Download Microcontroller Systems ELET 3232 Topic 14: Motion Control

Microcontroller Systems
ELET 3232
Topic 14: Motion Control
Objectives



To gain an understanding of the operation of
a stepper motor
To develop a means to control a stepper
motor
To gain an understanding of servo motors


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1/5/2011
Speed control
Position control
2
2
Stepper Motors [1]

Stepper motors are used in many devices we use
daily

They are used in:


printers, disk drives, toys, windshield wipers, vibrating pagers,
robotic arms, and video cameras.
We’ll focus on the permanent magnet stepper

1/5/2011
It is simpler and more popular than the variable reluctance type
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
3
Stepper Motors [1]
Teeth with wire wound around them.
The wire wound around the teeth is referred to as
the winding, coil, or phase
The rotor is a permanent magnet that is free to
rotate about the center of the motor
Stator (stationary part of the motor)
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
4
Stepper Motors [1]
Because the current is flowing in this direction
through the winding, an electromagnet is produced
in these teeth, with the North end pointing up.
This magnetic field causes the rotor to rotate to this
point, aligning itself with the magnetic field.
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
5
Stepper Motors [1]
If we now apply current to winding 2, the rotor will
turn to this new position
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
6
Stepper Motors [1]
And now, if current is applied to winding 1 again,
but in the opposite direction as before, the rotor will
turn to this new position
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
7
Stepper Motors [1]
And now we have made one complete revolution by
applying current (in the opposite direction as
before) through winding 2
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
8
Stepper Motors [1]
We have completed one cycle of the electrical
stimulation to the windings and at the same time,
have made one revolution (cycle) of the rotor.
fe = fm
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
9
Stepper Motors [1]
We have completed one cycle of the electrical
stimulation to the windings and at the same time,
have made one revolution (cycle) of the rotor.
fe = fm
In general:
fe = fm
p
2
where p is the number of equally-spaced magnetic
poles on the rotor
and:
1/5/2011
Step =
180°
p
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
10
Stepper Motors [1]
We have completed one cycle of the electrical
stimulation to the windings and at the same time,
have made one revolution (cycle) of the rotor.
fe = fm
In general:
fe = fm
p
2
where p is the number of equally-spaced magnetic
poles on the rotor
and:
“It is common to find two-phase steppers
with anywhere between 12 and 200 poles,
which results in a stepping resolution of
anywhere between 15º and 0.9º.” [1]
1/5/2011
Step =
180°
p
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
11
Stepper Motors [1]
This is a two-phase, six-pole motor:
It has two windings (two-phase) for the four teeth
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
12
Stepper Motors [1]
This is a two-phase, six-pole motor:
It has two windings (two-phase) for the four teeth
and
six equally spaced permanent magnet poles
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
13
Stepper Motors [1]
Voltage is applied to stator windings #1 with a
polarity such that current will flow in a direction
that causes a North magnetic field at the top and
South at the bottom.
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
14
Stepper Motors [1]
Voltage is applied to stator windings #1 with a
polarity such that current will flow in a direction
that causes a North magnetic field at the top and
South at the bottom.
If we remove that voltage and apply a voltage to
windings 2 such that a North polarity forms on the
left (South on the right), the rotor will rotate
clockwise one position.
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
15
Stepper Motors [1]
Voltage is applied to stator windings #1 with a
polarity such that current will flow in a direction
that causes a North magnetic field at the top and
South at the bottom.
If we remove that voltage and apply a voltage to
windings 2 such that a North polarity forms on the
left (South on the right), the rotor will rotate
clockwise one position.
And then reapply voltage to winding one, but with
the opposite polarity we get another clockwise
rotation of one position.
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
16
Stepper Motors [1]
Voltage is applied to stator windings #1 with a
polarity such that current will flow in a direction
that causes a North magnetic field at the top and
South at the bottom.
If we remove that voltage and apply a voltage to
windings 2 such that a North polarity forms on the
left (South on the right), the rotor will rotate
clockwise one position.
And then reapply voltage to winding one, but with
the opposite polarity we get another clockwise
rotation of one position.
And finally, reapply voltage to winding 2 with the
opposite polarity as before, we get one more step.
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
17
Stepper Motors [1]

