Industrial Circuits Application Note Basics

A stepper motor is an electromechanical Disadvantages device which converts electrical pulses into 15° discrete mechanical movements. The shaft 1. Resonances can occur if not A or spindle of a stepper motor rotates in properly controlled.

D' discrete step increments when electrical 2. Not easy to operate at extremely B command pulses are applied to it in the high speeds. 1 proper sequence. The motors rotation has 6 several direct relationships to these applied 2 C'

C input pulses. The sequence of the applied Open Loop Operation 5

pulses is directly related to the direction of One of the most significant advantages 3

motor shafts rotation. The speed of the of a stepper motor is its ability to be 4

B' motor shafts rotation is directly related to accurately controlled in an open loop D the frequency of the input pulses and the system. Open loop control means no length of rotation is directly related to the feedback information about position is A' number of input pulses applied. needed. This type of control eliminates the need for expensive Figure 1. Cross-section of a variable- sensing and feedback devices such as reluctance (VR) motor. Stepper Motor Advantages optical encoders. Your position is and Disadvantages known simply by keeping track of the input step pulses. Advantages 1. The rotation angle of the motor is Stepper Motor Types proportional to the input pulse. There are three basic stepper motor types. They are : 2. The motor has full torque at stand- N N S N S N still (if the windings are energized) • Variable-reluctance 3. Precise positioning and repeat- • Permanent- ability of movement since good S N stepper motors have an accuracy of • Hybrid 3 – 5% of a step and this error is non cumulative from one step to Variable-reluctance (VR) S the next. This type of stepper motor has been 4. Excellent response to starting/ around for a long time. It is probably stopping/reversing. the easiest to understand from a structural point of view. Figure 1 Figure 2. Principle of a PM or tin-can 5. Very reliable since there are no con- shows a cross section of a typical V.R. stepper motor. tact brushes in the motor. stepper motor. This type of motor Therefore the life of the motor is consists of a soft iron multi-toothed simply dependant on the life of the and a wound . When the S bearing. stator windings are energized with DC 6. The motors response to digital current the poles become magnetized. input pulses provides open-loop Rotation occurs when the rotor teeth N control, making the motor simpler are attracted to the energized stator N N and less costly to control. poles. 7. It is possible to achieve very low Permanent Magnet (PM) speed synchronous rotation with a Often referred to as a “tin can” or load that is directly coupled to the “canstock” motor the permanent S shaft. magnet step motor is a low cost and 8. A wide range of rotational speeds low resolution type motor with typical step angles of 7.5° to 15°. (48 – 24 Figure 3. Cross-section of a hybrid stepper can be realized as the speed is motor. proportional to the frequency of the steps/revolution) PM motors as the input pulses.

1 has some advantages such as very low in many different applications. Some inertia and a optimized magnetic flow of these include printers, plotters, path with no coupling between the highend office equipment, hard disk two stator windings. These qualities drives, medical equipment, fax are essential in some applications. N machines, automotive and many more. S N S N S N

