Advanced Hydraulic Motion Control

Design Practices

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Good Design Practices Overview

Affect of system components on control

Compensating for design

deficiencies

Know where design can be modified to meet design

requirements

High-performance hydraulic motion

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Feedbac

k

Accumulato

r

Cylinder

Good Valve Hard Piping

Reservoi

r

Pump

Controller

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Good Design is Simple!

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Good Design Provides…

• Excellent Control!

– Accuracy < 0.001 inch, even while moving

– Complex motion profiles

– Synchronization (no flow dividers)

• Smooth startup

• Less maintenance

• Less tech support

• Higher Initial Cost (not always)

• May not be possible

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• Cost

– Valve: good valves are much more expensive

– Feedback: high resolution is more expensive

– Larger cylinder larger valve larger pump

• Availability

– Linear valves may have longer lead times (Rexroth)

• Special requirements

– Space constraints: valve location

– Environment: valve location

Real-life design pressures

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Real-life design pressures

Understanding

control features Meet Design

Requirement

s

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Closed-Loop Control Review

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Control Review

Target Position

Actual Position

Control Signal

(V)

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• Proportional Gain

• Integral Gain

• Differential Gain

• Velocity Feed

Forward

• Acceleration Feed

Forward

Each PID gain contributes to the control output

signal

Control Review

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Proportional Gain – position error

Target

Position

Actual

Position

Control Review

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Integral Gain – sum of position error

Target

Position

Actual

Position

Control Review

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Integral Gain can cause overshoot

Winds up during

error

If it winds up, it must wind down! Unwinding causes

overshoot

Control Review

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Differential Gain – velocity error

Target

Velocity

Actual

Velocity

Control Review

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Quantization Noise Affects D Gain

Actual Velocity is

“noisy”

This results in a “noisy”

Control Output

Because velocity is calculated from position, not measured directly

Control Review

TODO Output Filter

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Velocity Feed Forward – prior knowledge of

velocity

Target

Velocity

Because the actual velocity is

proportional to the voltage to the valve

Example:

1 V ≈ 3 in./sec

2 V ≈ 6 in./sec

3 V ≈ 9 in./sec

Etc.

Control Review

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Acceleration Feed Forward – prior knowledge of

acceleration

Target

Acceleration

Because F = m∙A

The extra “boost” needed is

proportional to the Target Acceleration

Control Review

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Feed Forwards – eliminating error

P, I and D Gains operate only if an error exists

PID-only will not follow target exactly

Feed Forwards eliminate error

Control Review

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Important!

Control Review

Gains and feed forwards are linear equations!

Therefore,

The hydraulic system should

be as linear as possible!

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How Components Affect Control

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Accumulator

• Size

Valve

• Type

• Size

• Close to cylinder

Feedback

• High resolution

Cylinder

• Size

Critical

Components

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Accumulator

Provides constant

supply pressure to valve

Locate close to valve.

Sizing is easy.

Helps provide

linear

system!

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Accumulator

Undersized

accumulator

Control Output increases

during constant velocity.

Not linear!

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Valve

Best Practice:

• Linear

• Zero-lapped

• High response

• Locate on cylinder

with hard pipe to rod

end

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Valve

Counterbalance

Valves

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Valve Linear vs. Non-linear

Spool

Linear Single knee Curvilinear

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Valve Linear Spool

Provides

linear system

Velocity Feed

Forward functions

properly Precise

Control!

Auto-tuning works well

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Valve Single-Knee Spool

Valve-

linearization

algorithm

Appears

linear to PID

and feed

forwards

Good

Control

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Valve Single-Knee Spool

Auto-tuning non-linear valves is difficult

because system exhibits two different gains.

Unfortunately , single-knee spools may

be more available than linear spools.

Historically, lower gain section used for

low flow, higher gain used for high flow.

Modern high resolution of motion

controllers and valves make this

reason nearly obsolete.

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Valve Curvilinear Spool

Auto-tuning is difficult because system

exhibits multiple gains.

Less expensive.

Typically used in less expensive valves

Valve Linearization is possible, but difficult.

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Valve Effects of Non-linear

Spool (especially curvilinear)

Imperfect Valve Linearization

Poor tracking during move

Integrator Windup

Overshoot

One Solution:

Turn of integrator during move. Won’t track

well, but will not overshoot and will hold final

position. 31

Valve Overlapped

Spools (deadband)

No motion until Control Output reaches deadband value.

Spool travel delay causes more problems.