Obviously, some type of control circuit needs to be
developed to control:



The sequence of voltages applied to the windings
The polarity of the voltages applied to the windings
An AVR is perfect for the “heart” of the control circuit


1/5/2011
But, an AVR cannot provide enough current to the windings to
turn the stepper motor
An H bridge will need to be connected to each winding to
provide the current
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
18
H-Bridge [2]
This image shows two H-Bridges, one to control
each of the two phases of the stepper motor.
Note: +V can be whatever is required for the stepper
motor and transistors.
1/5/2011
[2] Freescale, 2007, Stepper Motor, downloaded 3/16/08 from:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02nQXGrrlPZh3C
19
H-Bridge [2]
This image shows two H-Bridges, one to control
each of the two phases of the stepper motor.
If a small voltage is applied to the bases of Q1 and
Q4, then current will flow DOWN through phase 1.
The AVR voltages are only used to turn on the
transistors. The output port pins should be tied
through a resistor to the base of the transistors.
1/5/2011
[2] Freescale, 2007, Stepper Motor, downloaded 3/16/08 from:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02nQXGrrlPZh3C
20
H-Bridge [2]
This image shows two H-Bridges, one to control
each of the two phases of the stepper motor.
If a small voltage is applied to the bases of Q2 and
Q3, then current will flow UP through phase 1.
1/5/2011
[2] Freescale, 2007, Stepper Motor, downloaded 3/16/08 from:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02nQXGrrlPZh3C
21
H-Bridge [2]
This image shows two H-Bridges, one to control
each of the two phases of the stepper motor.
If a small voltage is applied to the bases of Q5 and
Q8, then current will flow DOWN through phase 2.
1/5/2011
[2] Freescale, 2007, Stepper Motor, downloaded 3/16/08 from:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02nQXGrrlPZh3C
22
H-Bridge [2]
This image shows two H-Bridges, one to control
each of the two phases of the stepper motor.
If a small voltage is applied to the bases of Q6 and
Q7, then current will flow UP through phase 2.
1/5/2011
[2] Freescale, 2007, Stepper Motor, downloaded 3/16/08 from:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02nQXGrrlPZh3C
23
H-Bridge [2]
In the lab we have used:
1. 2N3904 transistors
2. A 10 kΩ resistor between the base
of the transistor and the output of
the port.
1/5/2011
[2] Freescale, 2007, Stepper Motor, downloaded 3/16/08 from:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02nQXGrrlPZh3C
24
H-Bridge [2]
In the lab we have used:
1. 2N3904 transistors
2. A 10 kΩ resistor between the base
of the transistor and the output of
the port.
1/5/2011
[2] Freescale, 2007, Stepper Motor, downloaded 3/16/08 from:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02nQXGrrlPZh3C
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Controller Outputs
Step 1:
Transistors: 87654321
00001001
So, the sequence we need to repeat is:
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Controller Outputs
Step 2:
Transistors: 87654321
00001001
01100000
So, the sequence we need to repeat is:
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Controller Outputs
Step 3:
Transistors: 87654321
00001001
01100000
00000110
So, the sequence we need to repeat is:
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Controller Outputs
Step 4:
Transistors: 87654321
00001001
01100000
00000110
10010000
So, the sequence we need to repeat is:
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Controller Outputs
Step 5:
Transistors: 87654321
00001001
01100000
00000110
10010000
Etc.
So, the sequence we need to repeat is:
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Controller Outputs
Step 5:
Transistors: 87654321
00001001
01100000
00000110
10010000
Etc.
It appears that each pin in the output port will need a square wave
output. All need the same frequency, and all will have a 25% duty cycle,
but they will all be out of phase.
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Controller Outputs
Port X b0
Port X b1
Port X b2
Port X b3
Port X b4
Port X b5
Port X b6
Port X b7
You have to leave the pulse high long enough for the stepper to
