Prepared By: Ahmad Firdaus Bin Ahmad Zaidi

Prepared By: Ahmad Firdaus Bin Ahmad Zaidi

A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical rotational movements. Stepper motor mainly used when motion and position have to be precisely controlled

Stepper motor rotate in discrete steps, each corresponding to a pulse supplied to one of its stator windings Depending on design stepper motor can advance by 90, 45, 18deg or a fraction of degree per pulse By varying pulse rate, the motor can be made to advance slowly, one step at the time or to rotate stepwise at high speed as 4000rpm Stepper can rotate cw or ccw depending upon sequence of pulses applied to windings

Whenever discrete steps are required High torque at low speeds When precision positioning is required If torque needs to be held To eliminate closed loop control (stepper motors can operate in open loop)

Machine Tools X-Y and X-Y-Z Positioning Process Control Main Conveyor Drive Assembly Line parts positioning Business Machines Copy Machine- lens positioning and paper feed Computer Peripherals Printer- positioning the matrix print head

Elementary stepper motor in figure consists stator having three salient poles and a 2-pole rotor made of soft iron Windings successively connected to a DC power supply by means of three switches A,B,C When all switch open, rotor can take up any position When switch A closed, resulting magnetic field created by pole 1 will attract rotor and so it will line up as in figure If open A while simultaneously close B, rotor will line up by pole 2 (rotor turn ccw by 60deg)

Thus rotor can be made to advance in ccw in 60deg steps by closing and opening switches in sequence A-B-C-A-B-C.. Rotation can be reversed by operating switches in reverse sequence A-C-B-A-C-B.. To fix final position of rotor, the last switch that was closed in the sequence must remain closed to hold rotor in last position and prevent it from moving under influence of external torque (motor then will remain locked provided external torque does not exceed holding torque)

Step angle is the angle of motor shaft turned at one step when given one pulse Step angle: (NST-NRT)/(NST x NRT) x 360 NST = number of teeth at stator NRT = number of teeth at rotor Steps per revolution is the number of steps needed for the motor shaft to turn one complete revolution or 360

1. Variable-reluctance 2. Permanent-magnet 3. Hybrid

Variable-reluctance motors are not very common but principle of operation is similar to elementary stepper motor Structure of rotor and stator is further modified to obtain smaller angular steps Rotor made of soft iron and can have multiple teeth The teeth created thereby constitute real salient poles of the rotor can be as many as 100 Stator wound type, often has four, five or eight main poles

Pole-faces of the stator also slotted to create a number of teeth, which are the real salient poles on the stator For given drive system, the number of teeth on the rotor and stator determines angular motion per step Steps 18deg, 15deg, 7.5deg, 5deg and 1.8deg is common

Figure: Construction of stator lamination stack assembly. Teeth are formed at pole face constitutes as real salient pole of the stator

Construction is similar with variable-reluctance type except rotor has permanent magnet with alternating north and south poles situated in a straight line parallel to the shaft Figure below shows permanent magnet motor having 4 stator poles and 6 rotor poles Due to permanent magnets, motor develops detent torque which keeps rotor in place even when no current flows in stator winding Stator wound type stator A1 & A2 coil in series, similar for B1 & B2 Direction of rotation depends on direction of current flow which affect polarity of stator poles

Hybrid stepper motor have two identical soft-iron armatures mounted on the same shaft Armatures are indexed so that the salient poles interlap Figure shows a hybrid stepper motor with two 5-pole armatures that are driven by a 4-pole stator Permanent magnet is sandwiched between the armatures This produces unidirectional axial magnetic field, resulting all poles in armature 1 are N while poles in armature 2 is S This type of motor also produces small detent torque because of permanent magnet Like PM type, direction of rotation depends on direction of current flow in stator windings

Open loop The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control. Brushless Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing. Incremental steps/changes The rotation angle of the motor is proportional to the input pulse.

Amount, speed, and direction of rotation of a step motor are determined by appropriate configurations of digital control devices. Major types of digital control devices are: Motor Drivers, Control Links, and Controllers. The Driver accepts clock pulses and direction signals and translates these signals into appropriate phase currents in the motor. The Indexer creates the clock pulses and direction signals. The computer or PLC (programmable logic controller) sends commands to the indexer.