Size and Power The Rotating Magnetic Field In addition to being classified by their Figure 4. Principle of a disc magnet motor When a phase winding of a stepper step angle stepper motors are also developed by Portescap. motor is energized with current a classified according to frame sizes magnetic flux is developed in the name implies have permanent which correspond to the diameter of stator. The direction of this flux is added to the motor structure. the body of the motor. For instance a determined by the “Right Hand The rotor no longer has teeth as with size 11 stepper motor has a body di- Rule” which states: the VR motor. Instead the rotor is ameter of approximately 1.1 inches. “If the coil is grasped in the right magnetized with alternating north Likewise a size 23 stepper motor has a hand with the fingers pointing in the and south poles situated in a straight body diameter of 2.3 inches (58 mm), direction of the current in the winding line parallel to the rotor shaft. These etc. The body length may however, (the thumb is extended at a 90° angle magnetized rotor poles provide an vary from motor to motor within the to the fingers), then the thumb will increased magnetic flux intensity and same frame size classification. As a point in the direction of the magnetic because of this the PM motor exhibits general rule the available torque out- field.” improved torque characteristics when put from a motor of a particular frame Figure 5 shows the magnetic flux compared with the VR type. size will increase with increased body path developed when phase B is ener- length. gized with winding current in the Hybrid (HB) Power levels for IC-driven stepper direction shown. The rotor then aligns The hybrid stepper motor is more motors typically range from below a itself so that the flux opposition is expensive than the PM stepper motor watt for very small motors up to 10 – minimized. In this case the motor but provides better performance with 20 watts for larger motors. The maxi- would rotate clockwise so that its respect to step resolution, torque and mum power dissipation level or south pole aligns with the north pole speed. Typical step angles for the HB thermal limits of the motor are seldom of the stator B at position 2 and its stepper motor range from 3.6° to 0.9° clearly stated in the motor manu- north pole aligns with the south pole (100 – 400 steps per revolution). The facturers data. To determine this we of stator B at position 6. To get the hybrid stepper motor combines the must apply the relationship P␣ =V ×␣I. motor to rotate we can now see that best features of both the PM and VR For example, a size 23 step motor may we must provide a sequence of type stepper motors. The rotor is be rated at 6V and 1A per phase. energizing the stator windings in such multi-toothed like the VR motor and Therefore, with two phases energized a fashion that provides a rotating contains an axially magnetized con- the motor has a rated power dissipa- magnetic flux field which the rotor centric magnet around its shaft. The tion of 12 watts. It is normal practice follows due to magnetic attraction. teeth on the rotor provide an even to rate a stepper motor at the power better path which helps guide the dissipation level where the motor case magnetic flux to preferred locations in rises 65°C above the ambient in still Torque Generation the airgap. This further increases the air. Therefore, if the motor can be The torque produced by a stepper detent, holding and dynamic torque mounted to a heatsink it is often motor depends on several factors. characteristics of the motor when com- possible to increase the allowable • The step rate pared with both the VR and PM power dissipation level. This is types. important as the motor is designed to • The drive current in the windings The two most commonly used types be and should be used at its maximum • The drive design or type of stepper motors are the permanent power dissipation ,to be efficient from magnet and the hybrid types. If a a size/output power/cost point of view. In a stepper motor a torque is devel- designer is not sure which type will oped when the magnetic fluxes of the best fit his applications requirements rotor and stator are displaced from he should first evaluate the PM type as When to Use a Stepper each other. The stator is made up of a it is normally several times less expen- Motor high permeability magnetic material. sive. If not then the hybrid motor may A stepper motor can be a good choice The presence of this high permeability be the right choice. whenever controlled movement is material causes the magnetic flux to required. They can be used to advan- be confined for the most part to the There also excist some special tage in applications where you need to paths defined by the stator structure stepper motor designs. One is the disc control rotation angle, speed, position in the same fashion that currents are magnet motor. Here the rotor is and synchronism. Because of the in- confined to the conductors of an elec- designed sa a disc with rare earth herent advantages listed previously, tronic circuit. This serves to concen- magnets, See fig. 5 . This motor type stepper motors have found their place trate the flux at the stator poles. The

