Transcript Stepper Motors and Artificial Muscles

Stepper
Motors
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)
S
N
• Principles of
operation
Use of successive
magnets
Stepper Motor
detailed analysis
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
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
How Does A Stepper Motor Work?
The top electromagnet (1) is charged, attracting the
topmost four teeth of a sprocket.
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°
How Does A Stepper Motor Work? (cont…)
The bottom electromagnet (3) is charged; another
3.6° rotation occurs.
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.
Reluctance (Stepper) Motors
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angle control
slow
usually no feedback used
accurate positioning
without feedback, not servos
easy to control
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
Digital Control of Stepper Motors
• There are many other types
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
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)
Transmission of
motion
Electric Motors: Mounting
1. When used with rotary joint systems, motors can
produce torque by:
1.
2.
being mounted directly on the joints
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
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
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
–
3.
For example, prismatic joints require linear translation rather than rotation from the
motor
Typically used to transform rotational to translational motion:
1.
2.
3.
4.
Leadscrews,
belt-and-pulley systems,
rack-and-pinion systems,
or gears and chains.
Leadscrews
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)
Motor and Encoder
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
Encoder Feedback
Another option:
potentiometer
US Digital
Use
datasheets
Use datasheets
Incremental shaft encoders
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
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
Artificial
Muscles
PNEUMATIC
Artificial Muscles
1.
During the past forty years a
number of attempts have been
made to build artificial muscles
2.
Muscles contract when
activated,
3.
1.
since they are
attached to bones on two
sides of a joint,
2.
the longitudinal
shortening produces joint rotation
Bilateral motion requires pairs of
muscles attached on opposite sides of a
joint are required to produce
Artificial Muscles: McKibben Type
1. The McKibben muscle was the
earliest attempt at constructing
an artificial muscle
2. This device consisted of a
rubber bladder surrounded by
a sleeve made of nylon fibers in a helical weave
3. When activated by pressurized air, the sleeve
prevented it from expanding lengthwise, and thus
the device shortened like living muscles
Artificial Muscles: McKibben Type
1.
2.
3.
In the 1960s there were attempts to use McKibben muscles to
produce movements in mechanical structures strapped to
nonfunctional arms of quadriplegics
The required compresses air was carried in a tank mounted on the
person’s wheelchair
These experiments were never completely successful
Artificial Muscles: McKibben Type
1. Since the 1960s there has been
several other attempts to develop
improved McKibben type artificial
muscles:
1. (Brooks, 1977) developed an
artificial muscle for control of the
arms of the humanoid torso Cog
2. (Pratt and Williamson 1995)
developed artificial muscles for
control of leg movements in a biped
walking robot
Artificial Muscles:
McKibben Type
• However, it is fair
to say that no
artificial muscles
developed to date
can match the
properties of
animal muscles
Artificial Muscles:
Shape Memory Alloys
1. Shape memory alloys (SMAs) have unusual
mechanical properties
2. Typically, they contract when heated,
– which is the opposite to what standard metals do when
heated (expand)
3. Furthermore, they produce thermal movement
(contraction) one hundred times greater than
that produced by standard metals
Shape Memory Alloy Robot
Artificial Muscles: Shape Memory Alloys
1. Because they contract when heated, SMA provide a
source of actuation for robots
2. After contraction, the material gradually returns to its
original length when the source of activation is
removed and it is allowed to cool
3. SMAs have two major problems when used as artificial
muscles:
1. They cannot generate very large forces
2. They cool slowly and so recover their original length
slowly, thus reducing the frequency response of any
artificial muscle in which they are employed
Northeastern University’s Robot Lobster
•A robot lobster developed
at Northeastern University
used SMAs very cleverly
– The force levels required
for the lobster’s legs are
not excessive for SMAs
– Because the robot is used underwater cooling is
supplied naturally by seawater
More on the robot lobster is available at: http://www.neurotechnology.neu.edu
Artificial Muscles: Electroactive Polymers
1. Like SMAs, Electroactive
Polymers (EAPs) also change
their shape when electrically
stimulated
2. The advantages of EAPs for
robotics are that they are able
to emulate biological muscles
with a high degree of
toughness, large actuation
strain, and inherent vibration
damping
3.
Unfortunately, the force actuation and
mechanical energy density of EAPs are
relatively low
Robotic face developed by a group
led by David Hanson. More
information is available at:
www.hansonrobotics.com