Magnetic actuators

1
Magnetic actuators

• (chapter 5, part C)

2
Energy density

• Actuators apply forces
• Forces are related to power and energy
• Power is the time rate of change in energy
• Force is the gradient (slope) of energy (I.e.,
  energy is the time integral of work and work is
  the line integral of force).
• Larger power from an actuator means it must have
  a larged energy density.

3
Electric and magnetic energy

• Electric actuators - Coulomb force
• Magnetic actuators - Lorentz force
• Magnetic actuators are more common
• Magnetic energy density is higher
• Easier to produce large magnetic fields
• Magnetic properties of materials more pronounced
• Easier to handle from a practical point of view

4
Electric and magnetic energy densities

• Electric energy density

• Magnetic energy density

5
Electric and magnetic energy densities -
comparison

• Electric energy density E105 V/m, er10?0
  8.845x10??? F/m (large values)

Magnetic energy density B 1T, m1000?0
1000x4?x10?? H/m
6
Electric and magnetic energy densities -
comparison

• Magnetic energy density is larger
• 3 orders of magnitudes or more
• Larger forces are attainable
• Smaller sized for actuators (larger forces per
  unit volume)
• Reason why we have magnetic motors and no
  electric motors (even though we call them
  electric)
• Electric forces are used in MEMs
• Small forces at small distances
• Electric forces on the atomic levels are very
  large

7
Voice coil actuators

• Voice coil actuators got their name from
  magnetically driven loudspeakers.
• In most applications, there is no use of voice
  only the similarity in operation.
• Based on the interaction between the current in a
  coil and the magnetic field of a permanent magnet
  or another coil.
• To understand this consider the basic structure
  of a loudspeaker driving mechanism shown next.

8
Construction of a loudspeaker
9
A small square loudspeaker
10
A loudspeaker coil
11
Voice coil actuators

• The magnetic field in the gap is radial.
• For a current carrying loop, the force is given
  by Lorenz force (F BIL)
• now L is the circumference of the loop and we
  assume a uniform magnetic field.
• With N turns, the force is NBIL.
• The field does not have to be uniform or
• The coil does not have to be circular

12
Voice coil actuators

• This is a simple configuration and is the one
  used in most speakers.
• The larger the current
• the larger the force
• the larger the displacement of the speakers
  cone.
• By reversing the current, the coil moves in the
  opposite direction.
• We should note a number of things

13
Voice coil actuators - notes

• The force is directly proportional to current for
  a given magnetic field. In this case (and in many
  voice coil actuators) it is linear with current.
• The larger the coil or the magnetic field, the
  larger the force.
• By allowing the coil to move, the displaced mass
  is small (compared with other actuators) and
  hence the mechanical response is quick. For this
  reason, a speaker can operate, say at 15 kHz
  while a motor driven actuator may take seconds to
  reverse.

14
Voice coil actuators - notes

• It is also possible to fix the coil and allow the
  magnet to move.
• The field in the actuator can be generated by an
  electromagnet if necessary.
• The voice coil actuator can be turned into a
  sensor by simply reversing the action.
• If we were to move the coil in the magnetic
  field, the voltage induced in the coil will be
  given by Fradays law of induction through Eq.
  (PP). The speaker becomes a microphone.

15
Voice coil actuators - notes

• In the absence of current, the actuator is
  entirely disengaged there is no intrinsic
  retaining or cogging force and no friction.
• The motion is limited
• Rotational motion can also be achieved by
  selection of coil and magnet configurations.
• The actuator is a direct drive device.

16
Voice coil actuators - notes

• From these properties, the main qualities
• Their small mass allows very high accelerations
  (upwards of 50g and for very short strokes up to
  300g)
• Operation at high frequencies
• Ideal candidates for fast positioning (example
  in positioning of read/write heads in disk
  drives).
• Forces achievable are modest in comparison to
  other motors (up to 5000 N) and the power they
  can handle is also significant.

17
Voice coil actuators - Applications

• Often used where very accurate positioning at
  high speeds is needed.
• They have no hysteresis and minimal friction
• Extremely accurate both as linear and as angular
  positioners.
• No other actuator matches their response and
  acceleration.
• Can also be used in less critical applications,
  mostly in positioning and control but also in
  valve actuation, pumps and the like.

