AN380 - STMicroelectronics

APPLICATION NOTE

HOW TO DRIVE DC MOTORS WITH SMART POWER ICs

by Herbert Sax

No other motor combines as many positive char-

acteristics as the direct current design: high effi-

ciency, ease of control & driving, compactness

without sacrificing performance and much more.

And DC motors can be controlled in many ways --

open loop current control, variable voltage control

or closed-loop speed control -- providing great

flexibility in operationalcharacteristics.

Before we turn to a detaileddiscussion of the va-

rious methods of control,it is worthwhile recalling

a fewbasics.

DC MOTOR BASICS

Generally speaking, the electric equivalent circuit

of a motor (figure 1) consists of three compo-

nents: EMF, L and RM.

The EMF is the motor terminal voltage, though

the motor is always a generator, too. It is of no

significance whether the unit operates as a motor

or a generator as far as the terminal voltage is

concerned.The EMF is strictly proportional to the

speed and has an internal resistance of zero. Its

polarity represents the direction of motion, inde-

pendentof the motor voltage applied.

The winding inductance, L, is the inevitable result

of the mechanicaldesign of the armature. Since it

hinders the reversal of current flow in the arma-

ture, to the detriment of torque as speed in-

creases, the winding inductance is an interfer-

ence factor for the motor. It also obstructs rapid

access to the generator voltage (EMF).

Motors of coreless, bell armature or pancake de-

sign are considerably less susceptible to winding

inductance.The smaller mass of these motors im-

proves their dynamic performance to a significant

extent. On the positive side, the winding induc-

tance can be used to store current in pulse-width

modulation(PWM) drive systems.

The winding resistance, RM, is purely an interfer-

ence variable because losses that reduce the de-

gree of efficiency increase as the load torque on

the motor shaft increases, the latter being propor-

tional to the current IM. It is also due to the wind-

ing resistance that the speed of the motor drops

as load increases while the terminal voltage Vs

remains constant.

Some of the mathematical relationships are

shownbelow in simplified form:

EMF = VS-(IM.RM)

Motor current IM= (Vs- EMF)/RM

Efficiency= EMF⋅IM

VS⋅IM=POUT

The drive torque at the motor shaft is proportional

to the motor current IM. Figure 2 shows the rela-

tionships graphed in a form commonly used for

DC motors. It is because of bearing and brush

friction that the efficiency tends towards zero at

lowload torques.

AN380/0591

There are many ways to control DC motors. Open-loop current control acts directly on torque

and thus protects the electronics, the motor and the load. Open-loop variable voltage control

makes sense if the motor and electronics are not overloaded when the motor stalls. Open-loop

variable voltage control with a current limiting circuit constitutes the simplest way of varying

speed.However, a closed-loopsystem is neededif precisionis called for in selecting speeds.

Figure1: Electricalequivalent circuit of a DC mo-

tor, consisitingof EMF, the winding in-

ductanceL andthe windingresistance

RM.

1/16

These basics show that essentially there are only

two parameters governing how an electrical

change can be made to act on the motor shaft:

a) with the current to vary the torque

b )with the mapping of the EMF on the speed

On account of the winding resistance RM, open

loop variable voltage control exercises no more

than an indirect effect on torque and speed and

can therefore be used only for simple functions

(speedvariation).

A number of sample applications using smart

power ICs and illustrating open-loop variable volt-

age or current control and closedloop speed con-

trol are discussed here. All of these circuits permit

the motor to run in both directions. The modifica-

tions needed for unidirectionaloperationare slight

and generally involve a simplification of the de-

sign.

OPEN-LOOP VARIABLE VOLTAGE CONTROL

In technical terms variable voltage control is the

simplest to implement. Its main scope of applica-

tionis in simple transportor drive functionswhere

exact speed control is not essential. Applications

of this kind are found, for example, in the automo-

bile industry for driving pumps, fans, wipers and

powerwindow lifts.

The circuit shown in figure 3 is an example of a

variable speed motor with digital direction control.

The motor voltage can be controlled via an ana-

log input. If the polarity of the control signal is the

variable that determines the motor’s direction of

rotation-- as is usually the case in servo systems,

for example -- the design shown in figure 4 can

be used.

One of the operational amplifiers is responsible

for the VM/VIN voltage and the other has an gain

of 1, so that the voltage losses Vs - VMare di-

vided evenly between the two parts of the

bridge.

Equivalents to the circuits in figures 3 and 4 are

shown in figure 5 and figure 6; these latter cir-

cuits, however, are switchmode and their effi-

ciency is thus improved to a considerableextent.

Figure2: Relationshipbetweenspeed, efficiency

and motor currentof a DCmotor.

