Stepper Motor

STEPPER MOTOR

Step Motors (often referred as stepper motors) are different from all other types of electrical drives in the sense that they operate on discrete control pulses received and rotate in discrete steps. On the other hand ordinary electrical A.C and D.C drives are analog in nature and rotate continuously depending on magnitude and polarity of the control signal received. The discrete nature of operation of a step motor makes it suitable for directly interfacing with a computer and direct computer control. These motors are widely employed in industrial control, specifically for CNC machines, where open loop control in discrete steps are acceptable. These motors can also be adapted for continuous rotation.

Construction

Step Motors are normally of two types:

(a) Permanent Magnet and

(b) Variable Reluctance Type.

In a step motor the excitation voltage to the coils is D.C. and the number of phases indicates the number of windings. In both the two cases the excitation windings are in the stator. In a permanent magnet type step motor the rotor is a permanent magnet with a number of poles. On the other hand the rotor of a variable reluctance type motor is of a cylindrical structure with a number of projected teeth.

(a) Permanent Magnet Step Motor

The principle of step motor can be understood from the basic schematic arrangement of a small permanent magnet step motor is shown in Figure 2.49. This type of motor is called a two-phase two-pole permanent magnet step motor; the number of windings being two (phase 1 and phase 2) each split into two identical halves; the rotor is a permanent magnet with two poles.

So winding, A is split into two halves A₁ and A₂. They are excited by constant D.C.voltage V and the direction of current through A₁ and A₂ can beset by switching of four switches Q₁, Q₂, Q₃ and where four switches are Q₅ -Q₈ are used to control the direction of current as shown in Figure 2.50(b). The directions of the currents and the corresponding polarities of the induced magnets are shown in Figure 2.49 Q₄ as shown in Figure 2.50(a). For example, if Q₁ and Q₂ are closed, the current flows from A₁ to A₂, while closing of the switches Q₃ and Q₄ sets the direction of current from A₂ to A₁. Similar is the case for the halves B₁ and B₂.

Now consider Figure 2.50. Let winding A be energized and the induced magnetic poles are as shown in Figure 2.51(a) we will denote the switching condition as (S₁ = 1). The other winding B is not energized. As a result the moving permanent magnet will align itself along the axis of the stator poles as shown in figure (a). In the next step, both the windings A and B are excited simultaneously, and the polarities of the stator poles are as shown in Figure 2.51(b). We shall denote S₁ = 1, for this switching arrangement for winding B. The rotor magnet will now rotate by an angle of 45° and align itself with the resultant magnetic field produced. In the next step, if we now make S₁ = 0 (thereby de-energizing winding A), the rotor will rotate further clockwise by 45° and align itself along winding B, as shown in figure (c). In this way if we keep on changing the switching sequence, the rotor will keep on rotating by 45° in each step in the clockwise direction. The switching sequences for the switches Q₁ to Q₈ for first four steps are tabulated in Table 1.

It is apparent from Table 1 and Figure 2.52 that for this type of switching the step angle is 45° and it takes 8 steps to complete a complete revolution. So we have 8 steps/ revolution. It can also be seen from Table 1 that a pair of switch (say Q₇ – Q₈) remains closed during consecutive three steps of rotation and there is an overlap at every alternate step where both the two windings are energized. This arrangement for controlling the step motor movement is known as half stepping. The direction of rotation can be reversed by changing the order of the switching sequence.

It is also possible to have an excitation arrangement where each phase is excited one at a time and there is no overlapping where both the phases are energized simultaneously, though it is not possible for the configuration shown in Figure, since that will require the rotor to rotate by 90° in each step and in the process, may inadventedly get locked in the previous position. But full stepping is achievable for other cases, as for example for the two-phase six-pole permanent magnet step motor as shown in Figure 2.52. In this case, the stator pitch q₁ = 90° and the rotor pitch θ_r = 60°; the full step angle is given by θ_fs =θ_s, θ_r = 30°and the half step angle θ_hs = (θ_s – 0_r)/2 = 15°. The desired direction of rotation can be achieved by choosing the sequence of switching.

The advantage of a permanent magnet step motor is that it has a holding torque. This means that due to the presence of permanent magnet the rotor will lock itself along the stator pole even when the excitation coils are de-energized. But the major disadvantage is that the direction of current for each winding needs to be reversed. This requires more number of transistor switches that may make the driving circuit unwieldy.

Another way of reducing the number of switches is to use unipolar winding. In unipolar winding, there are two windings per pole, out of which only one is excited at a time. The windings in a pole are wound in opposite direction, thus either N-pole or S-pole, depending on which one is excited.

(b) Variable Reluctance Type Step Motor

Variable reluctance type step motors do not require reversing of current through the coils, but at the same time do not have any holding torque. Compared to permanent magnet step motors, their step angles are also much smaller. Step angle as low as 1.8° can be achieved with this type of motors. Here the rotor is a cylindrical soft iron core with projected teeth. When a particular stator coil is excited, the rotor aligns itself such that one pair of teeth is along the energised stator coil, at the minimum reluctance path. The schematic arrangement of a three phase VR motor with 12 stator poles (teeth) and eight rotor teeth is shown in Figure 2.53.

When phase-I is energized, the rotor will align itself as shown in the Figure 2.53. In the next step, if phase-1 is switched off and phase-2 is switched on, the rotor will rotate in CCW direction by an angle of 15°. This can be understood from the following derivation:

Here the stator pole pitch

Similar to the earlier case, we can also have half stepping where step angle of 7.5° can be achieved. To switching sequence for rotation in the counter clockwise direction will half stepping would be 1-(1,2) – 2 - (2, 3) – 3 - (3, 1) - 1 ....