18
Voice coil actuators - Applications

• Interface with microprocessors is usually simpler
  than other types of motors
• Control and feedback is easily incorporated.
• Large variety of voice coil actuators available
• Most common
• Cylindrical actuator in Figure 5.38
• Rotary actuator in Figure 5.38b
• In the cylindrical linear actuator, the magnetic
  field is radial as in the loudspeaker.

19
Linear voice coil actuator
20
Angular voice coil actuator
21
Voice coil actuators - Applications

• Linear actuator
• The coil, attached to the moving, actuating shaft
  moves in/out from a center position
• The maximum stroke defined by the length of the
  coil and the length of the cylindrical magnet.
• For motion to be linearly proportional to
  current, the coil must be within the uniform
  magnetic field.
• Ratings of these actuators are in terms of
  stroke, force (in newtons), acceleration and
  power.

22
Motors

• Most common of all actuators
• Many types and variations.
• Will discuss some of the more salient issues
  associated with their use as actuators.
• Emphasis on modern, electronically controlled
  motors
• Will not discuss large motors
• Emphasis on DC and stepper motors

23
Motors - cont.

• Motors can be used and often are, as sensors.
• Many motors can be used as generators
• can sense motion, rotation, linear an angular
  position
• other quantity that affects these, such as wind
  speed, flow velocity and rate and many more.
• Some of these sensor applications will be
  discussed throughout this course
• Also common is to use them as dual -
  sensors/actuators

24
Motors - cont.

• Most motors are magnetic devices
• operate by attraction or repulsion between
  current carrying conductors or
• between current carrying conductors and permanent
  magnets in a manner similar to that of voice coil
  actuators.
• Motors include magnetic materials (mostly iron),
  in addition to permanents or electromagnets
• To increase and concentrate the magnetic flux
  density and to increase power and available
  torque at the smallest possible volume.

25
Motors - classification.

• For actuation purposes, there are three types of
  motors
• continuous rotational motors,
• stepper motors and
• linear motors.
• Best known is the continuous rotational motor.
• Stepper motors are much more common than one
  realizes
• Linear motors, are not as common, - specialized
  applications

26
Motors - cont.

• Variations in size and power they can deliver
  is staggering.
• Some motors are truly tiny. Example the motors
  used as vibrators in cell phones are about 6-8mm
  in diameter and no more than 20mm long.
• Motors delivering hundreds of MW of power are
  used in the steel industry, mining etc..
• The larger are generators in power plants these
  can be as large as 1000 MW or more.
• But there is no fundamental difference in
  operation between these devices.

27
Motors - principles

• Operation principles
• All motors operate on the principle of repulsion
  or attraction between magnetic poles.
• In its simplest form two magnets are kept
  separated vertically but the lower magnet is free
  to move horizontally.
• The two opposite poles attract and the lower
  magnet will move to the left until it is aligned
  with the upper magnet.

28
Magnetic attraction and repulsion
29
Principle of the motor
30
Motors - principles

• Operation principle more detailed
• The magnetic field (which may be produced by a
  permanent magnet or an electromagnet) is assumed
  to be constant in time and space (DC).
• If we apply a current to the loop as shown, and
  assuming the loop is initially at an angle to the
  field as shown in Figure 5.40b, a force will
  exist on each of the upper and lower members of
  the loop equal to BIL (Lorentz force in Eq. 11).

31
Motors - principles

• Force will rotate the loop to the right one half
  loop, (until the loop is perpendicular to the
  magnetic field).
• The Lorentz force is always perpendicular to both
  the current and the magnetic field.
• For a motor to operate continuously, when it
  reaches this position, the current in the loop is
  reversed (commutated)
• The force now will continue rotating it clockwise
  an additional half turn and so on.

32
Motors - principles

• Force on the loop is constant (independent of
  position)
• Torque is position dependent latter is
  T2BILrsin?, where r is the radius of the loop
• Maximum torque when loop is when loop is parallel
  to field
• Torque and force multiplied by N.

33
Motors - practical considerations

• This configuration requires commutation
• Commutation can be done mechanically or
  electronically.
• Figure 5.52 shows the same configuration with a
  mechanical commutator and a permanent magnet
  stator producing the magnetic field.
• This is a simple dc motor.

