Introduction to Stepper Motors

Nesh Basnet, Subash Shrestha, Lhakpa Dhondup, Lobsang Lekshey, Shiraz Macuff

Abstract

This document provides an introductions to stepper motors. The major types of stepper motors, their construction and required drive signals are explained. A comparison of each type of stepper motor and their application is also explained. Drive method for accurate load positioning and selection criteria for drive circuits is examined.

Introduction

Stepper motors are used where accurate load positioning, high speed positioning and fast acceleration is required without any position feedback control. However, feedback control is sometimes used to improve the characteristics of these motors. Permanent magnet (PM), variable reluctance (VR) ,  and hybrid stepper motors form the major groups of stepper motors in the industry. The permanent magnet motor have a permanent magnet as the rotor(rotating part) and the variable reluctance motors have a slotted  soft iron rotor. The rotor of the hybrid motor is a permanent magnet enclosed by laminated- slotted soft iron. The stator (stationary part) of the stepper motor is made from laminated iron and coils of magnet wire. Note that it is not the number of poles on the rotor that determine the type of motor but rather the way the motor is built and the way it operates.

PM Stepper motor rotor
Fig.1 PM Stepper motor rotor

VR Rotot
Fig.2 VR Rotot

Hybrid stepper motor Rotor
Fig.3 Hybrid stepper motor Rotor

The stator-rotor assembly and operation.

Fig. 4

The permanent magnet (PM) stepper motor consist of the rotor above and the stator assembly. A simplified schematic of a PM two phase stepper motor is shown . The phase of the motor will become apparent later. The magnetic poles of the rotor is labeled north (N) and south (S) which is encircled by the four stator electro-magnets. The stator magnets are wound and connected such that the magnetic field produced by a current flowing in one pair would be pointing in the same direction.Step 1 requires that the current entering the A+ terminal would produce a magnetic field pointing to the left. As a result the rotor would be held in the horizontal position.

Fig.5

Stepping (step 2) the motor requires stator coil A be de-energized and stator coil B be energized such that the magnetic field would point upwards. By allowing current to enter the B+ terminal (source) and exit the B- terminal (sink), the magnetic field would be pointing upwards. The change in magnetic direction will produce a torque on the rotor in the clock wise direction. This torque is what steps the rotor.

The third step requires that stator A be energized again. Now the current is reversed such that it enters the A- terminal. The current entering the A- terminal produce a horizontal magnetic flux pointing towards the right. This again produces a clock wise torque on there rotor which produces a step.

Subsequently (step 4) the magnetic field is rotated again by de-energizing the horizontal coil and energizing the vertical coil such that the magnetic field would point downwards. This again produces a 90 degree rotation due to the toque imposed on the rotor. This sequence is repeated to step the rotor in the clockclockwise direction. To rotate in the counterclockwise direction , the sequence is reversed. That is step4,step3,step2,step1,step4 is executed for counter clockwise rotation.

It is important to note that the motor depicted above is generalized and simplified. In reality the number of rotor pole can be two as shown above or there can be many more. Motors designed for high speed have less pole than those designed for high torque.

Variable reluctance stepper motor

drawing
Fig.6 VR motor

The variable reluctance stepper motor shown in figure 6 has as slotted soft iron  rotor encircled by electromagnetic stator coils which are wound on the stator teeth. The stator teeth that are geometrically opposing each other are wound to have a common magnetic field pointing in the same direction. The magnet field produced by the stator seeks a path of least resistance (reluctance). The magnetic path is provided by the rotor teeth and as a result couples to the teeth. A force is produced which is composed of two components. A tangential Ft and a normal force Fn. The tangential force produces a torque on the rotor which causes it to turn. The VR stepper motor is stepped like the PM stepper motor by energizing the coils in sequence. The difference here  is that the torque is result of the magnet field seeking a low reluctance path hence the name variable reluctance motor. Fig. 7 and Fig.8 both shows coils B and C energized sequentially producing  steps in the clockwise direction. Fig 9 shows coil A energized again but with the magnetic field pointing in the other direction when compared to figure 6.

drawingStep2
Fig. 7

drawingStep3
Fig. 8

drawingStep4
Fig. 9

Hybrid stepper motors

drawing-1
Fig. 10

Hybrid stepper motors combine both the characteristics of permanent magnet forces and magnetic reluctance forces.  Hybrid motors also differ in that the magnetic field produced by the permanent magnet on the rotor points not radially but lateral through the rotor. A closed magnetic loop is formed from the front- north pole of the rotor, laterally through the rotor, going radially through the soft iron front end bell, laterally through the soft iron stator body, through the soft iron rear end bell and back to the rotor’s south pole. The stator is wound to encourage the magnetic field lines from the stator poles to the rotor based on the required step.

