Dr. Pete Nelson Sierra Scientific Solutions, LLC Introduction to Control System Theory For Engineers This talk assumes: • No prior background in control systems • Working knowledge of Fourier Transforms and frequency-domain analysis • Some familiarity with complex numbers (will review) • The willingness to ask questions!
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Introduction to Control System Theory For Engineers
Introduction to Control System Theory For Engineers. This talk assumes: No prior background in control systems Working knowledge of Fourier Transforms and frequency-domain analysis Some familiarity with complex numbers (will review) The willingness to ask questions!. - PowerPoint PPT Presentation
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Dr. Pete NelsonSierra Scientific Solutions, LLC
Introduction to Control System
Theory
For Engineers
This talk assumes:• No prior background in control systems• Working knowledge of Fourier Transforms and
frequency-domain analysis• Some familiarity with complex numbers (will
review)• The willingness to ask questions!
Dr. Pete NelsonSierra Scientific Solutions, LLC
Section 1:
Introduction to Control System Theory:
Dr. Pete NelsonSierra Scientific Solutions, LLCTerminology: dB & the Complex Plane
R
Euler’s Equation:𝑖𝑅𝑠𝑖𝑛(𝜃)
Re
Im
𝑅𝑐𝑜𝑠(𝜃)
i
R
𝑿=𝟏𝟎𝑳𝒐𝒈𝟏𝟎 (𝑿 )
𝐿𝑜𝑔 ( 𝑋 ∗𝑌 )=𝐿𝑜𝑔 ( 𝑋 )+𝐿𝑜𝑔(𝑌 )
𝐿𝑜𝑔 ( 𝑋𝑁 )=𝑁∗𝐿𝑜𝑔(𝑋 )
𝐵𝑒𝑙𝑙=𝐿𝑜𝑔10( 𝑃𝑃0)
𝑑𝑒𝑐𝑖𝐵𝑒𝑙𝑙(𝑑𝐵)=10×𝐿𝑜𝑔10( 𝑃𝑃0)
𝑑𝐵=10× 𝐿𝑜𝑔10( 𝐴2
𝐴0❑2 )=10×𝐿𝑜𝑔10( 𝐴𝐴0
)2
=20×𝐿𝑜𝑔10( 𝐴𝐴0)
Some handy Amplitude ratios expressed in dB:0dB = 1x (often used instead of ‘unity gain’ in controls)
This already means we have made assumptions:• The magnitude of the transfer function (B/A) is constant independent of the
magnitude of A. Linearity and superposition• A single frequency input () produces only a single frequency output without
any harmonic distortion, etc. Linearity again….• The ratio of B()/A() and the phase () are constant in time. No saturation! • The input is unaffected by the output and the output is unaffected by load. ”The signal diode” assumption & ZERO output impedance!
..where B/A is the “Gain” at
Dr. Pete NelsonSierra Scientific Solutions, LLC
Basic Configuration of a Feedback System:
G(s)
H(s)
OutIn
Transfer function is:
)()(1
)(
)(
)(
sHsG
sG
sIn
sOut
-Becomes large when:
unitysHsG |)()(| 180))()(( sHsG
)()(1 sHsG-The “Characteristic Equation” is:
)()( sHsG-The “Loop Transfer Function” or “Loop Gain” is:
“G(s)” is the “Plant” transfer function“H(s)” is the “Compensation”
H3-The sensor / actuator with thehighest gain wins (you can’t havetwo loops controlling the same DOFat the same time.)
+
+
Dr. Pete NelsonSierra Scientific Solutions, LLC
Before we Look Under the Hood…
2) The Laplace Transform is used only in continuous-time models;Discretely-sampled (digital) systems require using the Z-Transform.
1) Question: What is on the cover of the Hitchhiker’s Guide to the Galaxy?
3) You will get into trouble if you try to use CT techniques in discrete-time(DT) systems. Discrete sampling effects such as aliasing becomevery significant and compensation filters need special techniques to design.
4) Sorry, but I won’t cover DT and Z-Transforms here. Understanding CTtechniques is critical to DT and they will get you “90%” of the way there.
Don’t PanicThe Laplace Transform IS the engine, but you can drive the car without itand you will never need to actually calculate one.
Dr. Pete NelsonSierra Scientific Solutions, LLCWhy Use the Laplace Transform?
)())((
)())(()(
21
21
N
N
PSPSPS
ZSZSZSKsTF
||||||
|||||||)(|
21
21
N
N
PSPSPS
ZSZSZSKsTF
1) The real frequency response is just theLaplace transform evaluated along the positiveimaginary axis in the S-plane: L(s) =F(ω) = L(iω)
Unstable!!!