Further reduction of step angle is possible by increasing the number of stator and rotor teeth. Besides, multi-stack stators are also used for achieving smaller step angle, where there are several stacks of stator windings skewed from each other by a certain angle. It has been already mentioned that the VR motors do not have any holding torque. It is natural because, when the stator coils are de-energised there is no magnetic force present and the rotor is free. Hybrid step motors are improved versions of single stack. VR motors, where the basic constructions are modified slightly in order to achieve holding torque. However this part will not be discussed in this lesson. Interested readers may consult the books given in the reference.

Fig. 2.53: Three-phase single-stack VR step motor with twelve stator poles (teeth) and eight rotor teeth

Hybrid Stepper:

The hybrid stepper motors have the combination of the best properties of variable reluctance and permanent magnet steppers, so they are more expensive than the PM stepper motor. The hybrid type stepper motors provide better performance with respect to step resolution, torque and speed.

The rotor of a hybrid stepper is multi-toothed like the variable reluctance steppers and it contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations de an even b in the air gap.

The most commonly used types of stepper motors are the hybrid and permanent magnet. The designers prefer permanent magnets unless their project requires the hybrid steppers, since the cost of permanent magnet are less than of hybrids.

Advantages of Stepper Motor:

1. The rotation angle of the motor is proportional to the input pulse.

2. The motor has full torque at standstill.

3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3-5% of a step and this error is non-cumulative from one step to the next.

4. Excellent response to starting, stopping and reversing.

5. Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependent on the life of the bearing.

6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.

7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.

8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Applications:

1. Industrial Machines - Stepper motors are used in automotive gauges and machine tooling automated production equipments.

2. Security - New surveillance products for the security industry.

3. Medical - Stepper motors are used inside medical scanners, samplers, and also found inside digital dental photography, fluid pumps, respirators and blood analysis machinery.

4. Consumer Electronics - Stepper motors in cameras for automatic digital camera focus and zoom functions. And also have business machines applications, computer peripherals applications.

Disadvantages:

1. Once motor is not controlled well, it can easily cause resonance vibration.

2. Hardly to run to the higher speed.

Characteristics of Stepper Motor

The stepper motor characteristics are classified as:

1. Static characteristics and

2. Dynamic characteristics

The static are at the stationary position of the motor while the dynamic are under running conditions of the motor.

(a) Static Characteristics

These characteristics include:

1. Torque displacement characteristics

2. Torque current characteristics.

1. Torque-Displacement characteristics:

This gives relationships between electromagnetic torque developed and displacement angle from steady state position. These characteristics are shown in the Figure 2.55.

2. Torque-Current Characteristics:

The holding torque of the stepper motor increases with the exciting current. The relationship between the holding torque and the current is called as torque-current characteristics. These characteristics are shown in the Figure 2.56.

(b) Dynamic Characteristics

The stepping rate selection is very important in proper controlling of the stepper motor. The dynamic characteristics gives the information regarding torque stepping rate. These are also called torque stepping rate curves of the stepper motor. These curves are shown in the Figure 2.57.

When stepping rate increases, rotor gets less time to drive the load from one position to other. If stepping rate is increased beyond certain limit, rotor cannot follow the command an starts missing the pulses.

Now if the values of load torque and stepping rate are such that point of operation lies to the left of curve I, then motor can start and synchronize without missing a pulse.

For example, for a load torque of T'L, the stepping rate selection should be less than f₁ so that motor can start and synchronize, without missing a step. But the interesting thing is that once motor has started and synchronized, then stepping rate can be increased e.g., upto f₂ for the above example. Such as increase in stepping rate from f₁ to f₂s without missing a step and without missing the synchronism. But beyond f₂, if stepping rate is increased, motor will loose its synchronism.

So point A as shown in the figure indicates the maximum starting stepping rate or maximum starting frequency. It is defined as the maximum stepping rate with which unloaded motor can start or stop without loosing a single step. While point B as shown in the figure indicates the maximum slowing frequency. It is defined as the maximum stepping rate which unloaded motor continues to run without missing a step. Thus area between the curves I and II shown hatched indicates, for various torque values, the range of stepping rate which the motor can follow without missing a step, provided that the motor is started and synchronized. This area of operation of the stepper motor is called slew range. The motor is said to be operating in slowing mode.

It is important that in a slew range the stepper motor can not be started, stopped or reversed without losing steps. Thus slew range is important for speed control application. In position control, to get the exact position the motor may be required to be stopped or reversed. But is not possible in a slew range. Hence slew range is not useful for position control applications.

To achieve the operation of the motor in the slew range motor must be accelerated carefully using lower pulse rate. Similarly to stop or reverse the motor without loosing acceleration and deceleration of the stepper motor, without losing any step is called ramping.

Important Definitions

1. Holding Torque: It is defined as the maximum static torque that can be applied to the shaft of an excited motor without causing a continuous rotation.

2. Detent Torque: It is defined as the maximum static torque that can be applied to the shaft of an unexcited motor without causing a continuous rotation. Under this torque the rotor comes back to the normal rest position even if excitation ceases. Such positions of the rotor are referred as the detent positions.

3. Step Angle: It is defined as the angular displacement of the rotor response to each input pulse.

4. Critical Torque: It is defined as the maximum load torque at which rotor does not move when exciting winding is energized. This is also called pullout torque.

5. Limiting Torque: It is defined for a given pulsing rate or stepping rate measured in pulses per second, as the maximum load torque at which motor follows the control pulses without missing any step. This is also called pull in torque.

6. Synchronous Stepping Rate: It is defined as the maximum rate at which the motor can step without missing Steps. The motor can start, stop or reverse at this rate.

7. Stewing Rate: It is defined as the maximum rate at which the motor can step unidirectionally. The stewing rate is much higher than the synchronous stepping rate. Motor will not be able to stop or reverse without missing steps at this rate.