic Control Systems... The author of the note wrote: "In general, applications with bandwidths of greater than 20Hz or control power greater than about 15kW, may be regarded as suitable for Servo Hydraulic Techniques"... <

It is hard to get any system up to 20 Hz. Most hydraulic systems are less than 10 Hz.

Hydraulic systems are preferred when
1. Power can be stored in an accumulator.
2. One power unit can power many actuators.
3. When the actuators must hold pressure or force.
4. When the size of the actuators must be minimized.
5. Areas where electrical components might be susceptible to radiation damage or can cause sparks in a flammable area.

Motors are preferred when:
1. The motion is continuous like a conveyor where power can't be stored or time averaged.
2. The motion is rotary.
3. Low power. It is hard to beat those small linear motors on the small applications.

> "Hydraulic system are mechanically "stiffer", resulting in higher machine frame resonant frequencies for a given power level"...
I disagree, hydraulic fluid compresses. However, I think the external mechanics has more to do with how stiff the system is.
>
> My questions are as follows:
> 1) What is the physical significance or meaning of 'bandwidth' in a Servo Hydraulic System?? I mean, in physical form what "reaction" of the Servo Hydraulic System will reflect bandwidth?? (I assume that a Servo Hydraulic System consists of a servo controller, a FCV, an actuator, a position feedback pot and a hyd power supply... Where is the 'bandwidth' associated in these components?) <

Bandwidth is measured the same way as anything else. For open loop bandwidth you send a analog sinusoid into the valve system and increase the frequency until the amplitude drops by 3 db or .707 of the max amplitude.

A term you hear more often is natural frequency. One increases the natural frequency by increasing the diameter of the cylinder and reducing the trapped volume of oil between the valve and the cylinder. This usually means mounting the valve on the cylinder for best results.

Closed loop bandwidth is measured in a similar way except one compares the amplitude of the feedback with the amplitude of the reference sinusoid.

If the moving mass is large, you had better not do this or you will attract a lot of negative attention. It is best to calculate the bandwidth.

> 2) What is the meaning of the word "stiffer" in the above paragraph?? <

Stiff as opposed to compliant? Stiff system do not give as much when an outside force is applied.

Peter Nachtwey
Delta Computer Systems, Inc
http://www.deltacompsys.com
 
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George Younkin

Bandwidth is a term related to servos. The best way to explain bandwith is to use an anology. Think of a high fidelity sound system. You want to have the audio system to acuately reproduce the sound from the recoded medium such as a CD. For example, in the 1812 overture you want to hear the cannons and the chimes. Thus we hear the very low frequency and also the high frequency. The range in frequencies we hear is refered to as the bandwidth in frequency. If you played the same music over a telephone the sound would be quite diffeent because of the limited range or bandwidth of a telephone.

Now in a servo we also want the servo output to faithfully repoduce the input command. The frequency response is a measure of how well the servo follows the input command for increasng frequency. As the frequency increases a point will be reached where the output can no longer keep up with the input and the servo output starts to lag behind. It is this point in frequency where the servo output starts to lag the input, is defined as the bandwidth of the servo. There are many reasons why the servo can't keep up with the input command. For example in a servo drive, the motor and conected inertia may be the limit in how fast the servo may respond. The bandwidth of a servo is a measure of how fast the servo can respond to changes in command. The bandwidth also tells how accurate the servo drive will be for rapid changes in velocity. This is a layman's definition of bandwidth. Of course the mathmatical definition is more complicated.

Geoge Younkin
[email protected]
 
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George Younkin

Grishma,

The layman's definition of servo stiffness can be related to a spring. Take a rotory spring and rotate it so many degrees. The spring has a stiffness constant K. Thus it takes so many "inch pounds" of torque the rotate it so many "radians or degrees".

With a servo, it acts like spring. It takes so many in pounds of torque to rotate the motor so many radians. The servo stiffness is proportional to the product of the srvo loop gains. I have documents that derive the stiffness for a number of servo configurations. If interested contact me at [email protected]

George Younkin
 
Thanks for your replies... But now I have another query... Agreed that the band of freq's for which the response is above 0.707 will be the bandwidth, but arent we dealing with DC signals (either 4-20mA current or some proportional voltages) in servo-hydraulic systems?? The PID controller will most likely have a DC output (current or voltage) so in such cases how will the freq component be physically reflected...??

Regards
Grishma
 
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William Sturm

You apply an AC signal into the system, such as a sinewave of +- 1 volt. The supply is 10 VDC, but the input signal is not DC.
 
Yes we do deal with dc signals. but a step change in DC value is made of number of ac signals with different amplitude and frequency, That is what is called fourier analysis. Now the servo system responds to each of these ac signals as per its(servo's)band width. Any change in DC value of the input is treated as bunch of ac signals.
 
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Peter Nachtwey

Yes, the signals are usually -10v to +10v to a proportional valve with on board electronics ( OBE ) 4-20ma is used sometimes in process control.

The bandwidth affects how tightly the motion controller can following a motion profile when there are fast accelerations or decelerations. You can think of every motion profile being made of up sine waves of different frequencies. If your system gain drops as frequency increases the sine waves that make up your motion profile are likewise attenuated the resulting sum of sine wave may not look like the original motion profile. This is why you need to keep high frequency components out of the motion profile.

Simple trapezoidal ramps will not do in high performance systems. To put it another way.. it is impossible to increase or decrease pressure and instantly in a hydraulic system. This means that one cannot instantly change force and acceleration.



 
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