Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems

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(continued )

set point

potentiometer

motor

tachometer

LM3524D

PWM

LMD15200

H-bridge

direction control

THREADED DESIGN EXAMPLE

DC motor position and speed controller—H-bridge driver and PWM speed control C.5

The figure below shows the functional diagram for Threaded Design Example C (see Section 1.3

and Video Demo 1.8), with the portion described here highlighted.

microcontrollers

SLAVE

PIC

MASTER

PIC

liquid crystal display

DC motor with

digital position encoder

quadrature

decoder

and counter

1 2 3

4 5 6

7 8 9

0 #

keypad

decoder

buzzer

H-bridge

driver

button

The complete circuit required for this part of the design is shown below. A commercially

available S17-3A-LV H-bridge was selected because of its ample current capacity and com-

patibility with the selected motor. It also exhibits a near linear relationship between PWM

duty cycle and motor speed over a fairly large range. Details about the H-bridge and motor can

be found at Internet Link 10.4. The two inputs to the H-bridge are a direction line to indicate

whether the motor turns clockwise or counterclockwise, and a PWM line to control the speed

of the motor via an appropriate duty cycle.

PIC16F88

RB7

RB6

DC motor

S17-3A-LV

H-bridge

J2-2

J2-3

J3-1

J3-6

DIR

PWM

VMOTOR

GND

Video Demo

1.8DC motor

position and

speed controller

Internet Lin

10.4S17-3A-LV

H-bridge

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452 CHAPTER 10 Actuators

The code used to rotate the motor at a desired speed is presented below. The subroutine

pwm_ periods calculates the pulse segment widths to create a duty cycle proportional to the

desired motor speed. The subroutine pwm_ pulse is called continuously by run_motor to keep

the motor rotating at the selected speed. The comments in the code explain some of the details

involved with the solution.

' Define I/O pin names

motor_dir Var PORTB.7 ' motor H-bridge direction line

motor_pwm Var PORTB.6 ' motor H-bridge pulse-width-modulation line

' Declare Variables

motor_speed Var BYTE ' motor speed as percentage of maximum (0 to 100)

motion_dir Var BIT ' motor direction (1:CW/Forward 0:CCW/Reverse)

on_time Var WORD ' PWM ON pulse width

off_time Var WORD ' PWM OFF pulse width

pwm_cycles Var BYTE ' # of PWM pulses sent during the position control loop

' Define constants

pwm_period Con 50 ' period of each motor PWM signal cycle (in microsec)

' (50 microsec corresponds to 20kHz)

' Initialize I/O and variables

TRISB.6  0 ' configure H-bridge DIR pin as an output

TRISB.7  0 ' configure H-bridge PWM pin as an output

motion_dir  CW ' starting motor direction: CW (forward)

motor_speed  50 ' starting motor speed  50% duty cycle

Low motor_pwm ' make sure the motor is off to begin with

' Subroutine to run the motor at the desired speed and direction until the

' stop button is pressed. The duty cycle of the PWM signal is the

' motor_speed percentage

run_motor:

' Set the motor direction

motor_dir  motion_dir

' Output the PWM signal

Gosub pwm_periods ' calculate the on and off pulse widths

While (stop_button  0) ' until the stop button is pressed

Gosub pwm_pulse ' send out a full PWM pulse

Wend

Return

'Subroutine to calculate the PWM on and off pulse widths based on the desired

' motor speed (motor_speed)

pwm_periods:

' Be careful to avoid integer arithmetic and

' WORD overflow [max65535] problems

If (pwm_period >655) Then

on_time  pwm_period/100

motor_speed

off_time  pwm_period/100

(100-motor_speed)

(continued )

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Else

on_time  pwm_period

motor_speed / 100

off_time  pwm_period

(100-motor_speed) / 100

Endif

Return

' Subroutine to output a full PWM pulse based on the data from pwm_periods

pwm_pulse:

' Send the ON pulse

High motor_pwm

Pauseus on_time

' Send the OFF pulse

Low motor_pwm

Pauseus off_time

Return

10.6 STEPPER MOTORS

A special type of DC motor, known as a stepper motor, is a permanent magnet or

variable reluctance DC motor that has the following performance characteristics: it

can rotate in both directions, move in precise angular increments, sustain a holding

torque at zero speed, and be controlled with digital circuits. It moves in accurate

angular increments, known as steps, in response to digital pulses sent to an electric

drive circuit. The number and rate of the pulses control the position and speed of the

motor shaft. Generally, stepper motors are manufactured with steps per revolution of

12, 24, 72, 144, 180, and 200, resulting in shaft increments of 30  , 15  , 5  , 2.5 , 2  ,

and 1.8  per step. Special micro-stepping circuitry can be designed to allow many

more steps per revolution, often 10,000 steps/rev or more.