34
DC motor with commutator
35
Motors - practical considerations

• The number of coils can be increased, say, to two
  as in Figure 5.53.
• In this case, there are four connections on the
  commutator to ensure that each coil is powered in
  the appropriate sequence to ensure continuous
  rotation.
• In practical motors of this type, many more loops
  are used spaced equally.
• This increases torque and makes for smoother
  operation due to commutation.

36
A two coil dc motor
37
Motors - practical considerations

• Most small dc motors are made in this
  configuration or a modification of it.
• One particular modification is to use
  electromagnets and to add additional poles for
  the magnetic field (also spaced equally).
• Figure 5.54 shows a small motor with a single
  stator pole and 8 rotating coils (note the way
  they are wound).
• The addition of the iron increases force and
  torque.

38
Rotor and stator of a universal motor
39
Motors - practical considerations

• This is called a universal motor
• can operate on DC or AC and
• most common in ac operated hand tools.
• can develop a fairly large torque
• very noisy.
• Typically the stator coils are connected in
  series with the rotor coils (series universal
  motors)
• Parallel connection is also possible.

40
Motors - practical considerations

• Commutated motors are very common
• Simple, inexpensive, high torque
• Problems common in these motor
• damage to the commutator due to sparks developed
  when brushes (carbon contacts) slide over the
  commutator in normal operation.
• Brushes wear out over time (need replacement)

41
Permanent magnet dc motors

• A modification of the basic configuration above
• The magnetic field produced by a pair (or more)
  of permanent magnets
• A number of poles produced by windings as shown
  in Figure 5.55.
• These are low power, simple motors

42
Small dc PM motors
43
Permanent magnet dc motors

• Figure 5.55a. In this case there are three poles
  on the rotor and two on the stator (seen as blue
  and white)
• Ensures the motor can never get stuck in a zero
  force situation.
• The commutator operates as previously but,
  because there are three coils, one or two coils
  are energized at a time (depending on rotary
  position).

44
Permanent magnet dc motors

• Figure 5.55b shows a similar rotor from a
  somewhat larger motor
• has 7 poles and the same number of contacts on
  the mechanical commutator.
• These motors are commonly encountered in tape
  drives and in toys, as well as in cordless tools.
  
• They can be reversed by simply reversing the
  polarity of the source.

45
Some small dc motors
46
Brushless dc motors

• Eliminates the mechanical commutator
• For very demanding applications, such as in disk
  drives a variation of the dc motor is used in
  which the commutation is done electronically.
• The physical structure is often different to
  allow fitting in tight spaces or incorporation on
  integrated circuits.
• These motors are often flat (hence the name flat
  motors) and often the rotor is a mere disk.
• An additional important aspect is that the coils
  are stationary and the magnets rotate.

47
Brushless dc motor
48
Brushless dc motors

• Flat motor with 6 coils forming the stator.
• The rotor has been taken out of its bearing and
  inverted to see both the coils and the structure
  of the rotor.
• These coils are placed directly on a printed
  circuit board (note also the 3 hall elements).
• The rotor, shown on the left has a ring made of 8
  separate magnets
• The sides facing the coil (up in this figure)
  alternate in their magnetic field.

49
Rotor - flat brushless motor
50
Brushless dc motors

• The individual magnets can be distinguished by
  the brighter lines separating them.
• The operation of the motor relies on two
  principles.
• First, the pitch of the stator and rotor are
  different.
• Second, the position of the magnets are sensed
  and this sensing is used both to drive the coil,
  measure the speed and reverse the sense of
  rotation.
• By driving sequentially pairs of coils the device
  can be made to rotate in one direction or the
  other.

51
Brushless dc motor - operation

• Initial condition sensed by the hall elements
• Sequence starts with driving coils 1 and 4
• Polarity as shown in Figure 5.57a
• Coil 1 will repel magnet 1 and attract magnet 2
• Coil 4 will repel magnet 5 and attract magnet 6.
• This will rotate the rotor (magnets) to the left
  until coil 1 is centered with magnet 2 and coil 4
  is centered with magnet 6.

52
Sequence for the flat motor
53
Brushless dc motor - operation

• Next step
• Coils 2 and 5 are driven in the same way (as
  shown in Figure 5.57b.
• Coil 2 will repel magnet 3 and attract magnet 4
• Coil 5 will repel magnet 7 and attract magnet 8.
• Again, the magnets are forced to rotate left
  until coil 2 is centered with magnet 4 and coil 5
  with magnet 8. (Figure 5.57c.)