)()()()()()())(( 2121 NN PSPSPSZSZSZSsTF
Pole-Zero representation:The factorized transfer function takes the form:
Then the magnitude of the frequency response is:
And the phase angle of the frequency response is:
iAeZ
Think:
S
2) The S-plane provides a simple and absolutestability criterion: if any right-half-plane (RHP) polesexist, the system is unstable!!! X
X
X
X
0
0
0
0
0
X
X
Re
Im
S
X
0
0
.(S-P1)
S = iω
1
Re
Im
Dr. Pete NelsonSierra Scientific Solutions, LLC
Figuring Out Transfer FunctionsMag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
M
LKF
Xin
Xout
0
0
Minimal-Phase (θ=90*slope exponent):
(plus damping)
f -2
f 0
θ = -2 x 90
θ = 0 x 90
S
X
X
Re
Im
Non-Minimal-Phase (RHP zeros or poles):
Examples:
All-pass filters Time delays (DT) “Zero-order hold” (DT) FIR filters (DT)
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
Non-minimal phaseis always bad in control systems!
f 0
S
X 0Re
Im
Dr. Pete NelsonSierra Scientific Solutions, LLC
The Nyquist Stability Criterion:Recall:
)()(1
)(
)(
)(
sHsG
sG
sIn
sOut
)()(1 sHsGIf the Characteristic Equation:
Has any RHP zeros, the closed-loop transferfunction will be unstable.
S-plane1+G(s)H(s)
X0 Re
ImPolar plot of Loop Transfer Function GH:
0
Real frequency response
-0 freq.
Conjugate frequency response
Contour enclosingall RHP poles and zeros
+0 freq.
+∞ freq.
-∞ freq.
A B
D
C
0
A
B
CD
UNSTABLE!
Gain=1
-1 point!
)()()()()()())(( 2121 NN PSPSPSZSZSZSsTF
Dr. Pete NelsonSierra Scientific Solutions, LLC
A Simple Example…..
Gain=1
STABLE!
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dBf -1
f -2
-0 freq.+0 freq.
Gain=1
UNSTABLE!
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
f -1
f -3
-0 freq.
+0 freq.
Dr. Pete NelsonSierra Scientific Solutions, LLC
Phase is Enemy #1 – Time Delays
Gain=1
UNSTABLE!
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
f -1
phase w/o delay
-0 freq.
+0 freq.Time delays cause rapidly dropping phase with higherfrequency
Alternate Bode-based stability requirement:The phase must be less than ±180 degreesat unity gain.
Dr. Pete NelsonSierra Scientific Solutions, LLCSources of Non-Minimal Phase:
The Zero-Order Hold (ZOH) and Transport Delays
100
101
102
103
-60
-50
-40
-30
-20
-10
0Phase lag vs oversampling factor for ZOH
Oversampling factor (fsamp/f)
Pha
se la
g, d
egre
es
Based on 4th order Pade approximation
10-3
10-2
10-1
-40
-35
-30
-25
-20
-15
-10
-5
0Phase lag vs time delay
Time delay (1/f)
Pha
se la
g, d
egre
es
Based on 4th order Pade approximation
∆t
ADC In DAC out
t
Cts.
Sampling at 10x unity gaingives ~18 degrees of extraphase!
A time delay of 10ms gives ~36 degrees of extra phase at 10Hz!
Both effects exist in all digital control systems!
Dr. Pete NelsonSierra Scientific Solutions, LLC
Summary of Section 1: Theory
• Plants with RHP zeros or poles. These are unusual, but can occur, often deliberately! This requires the ‘full version’ of the Nyquist criteria…. (yes, I have lied to you….)
• “MIMO” systems: Multiple Input, Multiple Output. This talk only covers “SISO”.• Digital systems require use of the “Z-Transform”. We don’t cover, but its important to
learn when you are ready.
Things we didn’t cover:
• Pick your Plant (G) to represent the inputs and outputs you care about.
• System Magnitude and Phase responses are represented by complex numbers. In the S-Plane (Laplace Transform Space) by complex poles and zeros.
• Systems with Right Half Plane (RHP) poles are unstable.
• Introduced a working form of the Nyquist Stability Criterion: || < 180• → Phase is the enemy. Beware time delays and ZOH!
The “God Equation”)()(1
)(
)(
)(
sHsG
sG
sIn
sOut
• Introduced Minimal Phase Networks: = n*90, n=power of slope.
• Non-minimal phase networks are useless as compensation (exercise for reader).
