Exploring Engineering
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Transcript of Exploring Engineering
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Exploring Engineering
Chapter 10
Control Systems and Mechatronics
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Topics to be Covered
Block Diagrams Transfer functions
Control systems Steady State Transient models
Mechatronics
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Block diagramsMathematically mimic a small piece of a physical
process E.g., a stereo amplifier … see:
www.engr.uky.edu/.../index_files/image003.jpg
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Block diagrams Notice the blocks show the connectivity and
gross function. They do not show the actual wires, printed circuits
etc that make a stereo receiver. The blocks can be broken down much further in
increasing detail of what is in a block
To be really useful, the blocks can be given a simple mathematical description that emulates just what they do
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Transfer (Response) Function
Relates what signal goes in (e.g., volts. pressure, light source) to what goes out (e.g., amperes, mechanism movements, volts etc.) Simplest transfer function:
Transfer function:Output = Gain x input
Gain = 5
Simple gain transfer block
Input = 1(volts, amps,temperature,etc.
Output = 5(volts, amps,temperature,etc.
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Transfer (Response) Function Cruise control – a string of blocks
representing the physical functions
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Transfer (Response) Function If the transfer functions to first power
(a.k.a. “linear”) you can multiply them together
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Open and closed loop controlFor your first cruise control, just put a
brick on the accelerator and sit back …
Brick controller
Engine system
Surroundings
DesiredSpeed, So
Fuel & air
ActualSpeed, S
Power
• If surroundings are head wind vs. tailwind, hills vs. flat,etc., will actual speed equal desired speed?
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Open and closed loop controlOpen loop controls do not work wellClosed loop or feedback control is near
universal Feedback is made possible by a “comparator”
The desired controlled variable is called the “set point”
Desired Speed, So +
-
Actual Speed, S
Difference or error signalS0 -S
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Feedback control
Comparator
Surroundings
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Mathematics of Feedback Control
Collapse all the blocks; the gain Gp is the product of all the linear gains of the blocks The control is proportional is the output is a simple
multiple of the input.
ProportionalController; gain Gp
+-
SActS0
S0 - S
ActpAct SSGS 0
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Mathematics of Control Blocks
ppAct
p
pAct
GG
SSS
G
SGS
1
100 error % the&
1-
iserror statesteady that theso 1
00
0
ActpAct SSGS 0
• Ops! Steady state error
• The moral is to watch your gains!
• So is an infinite gain the solution?
Gp % error
1 50
10 9.1
100 0.99
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Transient Behavior If you have a steady state feedback loop
given by one or more transfer functions, that solution is a snapshot in time If you change the set point to another value, that
gives another snapshot of the state of the system What happens during the transient interval
between steady states? Can your model accommodate transients?
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Transient Behavior Your model needs transient behavior built in
– which so far the proportional controller does not have
At a minimum for a cruise control you needa) The inertia of the car (it will not accelerate
instantaneously)
b) The wind resistance that varies as S3 and keeps the
car from speeding to speed
c) Perhaps an allowance for hills?
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Transient BehaviorExcluding hills,a simple model would
include at least these blocks
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Transient BehaviorWithout doing the arithmetic, results of
this model are as shown: a) low gain, b) medium gain and c) high gain
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Transient Behavior Notice the sensitivity to the overall gain:
a) Too low and the transient is sluggish
b) Medium and it has some overshoot but settles down
c) Oscillatory behavior
Moral is watch your gains! High and low gains have their drawbacks!
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Mechatronics Mechatronics is a
synthesis of mechanics, electronics, control engineering and computers
http://gizmodo.com/5342497/self+balancing-enicycle-is-like-a-segway-for-the-circus
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Mechatronics Instead of first
doing a mechanical design, followed by an electronic design, followed by a control systems design they are all done coequally
Stepper motors are often mechatronic components
1
1
2
2
3
3
Windings and poles
Rotor
Figure 12: Principle of a stepper motor
1
1
2
2
3
3
Windings and poles
Rotor
1
1
2
2
3
3
Windings and poles
Rotor
Figure 12: Principle of a stepper motorPrinciple of a stepper motor
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MechatronicsCan use a variant on a stepper motor
to replace two separate systems, a throttle and cruise control on a carhere’s how its done: http://video_demos.colostate.edu/mechatr
onics/index.html Go to: stepper motor PIC
-based position and speed controller
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MechatronicsSuch technology will become common
place on cars as part of “fly-by-wire” methodology (used on most new passenger aircraft)
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Summary Control depends on some simple abstractions:
Block diagrams that simulate an element in the control linkages (whether mechanical or otherwise)
Simple mathematical representation of the block’s function
A comparator to generate an error signal Feedback to correct the instantaneous value of the
controlled variable High proportional gain to reduce steady state error and
low gain to reduce unsteady transient behavior.
Mechatronics is an integrated method of design including mechanical, electronic and control elements.