54
Brushless dc motor - operation

• Third step
• Coils 3 and 6 are driven.
• Rotation to the left is obtained until the coils
  and magnets are as in Figure 5.57d.
• This is identical to Figure 5.57a - repeats.
• The 3 seps are called phases. This is a 3 phase
  operation and can be done digitally - all it
  requires is to ascertain the location of the
  magnets and drive the opposite coils according to
  the sequence above.
• By reversing the coils currents, the north (N)
  poles are operating against the magnets and
  rotation is in opposite direction.

55
Brushless dc motor - operation

• The common choice in most digital devices
• disk drives
• CD drives
• video recorder heads,
• tape drives and many others
• Controlled very easily and its control is
  essentially digital. (sometimes geared, mostly
  direct drive)
• speed is controlled by timing the three phases at
  will.
• There are many variations in terms of the actual
  construction, shape and number of magnets and
  coils, etc.

56
A CD drive motor
57
A floppy drive motor
58
AC motors

• There is a large variety of ac motors
• The most common of the conventional motors is the
  induction motor in its many variants.
• The induction motor may be understood by first
  returning to Figure 5.51 but now the magnetic
  flux density is an ac field.
• The rotating coil is shorted (no external
  current)
• The ac field and the coil act as a transformer
  and an ac current is induced in the coil because
  it is shorted.

59
AC motors

• According to Lentzs law, the current in the coil
  must produce an opposing field which then forces
  the coil to rotate.
• There is no commutation, continuous rotation is
  achieved by rotating the field.
• By using the phases of the ac power supply a
  rotating field is produced.
• This is shown schematically in Figure 5.59 for a
  three phase ac motor (a magnet is shown for the
  rotor but the shorted coil acts exactly as a
  magnet).

60
Principle of the rotating field
61
Induction motors - notes

• Induction machines are common in appliances
• very quiet, efficient
• rotate at constant speeds which depend on the
  frequency of the field and number of poles.
• Also used in control devices where constant speed
  is important. (example clocks)
• Control of induction motors is much more involved
  than dc motors.
• Other types of motors exist.

62
A small induction motor
63
Stepper motors

• Actuation requires control of a motor
• exact and repeatable positioning
• requires some means of feedback,
• counting rotations,
• sensing position etc.
• Motors which incorporate these means are called
  servomotors
• They have been, to a large extent, replaced by
  stepper motors.

64
Stepper motors

• A stepper motor is an incremental rotation or
  motion motor.
• They are often viewed as digital motors, in the
  sense that each increment is fixed in size and
  increments are generated by a train of pulses.
• Very simple to control
• Usually relatively small, low power motors

65
Stepper motors - operation

• Start with a simple PM motor
• 2 phase stepper motor and uses a permanent magnet
  as the rotor.
• This allows simple description of the operation.
• The rotor can be made to rotate in steps by
  proper driving the two coils which in turn define
  the magnetic poles of the stator.

66
A 2 phase stepper motor - principle
67
Stepper motors - operation

• By driving the two vertical coils, the magnet is
  held vertically.
• If both coils are driven as in Figure 5.62b, the
  rotor will be at rest at 45?, rotating to the
  right.
• This is called a half step and is the minimum
  rotation or step possible in a stepper motor.
• The rotor remains fixed until the phases are
  changed.

68
2-phase stepper motor
69
Stepper motors - operation

• If now the vertical coil is de-energized, but the
  horizontal coil is kept energized, the magnet
  rotates an additional quarter turn to the
  position in Figure 5.62c.
• In the next step, the current in the vertical
  coil is negative, in the horizontal coil it is
  positive and the situation in Figure 5.62d is
  obtained.
• Finally, by reversing the vertical coil current
  and setting the horizontal coil to zero (no
  current) a full rotation has been completed.

70
2-phase stepper motor
71
2-phase stepper motor
72
Stepper motors - operation

• This simple motor steps at 45?
• Requires 8 steps to rotate a full turn.
• To rotate in the opposite direction the sequence
  must be reversed as shown in Table 5.6.
• Half steps or full steps (90 ?) can be used

73
Sequence for rotation
74
Stepper motors - notes

• The size of the step (number of steps) depends on
  number of coils and number of poles in the rotor.
  