Dr. Pete NelsonSierra Scientific Solutions, LLC
Section 2:
Performance
Dr. Pete NelsonSierra Scientific Solutions, LLC
Dynamic Response: Phase and Gain Margins
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
Phase Margin
Gain MarginGain=1
(STABLE)
Phase Margin
Gain Margin
• Phase Margin usually dominates the closed-loop response
• All the information required for dynamic response is in the Bode diagram
)()(1
)(
sHsG
sG
Dr. Pete NelsonSierra Scientific Solutions, LLCA Simple Phase-Margin Calculation:
0 10 20 30 40 50 60 70 80 9010
-1
100
101
102
103
Phase Margin, Deg.
Am
plifi
catio
n at
Uni
ty G
ain
Amplification of Control System Responseat the Unity Gain Frequency vs Phase Margin
)(2222 COScbcba
Phasor diagram for 1+GH:
The Law of Cosines:
1+GH
GH at |GH|=1
The vector (1)
θ
Gives the amplification:
when |GH|=1 (at unity gain)
Phase Margin
))cos(1(2
1
1
1
GH
But the real situation is a bit more complex…..
Dr. Pete NelsonSierra Scientific Solutions, LLCDynamic Response: The Nichols Chart
G
H
OutIn
The Nichols chart plots:
)()(1
)()(
sHsG
sHsG
which is also called the control
signal
Dr. Pete NelsonSierra Scientific Solutions, LLC
The Designer’s Choice – No Right Answer…
10-1
100
101
-40
-20
0
20
40Loop Transfer Function (GH) of System
Frequency, Hz
Mag
nitu
de,
dB
10-1
100
101
-200
-150
-100
-50
0
Frequency, Hz
Pha
se,
Deg
.
Phase margins of: 10, 30, 50, 70 and 90 Degrees
10-1
100
101
-40
-30
-20
-10
0
10
20System Response vs Phase Margin
Frequency, Hz
Res
pons
e, d
B
Phase margins of: 10, 30, 50, 70 and 90 Degrees
0 0.5 1 1.5 2 2.5 3-6
-5
-4
-3
-2
-1
0
1
2
3
4Impulse Response vs Phase Margin
Time, sec
Impu
lse
Res
pons
e
Phase margins of: 10, 30, 50, 70 and 90 Degrees
Dr. Pete NelsonSierra Scientific Solutions, LLC
1/f^2 : The Optimal Servo?
- saturation
+ saturation
No gain(or negative!)
Nom. gain
Reduced gain
sensor out
sensor in
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
f -3
f -<2
f -3
Sensor (or actuator) Saturation:NEW 0dB
UNSTABLE!
Solution: Keep the slope above unity gain to less than 2 powers of frequency.
This is called a “Conditionally Stable” servo
Dr. Pete NelsonSierra Scientific Solutions, LLCHigh Bandwidth & Sensor Co-Location:
F …
H
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
Gain=1
+0 freq.
180 degreesfor each resonance
4 New unity-gain points!~10x
• Eliminate resonances between sensing and actuation• Damp resonances if possible• Increase resonant frequencies (hard!)• Can use filters for isolated resonances
OK
Dr. Pete NelsonSierra Scientific Solutions, LLC
Sensors Lie #1: Representation
Filt & Amp
Consider a temperature-control servo:
Outer housing
Thermistor
Inner housing
Heater windings
Insulation
Heater ground
~60C set pt.
~30C outside
Conclusion:Thermal performance is limited by the balancing ofheat loss and heat input to ~10% (?!?!). This means servo gains higher than ~20-30dB are a waste!!
Dr. Pete NelsonSierra Scientific Solutions, LLC
Sensors Lie #2: Sensor Noise
MXout
0
F H
+
Xn
M1
X2
0
F
M2
+
Xn
H
X1
0
(Sensor Out)X1
Log(f)
f 2f 0
0 dB
Conclusions:1. Mass tracks sensor noise2. Sensor output (noise) suppressed
by the loop gain
Conclusions:1. Sensor noise is amplified by 1/f2 below the
M2 resonance!!!!!2. With ‘1/f’ noise in sensor, mass motion
grows by 1,000x each decade (down) to unity gain.
3. Sensor output (noise) suppressed by gain.
Servo wants to minimizethe signal here...
Dr. Pete NelsonSierra Scientific Solutions, LLC
Summary of Section 2: Performance
• Integrators can reduce errors to zero, and the more integrators the better this works. However ‘integrator wind-up’ is a difficult problem.