2 torque output produced by the motor increase the number of steps per is proportional to the intensity of the revolution of the motor, or in other magnetic flux generated when the words to provide a smaller basic (full 8 1 winding is energized. 7 step) stepping angle. The permanent S 2 The basic relationship which magnet stepper motor contains an S Rotor N Phase A 6 N defines the intensity of the magnetic equal number of rotor and stator pole 3 flux is defined by: pairs. Typically the PM motor has 12 5 4 H = (N × i) ÷ l where: pole pairs. The stator has 12 pole pairs per phase. The hybrid type stepper Stator A N = The number of winding turns motor has a rotor with teeth. The Stator B i = current rotor is split into two parts, separated IB by a permanant magnet—making half Phase B H = Magnetic field intensity of the teeth south poles and half north l = Magnetic flux path length poles.The number of pole pairs is Figure 5. Magnetic flux path through a equal to the number of teeth on one of This relationship shows that the two-pole stepper motor with a lag between the rotor halves. The stator of a hybrid the rotor and stator. magnetic flux intensity and conse- motor also has teeth to build up a quently the torque is proportional to higher number of equivalent poles the number of winding turns and the (smaller pole pitch, number of current and inversely proportional to equivalent poles = 360/teeth pitch) N 8 1 the length of the magnetic flux path. compared to the main poles, on which 7 From this basic relationship one can Phase A S the winding coils are wound. Usually 2 see that the same frame size stepper VM S Rotor N 4 main poles are used for 3.6 hybrids I 6 motor could have very different torque A N and 8 for 1.8- and 0.9-degree types. Phase A 3 output capabilities simply by chang- It is the relationship between the 5 4 ing the winding parameters. More S number of rotor poles and the equival- Stator A detailed information on how the ent stator poles, and the number the winding parameters affect the output number of phases that determines the Stator B capability of the motor can be found full-step angle of a stepper motor. IB in the application note entitled “Drive Phase B Phase B ÷ × VM Circuit Basics”. Step angle=360 (NPh Ph)=360/N

NPh = Number of equivalent poles per N IA phase = number of rotor poles 8 1 Phases, Poles and Stepping 7 Ph = Number of phases S Angles S 2 N Phase A 6 Rotor Usually stepper motors have two N = Total number of poles for all N 3 phases, but three- and five-phase phases together 5 4 motors also exist. If the rotor and stator tooth pitch is S A bipolar motor with two phases unequal, a more-complicated relation- Stator A has one winding/phase and a unipolar ship exists. motor has one winding, with a center Stator B I tap per phase. Sometimes the unipolar B stepper motor is referred to as a “four- Stepping Modes Phase B phase motor”, even though it only has The following are the most common Figure 6. Unipolar and bipolar wound two phases. drive modes. stepper motors. Motors that have two separate • Wave Drive (1 phase on) windings per phase also exist—these motors with the same winding param- can be driven in either bipolar or • Full Step Drive (2 phases on) eters this excitation mode would result unipolar mode. in the same mechanical position. The • Half Step Drive (1 & 2 phases on) A pole can be defined as one of the disadvantage of this drive mode is that regions in a magnetized body where • Microstepping (Continuously in the unipolar wound motor you are the magnetic flux density is con- varying motor currents) only using 25% and in the bipolar centrated. Both the rotor and the motor only 50% of the total motor For the following discussions please stator of a step motor have poles. winding at any given time. This refer to the figure 6. Figure 2 contains a simplified picture means that you are not getting the In Wave Drive only one winding is of a two-phase stepper motor having 2 maximum torque output from the energized at any given time. The poles (or 1 pole pairs) for each phase motor on the stator, and 2 poles (one pole stator is energized according to the pair) on the rotor. In reality several sequence A → B → A → B and the more poles are added to both the rotor rotor steps from position 8 → 2 → 4 and stator structure in order to → 6. For unipolar and bipolar wound