Suppose motor operates at no-load, rotor has low inertia, small amount of bearing friction and initially facing pole 1 At the moment A opens and B closes, rotor will start accelerating ccw toward pole 2 It rapidly picks up speed and soon reaches centerline of pole 2 Instead of resting, it overshoot centerline, and thus magnetic field in pole 2 pull it in opposite direction, braking rotor Rotor will halt and start moving in cw direction, and again will overshoot centerline pole 2 and field will again exert a pull in ccw direction Rotor will oscillate like pendulum around center line of pole 2 and the oscillation will die out because bearing friction

In moving from pole 1 to pole 2, rotor oscillates around its 60deg position before coming to rest. Speed is zero when rotor reach limit of overshoot.

When inertia greater (by mounting flywheel on shaft) overshoot is greater and rotor takes longer to settle down.

Oscillations can be damped by increasing friction. Damping is accomplished using eddy-current brake or viscous damper. Viscous damper uses fluid such as oil or air to brake rotor from moving. Braking effect in viscous damping is proportional to speed. Figure shows previous system response when viscous damping added.

The system response when rotor is coupled to mechanical load while it is moving. It takes longer to attain 60deg position. Overshoot is smaller and oscillations are damped more quickly.

In summary, mechanical load and inertia will increase stepping time. Oscillations also prolong the time before rotor settles down. Therefore, for fast stepping response, inertia and load should be small as possible while oscillations can be suppressed using viscous damper Stepping time can also be reduced by increasing current in the winding However current is limited by the thermal consideration of copper loss

Figure shows current pulses Ia, Ib and Ic and instantaneous position of its rotor when motor makes one half revolution The motor is assumed to have some inertia and its driving mechanical load In the figure, duration of pulse is 8ms Hence stepping rate is 125 per sec One revolution requires 6 steps, so it takes 6/125 = 0.048 sec to complete one turn Average speed is 60/0.048 = 1250rpm Stepper motor rotates in start-stop jumps

In start-stop operation, there is upper limit to permissable stepping rate If pulse rate of current in winding too fast, rotor may not be able to accurately follow pulses and steps will be lost To maintain synchronism rotor must be allowed to settle down before advancing to next position Refer to previous figure, interval between successive steps must be at least 6ms due to settling down of 6ms This means stepping rate limited to 167 sps

Figure shows start-stop and slewing characteristic of typical stepper motor Curve1: start-stop curve with only inertia Curve2: same as 1 but with added load inertia 2kg.cm2 Curve3: slewing curve Start-stop stepping mode sometimes referred as start without error and its characteristics shown in curve1

Stepper motor can be made to run at uniform speed without starting and stopping at every step = slewing mode At uniform speed, inertia effect is absent For a given stepping rate, motor can carry greater load torque when slewing Next figure shows the difference between startstop mode and slewing mode, where in both cases motor is turning at average speed of 250sps Motor will cover same number of sps, namely 1 step every 4ms but angle increases smoothly when slewing

When motor is carrying a load, it cannot suddenly go from zero to 5000sps In the same way, motor that is slewing at 5000sps cannot brought to dead stop in a step Thus to bring up motor speed ans similarly to stop a motor running at high speed, it must be accelerated/decelerated gradually-subject to condition that instantaneous position of rotor must correspond to number of pulses This process is called ramping During acceleration phase, ramping consist of progressive increase in number of driving pulse per sec Ramping usually completed in fraction of second Generated by power supply that drives the stepper motor

Stepper motors use either bipolar or a unipolar winding on the stator In 4-pole stator, bipolar winding consists of two coil sets A1, A2, B1 and B2 as shown in figure

Current Ia reverses periodically and the same goes for Ib Coils are excited by a common dc source, and because current pulses Ia and Ib must alternate, switching is required Switches are represented by Q1 to Q8 Coils can be excited sequentially in 3 different ways: Wave drive Normal drive Half-step drive Micro-step drive

In wave drive only one set of coils is excited at a time Below is switching sequence for cw rotation Flux produced by Ia and Ib rotates by 90deg per step 1 2 3 4 1 Q1 Q2 on on Q3 Q4 on Q5 Q6 on Q7 Q8 on

In normal drive, both sets of coils are excited at a time Below is switching sequence for cw rotation Flux is oriented midway between poles as each step, however it still rotates 90deg per step Slightly greater torque than wave drive 1 2 3 4 1 Q1 Q2 on on on Q3 Q4 on on Q5 Q6 on on on Q7 Q8 on on

