Rotary actuators/DC motors
The conventional permanent magnet DC motor is still commonly used in vehicle actuation systems such as washer pumps, fuel pumps, window and sunroof open and closure, seat adjustment etc. This type of motor is very compact but high speed is needed to generate sufficient usable power. This tends to compromise the reliability and also means that for most applications a gearbox is needed to adapt speed and torque for the application. Cost and weight are important and the gearbox is generally constructed of plastic for cost reasons, although this also has a negative effect on durability.
Many applications now require high-precision, durable motor actuator systems and for these the brushless or electronically commutated DC motor is becoming more widespread. Typical applications are where high reliability is important, such as:
● gear shift actuators
● electrical power-steering systems ● electrical clutch actuators.
The basic construction of this motor incorporates a permanent magnet rotor that rotates inside a static stator winding. This consists of a number of separate field windings. Note that this is the opposite arrangement to a classically commutated DC motor where the field is static and the winding rotates. The field windings are energised in sequence by the motor drive electronics and this creates a rotating magnetic field. The permanent magnet rotor is locked by magnetism to this rotating field and hence drives an output shaft onto which it is mounted.
Acuators for chassis and body systems 67
The unit consists of a small DC motor combined with reduction gears (to increase force/torque) and a rack mechanism (to convert rotary to linear motion). Small, high-speed DC motors are very common and cheap to produce. When combined with plastic components in the gearbox/rack mechanism a very compact and light actuator can be produced cheaply and it has a long
68 Sensors and actuators Fundamentals of Motor Vehicle Technology: Book 3
The frequency with which the field winding is energised dictates the motor speed, and the current in the windings dictates the torque that the motor produces. This is monitored and controlled in the drive electronics according to the application requirements. An important factor is that the drive electronics must know the position of the rotor in order to energise the windings in the correct sequence. This can be achieved by fitting a rotor position sensor which feeds rotor position information back to the electronic drive system. Alternatively, where lower precision/reduced cost is required, the rotor position can be inferred from the emf that is induced in the non-energised windings whilst the motor rotates.
The main advantage is that this motor dispenses with the commutator and carbon brush arrangement. This means a reduction in noise and also an increase in reliability. These motors are basically maintenance free for the whole of their service life. The only moving parts are the bearings. The electronic drive system can incorporate a high degree of built-in intelligence for enhanced operation, for example:
● infinitely variable speed/torque control
● direction reversal
● motor protection – soft starting, protection against
overload and locking
● in-built diagnostic capability. Stepper motors
The introduction of digital electronics in vehicles has been accompanied by the adoption of stepper motors in numerous actuator applications. These respond to electrical pulses provided by an electronic driver circuit, normally built into an ECU. For precise control applications a stepper motor can move small angles in either direction in response to signals from the controller via the drive circuit. The type of motor governs the smallest step angle. Typical applications use step angles of 1.8, 2.5, 3.75, 7.5, 25 and 30 degrees. There are three main types of stepper motor.
Permanent magnet
In this type of motor the active rotor is a two-pole permanent magnet. The stator has two pairs of independent windings AA1 and BB1, through which current from the driver circuit may pass in either direction and this will turn the rotor by 90 degree steps. When current passes through BB1 the magnetic laws of attraction and repulsion align the rotor with the active poles of the stator (see Figure 2.51).
Complete rotation is obtained by applying to the motor four electrical pulses of suitable polarity. Direction of rotation depends on the polarity of the stator during the first pulse, e.g. if the current direction in Figure 2.51(b) is reversed, the rotor will move clockwise. The frequency of the step pulses will dictate the speed of rotation. By increasing the number of rotor/stator poles the step angle can be reduced. This can be calculated as follows:
Step angle = 360
number of step positions
Each winding has two directions of current flow so the number of step positions will always be an even number. Motors of this type normally have step angles of 7.5–120 degrees.