Stepper motors are either bipolar, requiring two power sources or a switchable

polarity power source, or unipolar, requiring only one power source. They are pow-

ered by DC sources and require digital circuitry to produce coil energizing sequences

for rotation of the motor. Feedback is not always required for control, but the use of

an encoder or other position sensor can ensure accuracy when exact position control

is critical. The advantage of operating without feedback (i.e., in open-loop mode) is

that a closed-loop control system is not required. Generally, stepper motors produce

less than 1 hp (746 W) and are therefore used only in low-power position control

applications. Video Demos 9.2 and 10.16 show interesting student project examples

of stepper motor applications.

A commercial stepper motor has a large number of poles that define a large

number of equilibrium positions of the rotor. In the case of a permanent magnet step-

per motor, the stator consists of wound poles, and the rotor poles are permanent mag-

nets. Exciting different stator winding combinations moves and holds the rotor in

different positions. The variable reluctance stepper motor has a ferromagnetic rotor

rather than a permanent magnet rotor. Motion and holding result from the attraction

of stator and rotor poles to positions with minimum magnetic reluctance (magnetic

field resistance) that allow for maximum magnetic flux. A variable reluctance motor

Video Demo

9.2Automated

laboratory rat

exercise machine

with IR sensor and

stepper motor

10.16Multifunction

slot machine

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454 CHAPTER 10 Actuators

has the advantage of a lower rotor inertia and therefore a faster dynamic response.

The permanent magnet stepper motor has the advantage of a small residual holding

torque, called the detent torque, even when the stator is not energized.

To understand how the rotor moves in an incremental fashion, consider a simple

design consisting of four stator poles and a permanent magnet rotor as illustrated

in Figure 10.21. In step 0, the rotor is in equilibrium, because opposite poles on the

stator and rotor are adjacent to and attract each other. Unless the magnet polarities

of the stator poles are changed, the rotor remains in this position and can withstand

an opposing torque up to a value called the holding torque. When the stator polari-

ties are changed as shown (step 0 to step 1), a torque is applied to the rotor, causing

it to move 90



in the clockwise direction to a new equilibrium position shown as

step 1. When the stator polarities are again changed as shown (step 1 to step 2), the

rotor experiences a torque driving it to step 2. By successively changing the sta-

tor polarities in this manner, the rotor can move to successive equilibrium positions

in the clockwise direction. The sequencing of the pole excitations is the means by

which the direction of rotation occurs. Counterclockwise motion can be achieved by

applying the polarity sequence in the opposite direction. The motor torque is directly

related to the magnetic field strength of the poles and the rotor.

The dynamic response of the rotor and attached load must be carefully consid-

ered in applications that involve starting or stopping quickly, changing or ramping

speeds quickly, or driving large or changing loads. Due to the inertia of the rotor and

attached load, rotation can exceed the desired number of steps. Also, as illustrated

in Figure 10.22 , a stepper motor driving a typical mechanical system through one

step will exhibit an underdamped response. If damping is increased in the system,

for example, with mechanical, frictional, or viscous damping, the response can be

modified to reduce oscillation, as shown in the figure. Note, however, that even with

an ideal choice for damping, the motor requires time to totally settle into a given

position, and this settling time varies with the step size and the amount of damping.

It is also important to note that the torque required from the motor increases with

added damping. Video Demo 10.17 shows an example of a typical underdamped

second-order system response of a stepper motor with a fairly large step size. It also

shows how the response changes as the step rate increases. Video Demo 10.18 shows

step 0 step 1

step 2

permanent magnet rotor

stator with reversible polarity poles

N or S

N or SN or S

N or S

Figure 10.21 Stepper motor step sequence.

Video Demo

10.17Stepper

motor step response

and acceleration

through resonance

10.18High-speed

video of medium

speed response

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slow-motion footage of the motor turning at a medium speed to show how the step

response effects are less pronounced at higher step rates.