• Full stepping (90? in this case) is accomplished
  using only one of the stator coils (single phase)
• More coils and more poles in the rotor will
  produce smaller steps.
• The number of poles in the rotor and in the
  stator must be different (fewer/more poles in the
  rotor)
• The magnetic field in the rotor can be generated
  by permanent magnets or by coils or by variable
  reluctance

75
Variable reluctance stepper motor

• The permanent magnet in the rotor is replaced
  with a piece of iron (non magnetized).
• The operation indicated above is still valid
  since the magnetic field produced by the stator
  coils will magnetize the iron (i.e. a magnet will
  attract a piece of iron).
• This simplifies matters considerably since now
  the rotor is much simpler to make.
• This type of stepper motor is called a variable
  reluctance stepping motor

76
Variable reluctance stepper motor

• VR is a common way of producing stepper motors.
• A practical motor is shown in Figure 5.64.

77
VR stepper motor - operation

• Coils marked as 2 are first energized.
• This moves the rotor one step to the left.
• Coils marked as 3 are next energized, moving one
  step to the left and so on.
• Opposite direction is obtained by inverting the
  sequence (driving coil No. 3 first then 2 and so
  on).

78
VR stepper motor - operation

• Steping size
• Assuming there are ns stator poles and nr rotor
  poles (teeth in this case). The stator and rotor
  pitches are defined as

Stepping size is given as an angle
79
VR stepper motor - operation

• Example 12 poles in stator, 8 in rotor

The stepper motor steps at 15? increments A
three phase stepper motor The number of poles in
the stator is larger than in the rotor. The
opposite is just as valid.
80
VR stepper motor - practical construction

• The rotor is made of nr teeth as above and the
  stator is made of a fixed number of poles, say 8,
  
• Each pole is toothed as shown in Fig. 5.64.
• In this case there are more teeth in the rotor
  (50) than in the stator (40).
• This produces a step of 1.8? (360/40-360/50).
• The motor in this figure is a 4 phase motor.

81
A practical 1.8? stepper motor
82
VR stepper motor - notes

• Variable reluctance stepping motors are simpler
  and less expensive to produce.
• However, when not powered, their rotor is free to
  move and hence they cannot hold their position.
• Permanent magnet stepper motors have some holding
  power and will maintain their position under
  power off conditions.

83
Multiple stack stepper motors

• Multiple rotors on a single shaft
• Decrease the step size
• Called multiple stack motors
• Now the pitch varies between stacks
• The driving sequence is more complicated than in
  a single stack motor.
• Usually, the stator and each rotor has the same
  number of teeth but the two rotors are shifted
  one half tooth apart.

84
Multiple stack stepper motors

• An example of an 8 pole (stator), double stack
  motor is shown in Figure 5.65.
• This motor has 50 teeth on each rotor and 50
  teeth on the stator. (rotors are magnetized)
• The rotors are magnetized and the motor shown has
  a 1.8? step.
• This particular motor was used to position the
  heads in an older floppy drive.
• In all other respects - same as single stack
  motors

85
Double stack, 1.8 ? stepper motor
86
Stepper motors - notes

• Stepper motors come in all sizes from tiny to
  very large and
• Currently the choice motors for accurate
  positioning and driving.
• More expensive and lower powered than other
  motors such as DC motors.
• The extra cost is usually justified by their
  simple control and accuracy and by the fact that
  they can be driven from digital controllers.

87
Stepper motors - notes

• Application
• Industrial control
• Consumer products such as printers, scanners and
  cameras.
• In these applications, the ability of the motor
  to step through a predictable sequence with
  accurate, repeatable steps, is used for fast
  positioning.
• The motors have typically low inertia, allowing
  them to respond quickly in both directions.

88
Stepper motors - examples
89
Linear motors

• A linear motor, either continuous motion or
  stepper motor can be viewed as a rotary motor
  that has been cut and flattened so that the rotor
  can now slide linearly over the stator. The rotor
  now becomes a slider or a translator.

