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2 . Stepper Motors
1. When incremental rotary motion is required in a robot, it is
possible to use stepper motors
2. A stepper motor possesses the ability to move a specified
number of revolutions or fraction of a revolution in order
to achieve a fixed and consistent angular movement
3. This is achieved by increasing the numbers of poles on
both rotor and stator
4. Additionally, soft magnetic material with many teeth on
the rotor and stator cheaply multiplies the number of
poles (reluctance motor)
3 .S • Principles of
N operation
Use of successive
magnets
4 . Stepper Motor
detailed analysis
5 . Stepper Motor Design
1. This figure illustrates the design
of a stepper motor, arranged
with four magnetic poles
arranged around a central rotor
2. Note that the teeth on the rotor
have a slightly tighter spacing
to those on the stator,
– this ensures that the two sets of teeth are close to
each other but not quite aligned throughout
6 . Use of successive magnets
1. Movement is achieved when
power is applied for short
periods to successive magnets
2. Where pairs of teeth are least
offset, the electromagnetic
pulse causes alignment and a
small rotation is achieved, typically 1-2o
7 .How Does A Stepper Motor Work?
The top electromagnet (1) is charged, attracting the
topmost four teeth of a sprocket.
8 .How Does A Stepper Motor Work? (cont…)
The top electromagnet (1) is turned off, and the
right electromagnet (2) is charged, pulling the
nearest four teeth to the right. This results in a
rotation of 3.6°
9 .How Does A Stepper Motor Work? (cont…)
The bottom electromagnet (3) is charged; another
3.6° rotation occurs.
10 . How Does A Stepper Motor Work? (cont…)
The left electromagnet (4) is enabled, rotating again by
3.6°. When the top electromagnet (1) is again charged, the
teeth in the sprocket will have rotated by one tooth
position; since there are 25 teeth, it will take 100 steps to
make a full rotation.
11 . Reluctance (Stepper) Motors
• angle control
• slow
• usually no feedback used
• accurate positioning
• without feedback, not servos
• easy to control
12 . Stepper Motor Types
•A stepper motor can be incrementally driven, one step at a time, forward
or backward
•Stepper motor characteristics are:
– Number of steps per revolution (e.g. 200 steps per revolution = 1.8°
per step)
– Max. number of steps per second (“stepping rate” = max speed)
•Driving a stepper motor requires a 4 step switching sequence for full-step
mode
•Stepper motors can also be driven in 8 step switching sequence for half-
step mode (higher resolution)
•Step sequence can be very fast, then the resulting motion appears to be
very smooth
13 .Digital Control of Stepper Motors
14 .• There are many other types
15 . Advantages of Stepper Motors
1. Stepper motors have several advantages:
1. Their control is directly compatible with digital
technology
2. They can be operated open loop by counting
steps, with an accuracy of 1 step.
3. They can be used as holding devices,
• since they exhibit a high holding torque when the rotor is
stationary
16 . Stepper Motors Advantages and
Disadvantages
• Advantages
– No feedback hardware required
• Disadvantages
– No feedback (!)
Often feedback is still required,
e.g. for precision reasons, since a stepper motor can “lose” a step signal.
• Requires 2 H-Bridges plus amplifiers instead of 1
• Other
– Driving software is different but not much more complicated
– Some controllers (e.g. M68332) support stepper motors in firmware
(TPU)
17 .Transmission of
motion
18 . Electric Motors: Mounting
1. When used with rotary joint systems,
systems motors can
produce torque by:
1. being mounted directly on the joints
2. or by pulling on cables
2. The cables can be thought of as tendons that
connect the actuator (muscle) to the link being
moved
3. Since cables can apply force only when pulled, it is
necessary to use a pair of cables to obtain
bidirectional motion around a joint,
– this implies mechanical complexity
19 . Electric Motors: Mounting (cont…)
•Mounting motors directly on joints allows
for bidirectional rotation,
–but such mounting may increase the physical
size and weight of the joint,
– and this may be undesirable in some
applications
20 . Electric Motors: Linear Movement
1. The fact that electric motors produce rotational motion raises an issue with regard to their use in robots
2. For linear translation it is necessary to translate rotational to linear motion
– For example, prismatic joints require linear translation rather than rotation from the motor
3. Typically used to transform rotational to translational motion:
1. Leadscrews,
2. belt-and-pulley systems,
3. rack-and-pinion systems,
4. or gears and chains.
Leadscrews
21 . Motor and Encoder
Motor and Encoder
• Motor speed determined by:
supplied voltage
• Motor direction determined by:
polarity of supplied voltage
• Difficult to generate analog power signal
(1A ..10A) directly from microcontroller
→ external amplifier (pulse-width modulation)
23 . Motor and Encoder
• Encoder disk is turned once for each rotor revolution
• Encoder disk can be optical or magnetic
• Single detector can determine speed
• Dual detector can determine speed and direction
• Using gears on motor shaft increases encoder accuracy
24 .Encoder Feedback
Another option:
potentiometer
US Digital
27 .Incremental shaft encoders
28 . Pulse-Width
Modulation
• A/D converters are used for reading analog
sensor signals
• Why not use D/A converter for motor
control?
– Too expensive (needs power circuitry)
– Better do it by software, switching power
on/off in intervals
– This is called “Pulse-Width Modulation” or
PWM
29 . Pulse-Width Modulation
• How does this work?
– We do not change the supplied voltage
– Power is switched on/off at a certain pulse ratio matching the desired output power
• Signal has very high frequency (e.g. 20kHz)
• Motors are relatively slow to respond
– The only thing that counts is the supplied power
– ⇒ Integral (Summation)
• Pulse-Width Ratio = ton / tperiod
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