The torque-speed characteristics for a stepper motor are usually divided into

two regions as illustrated in Figure 10.23 . In the locked step mode, the rotor decel-

erates and may even come to rest between each step. Within this region, the motor

can be instantaneously started, stopped, or reversed without losing step integrity. In

the slewing mode, the speed is too fast to allow instantaneous starting, stopping, or

reversing. The rotor must be gradually accelerated to enter this mode and gradually

decelerated to leave the mode. While in slewing mode, the rotor is in synch with the

stator field rotation and does not settle between steps. The curve between the regions

in the figure indicates the maximum torques that the stepper can provide at different

speeds without slewing. The curve bordering the outside of the slewing mode region

represents the absolute maximum torques the stepper can provide at different speeds.

Figure 10.24 illustrates a unipolar stepper motor field coil schematic with exter-

nal power transistors that must be switched on and off to produce the controlled

sequence of stator polarities to cause rotation. The schematic in Figure 10.24 shows

six wires connected to the motor. Because the second and fifth wires are usually

connected externally as shown, manufacturers sometimes connect them inside the

motor, in which case the motor only has five external wires. The wires are usually

color coded by the manufacturers to help the user make a correspondence to the

schematic. Figure 10.24 includes a common color scheme used for a six-wire unipo-

lar stepper motor: yellow (coil 1), red (1/2 common), orange (coil 2), black (coil 3),

green (3/4 common), brown (coil 4). Another common six-wire color scheme is

Figure 10.22 Dynamic response of a single step.

with added damping

rotation

step

size

Figure 10.23 Stepper motor torque-speed curves.

torque

speed

locked

step

mode

slewing

mode

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456 CHAPTER 10 Actuators

green (coil 1), white (1/2 common), blue (coil 2), red (coil 3), white (3/4 com-

mon), black (coil 4). A common color scheme for a five-wire unipolar stepper is

red (coil 1), green (coil 2), black (common), brown (coil 3), white (coil 4). If you

come across a motor for which you have no documentation and the color scheme is

unknown, there is a testing procedure you can follow to determine the wire identi-

ties (e.g., see Internet Link 10.5 , being aware that the coils are numbered differently

in the linked Web page. Coils 3-2-1-4 in the Web page correspond to coils 1-2-3-4

or φ

-φ

- φ

here).

Figure 10.25 illustrates the construction of and stepping sequence for a four-

phase unipolar stepper motor. It consists of a two-pole permanent magnet rotor and

a four-pole stator, with each pole wound by two complementary windings (e.g., φ

and φ

wound in opposite directions on the top left pole). Table 10.1 lists the phase

sequence required to step the motor in full steps, where two of the four phases are

energized (ON) and each stator pole is polarized. Table 10.2 lists the phase sequence

for half-stepping, where between each full step only one phase is energized (ON) and

Figure 10.24 Standard unipolar stepper motor field

coil schematic.

YEL RED OR

BLK

GRN

BRN

Figure 10.25 Example of a unipolar stepper motor.

stator poles/

windings

step 1

(φ

, φ

: ON)

step 2

(φ

, φ

: ON)

step 1.5

(φ

: ON)

rotor

Internet Lin

10.5How to

identify the

different wires of a

stepper motor

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only two stator poles are polarized. The resolution or number of steps of the motor

is twice as large in the half-step mode (8 steps/rev at 45  ) than in the full-step mode

(4 steps/rev at 90 ), but the holding torque and drive torque change between two

values on alternate cycles in half-step mode. Another technique for increasing the

number of steps is called micro-stepping, where the phase currents are controlled

by fractional amounts, rather than just ON and OFF, resulting in more magnetic

equilibrium positions between the poles. In effect, discretized sine waves are applied

to the phases instead of square waves. The most common commercially available

stepper motors have 200 steps/rev in full-step mode and are sometimes referred to as

1.8 (360  /200) steppers. In micro-stepping mode, 10,000 or more steps per revolu-

tion can be achieved.

Figure 10.26 illustrates a bipolar stepper motor schematic with external power

transistors that must be switched on and off in ordered combinations to produce the

controlled sequence of stator polarities to cause rotation. There are four wires con-

nected to the motor, which easily distinguishes it from a unipolar stepper, which has

either five or six wires. The wires are usually color coded by the manufacturer to

help make a correspondence to the schematic. Figure 10.26 includes a common color

scheme used for a bipolar stepper motor: red (  ) and grey (  ) for the coil A, and

yellow (  ) and black (  ) for coil B.