Unipolar and bipolar motors

Fig.11
Fig.11

The direction of the magnetic field produced by each stator coil has to be reversible to get complete rotation of the rotor. The two ways that is accomplished is by 1) changing the direction of the current flowing the coil or 2)  by having two coils on the same stator pole wound in different direction and alternately energizing them for the required magnetic direction. When motors are constructed such that the direction of the current has to be revered, the motor is called a bipolar motor. Likewise a motor wound with

Fig.12
Fig.12

dual coils on the same pole with different direction is called a unipolar motor. Driving a phase of the bipolar motor requires the use of four switched configured as an H-bridge as shown in figure 11. Turning on switches Q1 and Q 4 results in current flow trough the phase as indicated by i1. Likewise turning on switched Q2 and Q3 results in current i2. The uni-polar phase winding is shown in figure 12. By turning on switch Q1, the current flowing from VDD to VSS would create a magnetic field pointing to the right. Likewise turning on Q2 would produce a magnetic field pointing to the left. Note that the drive circuitry for a uni-polar motor is significantly simpler compared to the bipolar circuit. Only half of the winding is used on the unipolar when compared to the bipolar motor. This results in the unipolar motor only having half the current handling capacity than the bipolar motor of the same size.

Accurate load positioning

As mentioned before stepper motors are generally used where accurate mechanical load positioning is required . They can be stepped in three major ways: full stepping , half stepping and micro stepping. A controller can have one , two or all three types of stepping capabilities. Full stepping the motor requires that one phase be completely energized while the other is de-energized as shown previously. A motor with  200 steps per revolution would would turn 1.8 degree with each full step. Half stepping requires that the two of the phases be turned on some of the time. Half stepping results in 0.9 degree turn on a motor with 200 steps per revolution. With micro stepping a finer resolution is attained per micro step. The resolution is determined by the resolution of the driver and is published in the driver’s data sheet. Selecting the correct driver is based on the end application requirements. For example a printer head that is driven by a stepper motor attached to a toothed belt, would certainly require micro stepping since the longitudinal increment may need to be as high as  100 discrete positions per inch or higher. A lead screw assembly on the other hand that gives a longitudinal movement of 1 mill per 20 degree of turn may not even required half stepping! The end application determines the best driver based needed based on the actual requirement. For example , while designing a drive train for the lead screw assembly mentioned above, it would be wise to select a driver with correct set of features. Over-designing by using a driver for a laser cutting head assembly with micro-stepping would add little to the product’s value while adding substantial cost. Instead, a driver with built in feedback control, over current protection, motor temperature measurement and full step only features would add significantly more value to the end application . Some of these features are explain later and the others are explained in subsequent application notes.

Open and closed loop control

Open loop control systems are systems where the controller would send an instruction and the controller would not check that the system has executed instruction properly. On the other hand the closed loop system would check to see if the instruction was executed and if not, it would automatically adjust the next instruction to compensate for the error. Stepper motors are initially selected because of the ease at which they can be implemented as open loop control. In many cases the initially open loop design would show a need for some verification that the motor has actually moved the required number of steps. This happens as a result of the application engineer not factoring in the variation in rotor position when it’s loaded for example. Another aspect that is overlooked very easily is the impact of the system missing a step during a sequence of instruction. For a system that is being design to operate without human attendance for long periods of time would require closed loop control.

References:

1: AVR446 Linear speed control of stepper motor  from ATMEL
2: Electromechanics by James H.Harter
3: Stepping Motors by D.P Atherton and G.W. Irwin
4: Application note AN469 by ST Thomas Hopkins
5: www.shinano.com