90
PM linear motor - operation

• The slider or translator (equivalent to the
  rotor) may have as many poles as we wish 4 are
  shown
• Starting from the initial condition in Figure
  5.68a, the sliding poles are driven as shown and
  are therefore attracted to the right.
• As they pass past the stator poles, they are
  commutated and the polarities change, forcing
  motion to the right.
• This is merely a commutated DC machine. Motion to
  the left requires the opposite sequence.

91
PM linear motor - operation
92
VR linear motor

• This motor is equivalent to the rotary motor in
  Figure 5.50.
• The pitch is measured in units of length (so many
  mm per step).
• In this sequence, we assume that the stator poles
  are driven and that the rotor is a mere toothed
  iron piece (variable reluctance motor).
• The sequence is as follows

93
VR linear motor
94
VR linear motor

• Starting with Figure 5.68a, poles marked as 1 are
  driven alternately as N and S as shown.
• The slider moves to the right until teeth 1 are
  aligned with poles 1. (Figure 5.68b).
• Now poles 3 are driven as previously and the
  slider again moves to the right until teeth 2 are
  alighed with poles 3 (Figure 5.68c).
• Finally, poles 2 are driven in which the cycle
  completes and the relation of the slider and the
  stator is now as at the beginning of the
  sequence.

95
VR linear motor

• The same can be accomplished with permanent
  magnet poles in the rotor.
• From Figure 5.68, it should be noted that the
  pitch of the stator and slider are different -
  for every 4 poles in the stator there are 3 teeth
  in the slider.
• Each step equals the pitch of the stator (i.e. in
  each step a tooth moves either from the middle
  between two poles to the center of the pole or
  vice versa).
• By changing the number of teeth, one can change
  this pitch.

96
VR linear motor

• In the motor described here, the sequence is
  1-3-2 for motion to the right.
• Moving in the opposite direction is accomplished
  by reversing the sequence above to (2-3-1)
• In many linear stepping motors, it is more
  practical to drive the slider rather than the
  stator since the stator may be very long while
  the slider is usually small.

97
VR (PM) linear motor - 8 pole/4 teeth stator
98
VR (PM) linear motor - assempled
99
Magnetic solenoid actuators and magnetic valves

• Magnetic solenoid actuators are electromagnets
  designed to affect linear motion
• Exploit the force an electromagnet can generate
  on a ferromagnetic material.
• Principle a coil generates a magnetic field
  everywhere, including in the gap between the
  fixed and movable iron pieced.
• We shall call the movable piece a plunger.

100
Magnetic solenoid actuator - principle
101
Magnetic solenoid actuator - principles

• Force exerted on the plunger is

B the magnetic flux density in the gap,
generated by the coil S the cross-sectional
area of the plunger ?0 is the permeability of
free space. Force exerted on the plunger is
102
Magnetic solenoid actuator

• The plunger tends to close the gap
• This motion is the linear motion generated by the
  magnetic valve actuator.
• A more practical construction is Figure 5.70b
• This generates an axial field in the plunger but
  also closes the external field so that the total
  magnetic field available at the plunger is larger
  
• A modification of the linear plunger is the
  rotary or angular solenoid actuator.

103
Magnetic solenoid actuator

• In this form, the device is used as a simple
  go/nogo actuator.
• When energized, the gap is closed and when
  de-energized it is open.
• This type of device is often used for electrical
  release of latches on doors and as a means of
  opening/closing fluid or gas valves.
• Two examples of linear solenoid actuators and an
  angular actuator are shown next.

104
Magnetic solenoid actuators
105
Angular solenoid actuator
106
Magnetic solenoid actuator

• The basic solenoid actuator is often used as the
  moving mechanism in valves.
• A basic configuration is shown in Figure 5.73.
• These valves are quite common in control of both
  fluids and gases and exist in a variety of sizes,
  construction and power levels.
• Can be found in industrial processes but also in
  consumer appliances such as washing machines,
  dishwashers and refrigerators as well as in cars
  and a variety of other products.

107
Solenoid valve actuator
108
Magnetic solenoid actuator

• The actuating rod (plunger) in this case acts
  against a spring
• By properly driving the current through the
  solenoid its motion can be controlled as to speed
  and force exerted.
• Similar constructions can operate and control
  almost anything that requires linear (or
  rotational) motion.
• The travel of the actuating rods is relatively
  small, of the order of 10-20mm.

109
A fluid valve - magnetically actuated