3 Torque The excitation sequences for the The displacement angle is deter- Unstable above drive modes are summarized in mined by the following relationship: Table 1. TH Region ÷ π × ÷ X = (Z 2 ) sin(Ta Th) where: T In Microstepping Drive the a currents in the windings are Angle O continuously varying to be able to Z = rotor tooth pitch A Oa B C break up one full step into many Ta = Load torque Stable Unstable Stable smaller discrete steps. More T = Motors rated holding torque Point Point Point information on microstepping can be h found in the microstepping chapter. X = Displacement angle. Figure 7. Torque vs. rotor angular position. Therefore if you have a problem Torque vs, Angle with the step angle error of the loaded Characteristics motor at rest you can improve this by Torque The torque vs angle characteristics of changing the “stiffness” of the motor. a stepper motor are the relationship This is done by increasing the holding T H2 between the displacement of the rotor torque of the motor. We can see this and the torque which applied to the effect shown in the figure 5. TH1 rotor shaft when the stepper motor is Increasing the holding torque for a constant load causes a shift in the lag TLoad energized at its rated . An ideal stepper motor has a sinusoidal torque angle from Q2 to Q1. vs displacement characteristic as shown in figure 8. AngleO Positions A and C represent stable Step Angle Accuracy O1 O2 Figure 8. Torque vs. rotor angle position at equilibrium points when no external One reason why the stepper motor has different holding torque. force or load is applied to the rotor achieved such popularity as a position- shaft. When you apply an external ing device is its accuracy and repeat- ability. Typically stepper motors will force Ta to the motor shaft you in In Full Step Drive you are ener- essence create an angular have a step angle accuracy of 3 – 5% gizing two phases at any given time. displacement, Θ . This angular of one step. This error is also non- Θa cumulative from step to step. The The stator is energized according to displacement, a, is referred to as a the sequence AB → AB → AB → lead or lag angle depending on wether accuracy of the stepper motor is AB and the rotor steps from position the motor is actively accelerating or mainly a function of the mechanical 1 → 3 → 5 → 7 . Full step mode decelerating. When the rotor stops precision of its parts and assembly. results in the same angular movement with an applied load it will come to Figure 9 shows a typical plot of the as 1 phase on drive but the mechanical rest at the position defined by this positional accuracy of a stepper motor. position is offset by one half of a full displacement angle. The motor Step Position Error step. The torque output of the develops a torque, Ta, in opposition to unipolar wound motor is lower than the applied external force in order to The maximum positive or negative the bipolar motor (for motors with the balance the load. As the load is position error caused when the motor same winding parameters) since the increased the displacement angle also has rotated one step from the previous uses only 50% of the increases until it reaches the holding position. available winding while the bipolar maximum holding torque, T , of the Step position error = measured step h angle - theoretical angle motor uses the entire winding. motor. Once Th is exceeded the motor Half Step Drive combines both enters an unstable region. In this wave and full step (1&2 phases on) region a torque is the opposite Positional Error direction is created and the rotor The motor is stepped N times from an drive modes. Every second step only ° one phase is energized and during the jumps over the unstable point to the initial position (N = 360 /step angle) other steps one phase on each stator. next stable point. and the angle from the initial position The stator is energized according to the sequence AB → B → AB → A → AB → B → AB → A and the rotor steps from position 1 → 2 → 3 Table 1. Excitation sequences for different drive modes → 4 → 5 → 6 → 7 → 8. This results in angular movements that are half of Normal those in 1- or 2-phases-on drive Wave Drive full step Half-step drive modes. Half stepping can reduce a Phase 1234 1234 12345678 phenomena referred to as resonance A • ••• •• which can be experienced in 1- or 2- B •••••• phases-on drive modes. A • •• ••• B • •• •••