Table 10.3 lists the phase sequence required to step a bipolar stepper motor in

full steps. Note that the table is the same as for a unipolar stepper (see Table 10.1),

but here sets of two transistors are switched together for each ON or OFF state

Table 10.1 Unipolar full-step phase sequence

Step 



CW 1 ON OFF ON OFF

↓ 2 ON OFF OFF ON

CCW 3 OFF ON OFF ON

↑ 4 OFF ON ON OFF

Table 10.2

Unipolar half-step phase sequence

Step 



CW 1 ON OFF ON OFF

↓ 1.5 ON OFF OFF OFF

2 ON OFF OFF ON

2.5 OFF OFF OFF ON

CCW 3 OFF ON OFF ON

↑ 3.5 OFF ON OFF OFF

4 OFF ON ON OFF

4.5 OFF OFF ON OFF

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458 CHAPTER 10 Actuators

instead of just one with the unipolar sequence. The configuration of transistors is

called an H-bridge configuration (see Design Example 10.1 and Internet Link 10.3

for more information). By switching the transistors in pairs, the current direction can

be reversed. For example, in step 1, transistors 1 and 4 are ON and transistors 2 and 3

are OFF. This applies power to coil A in the forward direction, creating a positive

current (I

> 0). Likewise, transistors 5 and 8 are ON and 6 and 7 are OFF in step

1, creating a positive current in coil B (I

> 0). In step 2, transistors 5 and 8 are

OFF and 6 and 7 are ON, reversing the direction of the current in coil B (I

< 0).

Table 10.4 lists the phase sequence for half-stepping. As with the unipolar motor,

only half of the holding torque is available in the half-step positions, because only

one coil is energized.

Figure 10.27 illustrates the structure, pole geometry, and coil connections of an

actual stepper motor in more detail. This particular stepper motor can be wired as

Table 10.3 Bipolar full-step phase sequence

Step 

and 



and 



and 



and 

CW 1 ON OFF ON OFF

↓ 2 ON OFF OFF ON

CCW 3 OFF ON OFF ON

↑ 4 OFF ON ON OFF

Figure 10.26

Standard bipolar stepper motor field coil schematic.

RED

GRY

YEL

BLK

Internet Link

10.3H-bridge

resource

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Internet Lin

10.6Stepper

motor online

resource

Table 10.4 Bipolar half-step phase sequence

Step 

and 



and 



and 



and 

CW 1 ON OFF ON OFF

↓ 1.5 ON OFF OFF OFF

2 ON OFF OFF ON

2.5 OFF OFF OFF ON

CCW 3 OFF ON OFF OFF

↑ 3.5 OFF ON OFF OFF

4 OFF ON ON OFF

4.5 OFF OFF ON OFF

Figure 10.27

Typical stepper motor rotor and stator configuration.

(Courtesy of Oriental Motor, Torrance, CA)

Figure 10.28 Actual stepper motor rotor. (Courtesy of Oriental

Motor, Torrance, CA)

a four-phase unipolar motor or a two-phase bipolar motor. Figure 10.28 shows the

50-tooth split rotor with one side having north polarity and the other having south

polarity, creating a large number of effective poles.

See Internet Link 10.6 for links to many resources and manufacturers for vari-

ous stepper motor products.

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460 CHAPTER 10 Actuators

10.6.1 Stepper Motor Drive Circuits

A drive circuit for properly phasing the signals applied to the poles of the unipolar

stepper motor for rotation in full-step mode is easily and economically produced

using the components illustrated in Figure 10.29 . A similar drive circuit can be pur-

chased as a single monolithic IC (e.g., E-Lab’s EDE1200, Signetics’ SAA1027, or

Allegro Microsystems’ UCN 5804B). The discrete circuit includes 7414 Schmitt

trigger buffers, a 74191 up-down counter, and 7486 Exclusive OR gates. The Schmitt

triggers (see Section 6.12.2) produce well-defined control signals with sharp rise and

fall times in the presence of noise or fluctuations on the direction (CW/ CCW), ini-

tialization (RESET), and single-step (STEP) inputs. The up-down counter and the

XOR gates in turn create four properly phased motor drive signals. These four digital

signals ( φ

, φ

) are coupled to the bases of power transistors that sequentially

energize the respective motor coils connected to the DC motor supply, resulting in

shaft rotation. Each square-wave pulse received at the STEP input causes the motor

to rotate a full step in the direction determined by the CW/CCW input.

The timing diagram for the two least significant output bits B

and B

of the

counter and the phase control signals is shown in Figure 10.30 . Compare the signals

7486

DN/UP

74191

RST

CW/CCW

RESET

+5 V

15 101

+5 V

7414

STEP

Figure 10.29 Unipolar stepper motor full-step drive circuit.

Figure 10.30 Timing diagram for full-step unipolar stepper motor

drive circuit.

CW CCW

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