The main advantage for these motors is that the permanent magnet holds the rotor in position even when the windings are de-energised. This is known as detent torque and not all stepper motors have this feature. The disadvantage is they have relatively high inertia.
Variable reluctance
This type of motor has a soft iron rotor with radial teeth and a wound stator equipped with more poles than the rotor. Figure 2.52 shows a simplified layout of a three- phase, 15 degree step angle motor. This has eight rotor teeth and 12 stator poles around which the current flows in one direction only.
The number of step positions (N) is calculated via:
N = SR
(S – R)
where S = Slots in stator and R = Slots in rotor. For example, in this case:
N = (8 (12 – 8)× 12) = 964 = 24 Therefore:
Step angle = 360
24 =15 degrees
In Figure 2.52, (b) shows the winding arrangement for phase 1. When a current flows through one phase of the stator windings, the rotor aligns itself to give the shortest magnetic path, i.e. the path of minimum reluctance. In each step position, the rotor aligns with four stator poles and this gives the motor greater power.
An angular movement of one step from the position shown in Figure 2.52 (a) above is obtained by energising either phase 2 or phase 3 depending on the required direction. For a clockwise motion, the phases would be energised in the order 3–2–1–3–2–1. The angle turned by the rotor by these six current pulses is 90 degrees and the time taken for the total movement is governed by the time taken by the control circuit to energise the windings sufficient to move the rotor to the next step.
This type of motor is available with step angles between 1.8 and 15 degrees and has a fast response due to low rotor inertia. The motor can have a fast
Figure 2.52 Variable-reluctance stepper motor
Acuators for chassis and body systems 69
stepping rate but it does not have any detent torque and hence it has to be damped externally to prevent unwanted oscillation.
Hybrid type
As the name suggests, this type is a combination of the above two types. Figure 2.53 shows that the rotor is constructed in a similar manner to a vehicle alternator rotor. A permanent magnet with its poles coaxial to the rotor shaft is sandwiched between two iron claws which have teeth to form the poles.
The stator has eight main poles which are cut to form small teeth on the surface adjacent to the stator. Operation is similar to the permanent magnet type (above). The rotor aligns itself so that the magnetic reluctance is minimum.
The hybrid stepper motor has stepping angles as low as 0.9 degrees. It also has a relatively high torque and can operate at high stepping rates. The disadvantages are high inertia and resonance at some speeds.
Stepper motor control
All three types of stepper motor respond to digital signals. The direction of current flow through the appropriate stator winding governs the direction of rotor movement, and the speed at which the pulse signals are supplied controls the speed of rotor movement.
Taking the permanent-magnet type as an example, Figure 2.54(a) shows the pulses that are applied to turn the rotor. Note that the pulses do not overlap and the speed is controlled by the pulse frequency.
Figure 2.54(b) shows the pulse pattern needed to move the rotor forward through three steps (270 degrees) and then reverse to the original position. The input to the drive circuit, normally from a logic signal source, has low power and this must be amplified in the motor control electronics to a power level sufficient to drive the motor.
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Figure 2.53 Hybrid stepper motor
An actuator acts as an energy converter to convert electrical (or other) energy into physical movement or force
Actuators used in automotive systems are generally the linear or rotary type
A solenoid has a coil of wire around a former into which is fitted a soft iron plunger
A linear motor is similar to a solenoid but has a powerful magnetic plunger
K
ey P
oints
K
ey P
oints Figure 2.54 Pulse signals to control stepper motor
2.3.1 Introduction
A control system is an arrangement or sub-system which directs the operation of a main system. The commands given by the control system should ensure that the main system performs according to a given set of directions or commands, or performs its function within certain tolerances. An example is an engine control system in an ECU. In this case the system must achieve certain standards with respect to performance and economy of the engine and it is the control system’s task to achieve this.
The control system must be able to respond quickly and accurately to changes in the operating conditions. It must also maintain stable control and be able to separate valid input signals from those which are induced into sensing lines by electrical disturbance (known as ‘noise immunity’). There are two main types of control system.