4 is measured at each step position. If Torque vs, Speed Positional the angle from the initial position to Characteristics Angle Accuracy the N-step position is Θ and the Deviation ∆Θ N The torque vs speed characteristics are error is N where: the key to selecting the right motor ∆Θ ∆Θ × and drive method for a specific N = N - (step angle) N. application. These characteristics are The positional error is the difference dependent upon (change with) the of the maximum and minimum but is motor, excitation mode and type of Theoretical ± Position usually expressed with a sign. That driver or drive method. A typical is: “speed – torque curve” is shown in ±1 ∆Θ ∆Θ figure9. positional error = ⁄2( Max - Min) To get a better understanding of Hysteresis Error this curve it is useful to define the Hysteresis Positional Error different aspect of this curve. Figure 9. Positional accuracy of a stepper The values obtained from the measure- motor. ment of positional errors in both Holding torque directions. The maximum torque produced by the motor at standstill. Torque Holding Torque Mechanical Parameters, Pull-In Curve Pull-out Load, Friction, Inertia The pull-in curve defines a area refered Torque The performance of a stepper motor to as the start stop region. This is the Curve system (driver and motor) is also maximum frequency at which the Pull-in Torque Slew highly dependent on the mechanical motor can start/stop instantaneously, curve parameters of the load. The load is with a load applied, without loss of Region defined as what the motor drives. It is synchronism. Start-Stop Region typically frictional, inertial or a Speed Max Start Rate P.P.S. combination of the two. Maximum Start Rate Max Slew Rate Friction is the resistance to motion The maximum starting step frequency due to the unevenness of surfaces with no load applied. Figure 10. Torque vs. speed characteristics which rub together. Friction is of a stepper motor. constant with velocity. A minimum Pull-Out Curve torque level is required throughout The pull-out curve defines an area the step in over to overcome this refered to as the slew region. It defines The shape of the speed - torque friction ( at least equal to the friction). the maximum frequency at which the curve can change quite dramatically Increasing a frictional load lowers the motor can operate without losing syn- depending on the type of driver used. top speed, lowers the acceleration and chronism. Since this region is outside The bipolar type drivers increases the positional error. The the pull-in area the motor must which Ericsson Components produces converse is true if the frictional load is ramped (accelerated or decelerated) will maximum the speed - torque lowered into this region. performance from a given motor. Most Inertia is the resistance to changes motor manufacturers provide these in speed. A high inertial load requires Maximum Slew Rate speed - torque curves for their motors. a high inertial starting torque and the The maximum operating frequency of It is important to understand what same would apply for braking. In- the motor with no load applied. driver type or drive method the motor creasing an inertial load will increase The pull-in characteristics vary also manufacturer used in developing their speed stability, increase the amount of depending on the load. The larger the curves as the torque vs. speed charac- time it takes to reach a desired speed load inertia the smaller the pull-in teristics of an given motor can vary and decrease the maximum self start area. We can see from the shape of the significantly depending on the drive pulse rate. The converse is again true curve that the step rate affects the method used. if the inertia is decreased. torque output capability of stepper The rotor oscillations of a stepper motor The decreasing torque output as motor will vary with the amount of the speed increases is caused by the friction and inertia load. Because of fact that at high speeds the inductance this relationship unwanted rotor oscil- of the motor is the dominant circuit lations can be reduced by mechanical element. damping means however it is more often simpler to reduce these unwanted oscillations by electrical damping methods such as switch from full step drive to half step drive.

5 Single Step Response and Stepper motors can often exhibit a Resonances phenomena refered to as resonance at Angle The single-step response character- certain step rates. This can be seen as a istics of a stepper motor is shown in sudden loss or drop in torque at cer- tain speeds which can result in missed figure 11. O When one step pulse is applied to a steps or loss of synchronism. It occurs stepper motor the rotor behaves in a when the input step pulse rate coin- manner as defined by the above curve. cides with the natural oscillation The step time t is the time it takes the frequency of the rotor. Often there is a Time resonance area around the 100 – 200 motor shaft to rotate one step angle tT once the first step pulse is applied. pps region and also one in the high This step time is highly dependent on step pulse rate region. The resonance Figure 11. Single step response vs. time. the ratio of torque to inertia (load) as phenomena of a stepper motor comes well as the type of driver used. from its basic construction and there- Since the torque is a function of the fore it is not possible to eliminate it displacement it follows that the accel- completely. It is also dependent upon eration will also be. Therefore, when the load conditions. It can be reduced moving in large step increments a by driving the motor in half or micro- high torque is developed and stepping modes. consequently a high acceleration. This can cause overshots and ringing as shown. The settling time T is the time it takes these oscillations or ringing to cease. In certain applications this phenomena can be undesirable. It is possible to reduce or eliminate this behaviour by microstepping the stepper motor. For more information on microstepping please consult the microstepping note.

6