• The importance of modeling systems. Straight-line approximations only go so far…• PID controllers are horribly non-parametric and only work well for free-mass systems
(i.e.: moving stage & motion control). Otherwise they mostly suck. You can do better!• Issues with actually closing the loop on high gain servos – dynamic range and noise!
Things we didn’t cover:
• Showed Phase and Gain Margins are useful parameters to describe performance. There is ALWAYS a Phase Margin, but not always a gain margin.
• Overshoot (~Q) is ~3x at 20pm, ~2x at 30pm, and critically damped at ~60pm.
• Demonstrated there is no ‘right answer’ when it comes to choosing a phase margin.
• Proved that a transfer function slope of 1/f2 is the best performing system possible without introducing conditional stability.
• Showed mechanical resonances always limit bandwidths and that sensor co-location can mitigate that.
• Used the Nichols Plot to show that feedback systems rarely look like simple second-order systems. Will demonstrate in Section 3.
• Sensors are lying bastards. If you need to verify performance, use an independent sensor.
Dr. Pete NelsonSierra Scientific Solutions, LLC
Section 3:
Practical Servo Design
Dr. Pete NelsonSierra Scientific Solutions, LLCHow I Design a Servo:
Step 1:Model, then measure the planttransfer function (G):
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
f -2
f 0
θ = -2 x 90
θ = 0 x 90
Step 3:Measure loop transfer function(GH) to confirm.
G
Step 4:
Close the loop!
Step 2:Design a compensation (H) which pulls the phase down to -180º with enough phase lead at unity gain to give me the desired stability & impulse response.
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
f -2
f 0
H
phase lead
Dr. Pete NelsonSierra Scientific Solutions, LLCNow With Some More Detail:
M
Fin
Xout
0
F H
L
Remember from start of talk:
Requirements for our servo:• Have a spring-like restoring force• Provide damping• Always bring the sensor to null • Filter noise
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
Compensation
f -1
f 1
f -2
Null sensor Damping(phase lead)
Filter HF noise
H
Always start with physics: F=ma
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
Plant
f -2Units:X/F !
θ = -2 x 90 = -180
G
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
Loop Transfer Function
GH
f -2
f -1f -2
f -4
f -3
Dr. Pete NelsonSierra Scientific Solutions, LLCBut is it stable?
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
0dB
Loop Transfer Function
GH
f -2
f -1f -2
f -4
f -3
Gain=1
UNSTABLE?CCW encirclement?
-0 freq.
+0 freq.
Gain=1
STABLE
-0 freq.
+0 freq.
S
X0 Re
Im
0
Real frequency response
-0 freq.
Conjugate frequency response Contour enclosingall RHP poles and zeros
+0 freq.
+∞ freq.
-∞ freq.
0
We have THREE poles at the origin!
Recall:
Dr. Pete NelsonSierra Scientific Solutions, LLC
The Magic Disappearing Resonance:
Consider a mass on a spring:
MXout
Fin
0
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
f -2
f 0
θ = -2 x 90
θ = 0 x 90
G
ω0
Recall:)()(1
)(
)(
)(
sHsG
sG
sIn
sOut
Plant:
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
f -2
f 0
θ = -2 x 90
θ = 0 x 90
GH
ω0
LoopT.F.:
0dB
Closed-loopresponse:
Mag.(dB)
+180
+90
0
-90
-180
Phase
Bode Diagram:
Log(f)
Log(f)
ω0
Original resonance is GONE!
F G
Dr. Pete NelsonSierra Scientific Solutions, LLC
The “Super Spring” Servo
10-1
100
101
102
103
-400
-200
0
200Plant Transfer Function
Frequency, HzM
agni
tude
, dB
10-1
100
101
102
103
-200
-100
0
100
200
Pha
se,
Deg
Frequency, Hz
M
Xout
Sensor
0
F G
M
Mref
Xin
0
Optical Corner Cube
InjectTestSignal
Meas.Output
200Hz
Dr. Pete NelsonSierra Scientific Solutions, LLC
Summary of Section 3: Practical Design
• Did I already mention modeling? Good. I needed to at least twice!• Sensor noise can also limit bandwidth. In the Super Spring, it grows like 1/f3!!• So many other things. I hope this is just a start!
Things we didn’t cover:
• Start design with a simple, linear, minimal-phase approximation to get 1/f2.
• Work backwards from the Plant (G) and the desired LTF (GH) to get H.
• Closed-loop systems often show no sign of open-loop resonances.
• You can add 360 to the phase and get the same plot. Phase works on a circle! High-Freq. resonances can sometimes be tamed by ADDING phase!!!
• Use the alternate version of the Nyquist Stability criteria because polar plots make your head hurt too much.