Combining wave drive and normal drive Flux now rotates at 45degrees per step Main advantage: improving resolution of position tends to reduce problems of resonance 1 2 3 4 5 6 7 8 1 Q1 Q2 On On On On Q3 Q4 On On On Q5 Q6 On On On Q7 Q8 On on On

In practice, current pulse in a winding does not rise immediately to its rated value at beginning of pulse and does not drop immediately to zero at the end of pulse interval If winding has inductance of L in henrys and resistance of R ohms, time constant T 0 equal to L/R seconds Let the coil be connected to a dc source of E volts by means of transistor A diode is connected across the windings to prevent high induced voltage from destroying the switching transistor at the moment it interrupts current flow When transistor switched on, transient current i1 reaches rated value I=E/R after 3 time constants When transistor turns line current off, transient current i2 continues to flow in coil for about 3T0 secs

Two important facts here: Current does not immediately rise to final value when transistor turned on so initial torque developed by stepper motor is smaller than normal. Rotor does not move as quickly as expected. When transistor turned off, current i2 continues to circulate in the coil/diode loop. Effective duration of pulse is Tp + 3T 0. This means we cannot switch from one coil to the next as quickly as we thought

Shortest possible pulse that permits the current to rise to its value I has length 6T 0. Windings of stepper motor have time constants T 0 ranging from 1ms to 8ms. Duration of steps can be no shorter than 6 x 1ms = 6ms Corresponds to 166 sps, still considered slow. One way to quicken stepping rate is to reduce T 0 by adding external resistance to motor windings and raising dc voltage so same rated of current will flow

External resistor has value 4 times of the coil resistance R, and dc voltage raised from E to 5E volts T 0 drops by factor of 5 (L/R to L/5R) hence stepping rate can be increased by the same factor Drawbacks of solution: Power supply is more expensive to deliver 5 times power Efficiency of system is low because lots of power wasted in external resistor. Fast-acting stepper motor in 100W must be driven by other means

The basic function of a motor driver is to provide the rated motor phase current to the motor windings in the shortest possible time. There are currently three types of motor drivers: Bilevel drivers L/R drivers PWM (Chopper) drivers

Bilevel drive enable us to obtain fast rise and fall times of current without using external resistors In the figure, switches Q1 and Q2 represent transistors that open and close the circuit Winding assumed to have R=0.3Ω, inductance = 2.4mH and rated current=10a Power supply is 60V with a tap at 3V Switch Q1 initially closed (a). Current pulse initiated by closing Q2 (b). Time constant for this circuit (b) is 8ms

Initial rate of rise corresponds to straight line OP that reaches 200A in 8ms (c). Hence current in coil rises at rate of 25000A/s Time to reach 10A is 0.4ms As soon as current reach this rated value, Q1 opens, forces current to follow new path in (d) Current is now fed by 3V source and remains fixed at 10A Current stays at this value until we want to end the pulse

To end pulse, we open Q2, forcing current to flow in path in (e) 57V source tries to drive current through the coil that opposite to i, thus decreasing i to zero at rate of 23750A/s. Hence current becomes zero after 0.42ms The moment current reaches zero, Q1 closes Pulse shape and Q1,Q2 switching sequence is shown in (f)

Chopper drives are also used in addition to bilevel drives Most stepper motors are coupled to a lead screw which permits rotary motion converted to linear displacement Suppose a stepper motor having 200 steps per revolution coupled to lead srew having pitch 5threads per inch The motor has to make 200x5 = 1000 steps to advance linearly by 1 inch Hence each step contributes displacement of 0.001in

Microcontroller is normally used to perform the control for the bilevel drive and chopper drive operation for the stepper motor Microcontrollers are often used to control stepper motors because: Compatible with the discrete movements of steppers Fast Can easily be programmed to work with steppers of other types Can produce the waveform used to drive small stepper motors

Rotation angle is proportional to number of step of input pulse Motor has full torque at standstill Precise positioning and repeatability of movement Excellent response start/stop/reverse Open loop control possible Realize a wide range of speeds Inexpensive relative to other motion control systems Easy to set up and use Overload safe. Motor cannot be damaged by mechanical overload

Low efficiency. Motor draws substantial power regardless of load. Torque drops rapidly with speed. No feedback to indicate missed steps. Motor gets very hot in high performance configurations. Motor is audibly very noisy at moderate to high speeds. Low output power for size and weight.