move a step, but not too long if you want smooth movement.
1/5/2011
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H-Bridge [3]
Of course, you can always use an IC H-Bridge as shown above, but
you will still need the square wave outputs.
[3] Acroname Robotics, 2007, downloaded 3/16/08 from: http://www.acroname.com/robotics/parts/R27-18200.html
1/5/2011
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Unipolar Stepper Motors [1]
This is a wiring diagram of a unipolar stepper motor. As you can
see the center tap of the winding also has a lead (C).
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
34
Unipolar Stepper Motors [1]
This is a wiring diagram of a unipolar stepper motor. As you can
see the center tap of the winding also has a lead (C).
The controller circuit (and sequence of high/low outputs from the
controller) is a little different.
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
35
Unipolar Stepper Motors [1]
Black (+12 V)
Red (Coil 1)
Brown (Coil 3)
Green (Coil 2)
White (Coil 4)
When connecting a stepper motor to a circuit, you should look up
the specifications for that motor. The color codes above are typical
but not necessarily correct for your stepper motor.
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
36
Unipolar Stepper Motors [1]
Transistor Sequence: T1 T2 T3 T4
1 0 1
1 0 0
0 1 0
0 1 1
Etc.
0
1
1
0
Above is the sequence of outputs from the AVR port to get this type
of motor moving.
1/5/2011
[1] Simon, D., 2003, Get Your Motor Running, Embedded Systems Design, April 10, 2003
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Servo Motors [4]
Servo motors may also be used in applications
where exact positioning or speed is required.
1/5/2011
[4] Baldor Electric Company, Servo Control Facts
38
Servo Motors [4]
Servo motors may also be used in applications
where exact positioning or speed is required.
Servo motors are actually an assembly of
parts that include a motor, a control circuit,
and a feedback device.
1/5/2011
[4] Baldor Electric Company, Servo Control Facts
39
Servo Motors [4]
Servo motors may also be used in applications
where exact positioning or speed is required.
Servo motors are actually an assembly of
parts that include a motor, a control circuit,
and a feedback device.
For speed control a tachometer would be used
as the feedback device.
If position control is needed, a position encoder would be used. There are
several types of encoders including, absolute, directional, and incremental.
1/5/2011
[4] Baldor Electric Company, Servo Control Facts
40
Servo Motors [4]
This is an example of an incremental encoder. As the motor shaft rotates, the
disk also rotates. As the light shines through the disk and lines up with the
grid assembly, light will strike the photo sensor. This produces a pulse, which
can be counted
1/5/2011
[4] Baldor Electric Company, Servo Control Facts
41
Servo Motors [4]
For our purposes, we need to know
how to make the motor move, in this
case, to a specific position.
Our controller would only need to
provide a pulse of the required
duration to achieve the desired
position.
The values to the left are a good rule
of thumb. To maintain this position, the pulse would have to be repeated at
specified intervals. This information would be provided by the manufacturer.
1/5/2011
[4] Baldor Electric Company, Servo Control Facts
42
Servo Motors [4]
This is an example of pulse width modulation (PWM). In this example, the
motor would rotate at a rate defined by the voltage applied to it. The pulses of
different widths would provide different average voltage values, thus
producing different speeds.
1/5/2011
[4] Baldor Electric Company, Servo Control Facts
43
Servo Motors [4]
Pulse frequency modulation (PFM) works in much the same way. Again, this
motor would rotate at a rate defined by the voltage applied to it. The different
frequency of pulses would also provide different average voltage values, thus
producing different speeds.
1/5/2011
[4] Baldor Electric Company, Servo Control Facts
44
Summary



We discussed the operation of a stepper
motor
We discussed a means to control a stepper
motor
We also discussed the operation of servo
motors and:


1/5/2011
How to control speed
How to control position
45