Doesn’t accurately

reach position

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Valve Deadband Compensation

Always add positive or negative

Deadband value to Control Output

Deadband value

“Hunts” when holding position

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Valve Ratioing Deadband

to Reduce Hunting

Ratio deadband when

close to setpoint

Much less hunting

Deadband Tolerance value

does decrease position

accuracy

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Valve Overlapped Spools

Tuning is difficult

Typically less expensive

Not for rapid deceleration

Auto-tuning is nearly impossible

Not for rapid direction changes

Nearly leak-free at 0 volts

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Valve Valve Response 3 dB point

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Valve Valve Response

Must be significantly greater than actuator response

Pay attention to ±5% and ±90% specs

High response gives ability to overcome other deficiencies

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Position Sensors

Magnetostrictive rules!

• Rugged

• Non-contact

• Mounts in cylinder

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Sensors Use High Resolution

and fast response

Magnetostrictive State of the Art:

1µm (0.00004 in.)

• Balluff and MTS

• SSI output

High resolution provides accuracy

AND

controllability for difficult systems

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Sensors Magnetostrictive Outputs

SSI – digital, noise immune, accurate

Start/Stop or PWM – timing dependent,

Resolutions to 0.0005, better with recircs

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Sensors Magnetostrictive Outputs

Analog is noisy!

Beware “infinite” resolution

Is acceptable for short travel.

Filtering is possible, but introduces

phase delay.

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Sensors Other position sensor options

Externally mounted magnetostrictive

String Potentiometer

• String can oscillate

• Filtering is difficult because oscillation is similar to motion frequency.

• Use only if absolutely necessary

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Cylinders

Flow makes it

go

F = m ∙ A • Provide Force

• Provide Stiffness

Size Bore Diameter to…

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Cylinders Cylinder Stiffness

Narrow bore:

Wide bore:

Mass

Mass

Sloppy control

Tight control

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Cylinders Remote Valve is similar to

Narrow Cylinder

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Cylinders

Large Bore Small Bore

Cost Savings

Energy Savings

Force

Stiffness

Sizing is Difficult

Cylinder size affects:

• System Response

• Valve size

• HPU size

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Cylinders Sizing Calculation Methods

• Good for calculating steady state speed.

VCCM Equation

• Enhanced for motion dynamics.

• Produces acceleration times.

Enhanced VCCM

2*2*0*412

*2*0*412

1)1(*2*0*22*0*41*1

)1(*)12*0*41(*0)(

2

2

cecccccc

eccccctv

tccc

tccc

Thanks to George Keller, Jack Johnson, Peter Nachtwey 47

Controlling Sloppy Systems Low Natural Frequency

Mass

Spring absorbs

energy

during acceleration,

then releases it

Forc

e

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Low Natural

Frequency Jerk Feed Forward

Provides extra boost during beginning of

acceleration, pulls back during end of deceleration.

Velocity Feed Forward

Acceleration Feed Forward

Jerk Feed Forward

Mass

Target Accel

Jerk Feed Forward

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Double Differential Gain Low Natural Frequencies require extra control f the

acceleration. Double Differential Gain.

Low Natural

Frequency

Mass

Target

Accel

Actual

Accel

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Double Differential Gain

Quantization Noise

Low Natural

Frequency

Mass

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Observer Calculates actual acceleration based on the

mathematical system model.

This Actual Acceleration is very clean, allowing the

double differential gain to be used.

Low Natural

Frequency

Mass

Output Filter is also

possible.

It filters the result of the

Double Differential

Gain, does not filter

actual acceelration.

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Prerequisites to using

Double-Differential Gain

Low Natural

Frequency

Mass

• High resolution feedback

• High response, linear valve

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Cylinders Real-Life Cylinder Example Sawmill

• A number of years ago, sawmill OEM needed

assistance tuning during new sawmill startup

• Delta suggested increasing cylinder bore to

achieve proper stiffness

• OEM replaced cylinders

Cost: Several $100,000

• Now, with advanced tuning, it is possible to

control smaller cylinders.

• Higher-order tuning is complex, so should be

approached with caution.

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Cylinders Real-Life Cylinder Example Sheet metal brake

• Customer designed brake with one cylinder on

either end, one valve in middle. Approx. 6 ft hose

to each.

• Customer requested assistance tuning

• After 6 hours of tuning, met specs. This required

Double-differential gain.

• Pros: customer achieved competitive machine

price

• Cons: Difficult tuning, maintenance tuning

requires great expertise.

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Cylinders Real-Life Cylinder Example Radioactive cell

• Hydraulic cylinder in a radioactive cell

• Due to radiation, valve needed to be outside of

cell, resulting in long length of hose.

• Higher-order tuning was successful.

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Cylinders Real-Life Cylinder Example Large Pool

• In House of Dancing Waters in Macau, hydraulic

cylinders were in large pool.

• Valves needed to be outside of pool, resulting in

long length of hose.

• Higher-order tuning was successful.

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Summary

Proper hydraulic design saves time

and money

Real-life demands may require compromise

Modern motion controller features

compensate for design deficiencies

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Summary

1 Problem

Many Problems

• Undersized Cylinder or remote valve

• Poor Valve

• Noisy Feedback

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