Figure3: Circuit for drivinga variable-speed motor. Where the enable functionis needed,the type

L6242 can be used.

APPLICATION NOTE

2/16

Figure4: A typicalcircuit for driving servo system.

Figure5: Equivalent circuit to that in Figure 3, but using PWM.

Figure6: Equivalent circuit to that in Figure 4, but using PWM.

APPLICATION NOTE

3/16

OPEN-LOOP CURRENT CONTROL

Open-loop control is called for whenever a motor

has to supply a constant or variable torque. Appli-

cations include the head motors in tape recorders

or the motors used to tension threads when textile

fibers are wound onto spools. The speed of the

motor at any given time is of no significance. In

applications of this nature the motor shaft will

often rotate in the direction opposite to that deter-

minedby the current.

Two conditions are particularly important in a cur-

rent controlled application. The circuit will not op-

erate unless VMmax > EMF + (IMRM), if the motor

shaftis running in the same directionas the drive.

The equation applicableto a counter rotating mo-

tor shaft is: -VMmax -EMF ≤IMRM

Open-loop current control is often used in con-

junctionwith open-loopvariable voltagecontrol or

closed loop speed control. Such an arrangement

would be designed to:

♦limittorque to protect the load and the motor

♦protect thepower ICs against overload

♦obtainacceleration and deceleration

characteristicsindependentof speed.

Figure 7 shows the simplest form of open-loop

current control with a positive & negative supply.

Transferring the circuit to a bridge eliminates the

ground at one end of the shunt RSand a way of

differentially sampling the sense resistor voltage

must be found. One solution is shown in figure 8.

As in figure 4, the second half of the bridge oper-

ates as a voltage inverter.

Figure8: This circuit permits differentiated sampling of the voltage at the sense resistor.

Figure7: Current control circuit with bipolar voltage supplyin itssimplest form.

APPLICATION NOTE

4/16

When the principle behind the circuitshown in fig-

ure 8 is transferred to a switchmodecircuit (figure

9), a considerable degree of complexity is called

for to reduce power loss. For this reason the cir-

cuit is shownin slightly simplified form.

Operational amplifier 1 reconstructs the current

proportional voltage VRS to ground as shown in

figure 7. Two sense resistors are needed, as oth-

erwiseit would not be possibleto detect the direc-

tion of the currentin the bridge.

Operating as a PI controller and converting the

error signal in a PWM via comparator 3, OP2

compares the reference and feedback values.

One major advantage of a circuit such as that

shown in figure 9 is its high transfer linearity

maintained even in the vicinity of the zero current

crossing. Open-loop current control also functions

with a generator, the motor returning its own ki-

netic energy and that of the load to the supply

voltage in a controlledmanner. Braking is a case

in point, and for this reason circuits of this design

are usually found in servo positioning drives that

demand precise current control over a wide oper-

atingrange.

CLOSED-LOOP SPEEDCONTROL

Many circuits, often of completely different de-

sign, have been developedfor closed-loop speed

control. The most suitable system has to be cho-

sen on the basis of the requirements that a drive

concept has to meet. These requirements also

determine how the speed will be sensed and

processed.

Figure9: Operatingprincipleof the circuit of figure 8 transferredto a PWM arrangement.

APPLICATION NOTE

5/16

The table provides an overview of the most com-

mon principles of sensing and processing and their influence on control characteristics and sys-

tem costs.

CHARACTERISTICS

PRINCIPLE OF SPEED SENSING SIGNAL PROCESSING ACREFERENCE

Tacho V-I

Control EMF

Sense AC

Tacho

Commu

tation

Sense

Control PI

Control PID

Control PLL

Control Digital

Sensor

CONTROL

ACCURACY

HIGH •• ••

MEDIUM •• ••

LOW ••

EXTENDED CONTROL

RANGE POSSIBLE •• •

CONTROL

REACTION FAST ••• •••

SLOW •• •

GOOD CONTROL

CHARACTERISTICS

AT LOW SPEEDS •• ••

SUITABLE FOR

SERVO DRIVES •••••

SYSTEM

COST

HIGH •••

MEDIUM •• •

LOW ••••

CLOSED-LOOP CONTROL PROCESSES

DC Tachogenerator

Since a control circuit with a DC tacho-generator

yields a direct voltage that is proportional to

speed, the circuit itself is less complex than all

other designs. Nonetheless, high precision -- a

constant voltage with low ripple -- signifies high

cost. On the otherhand, the actual electroniccon-

trol circuit is simplicity itself, as figure 10 shows.

The bridge extension for a simple supply voltage

is identical to that shown in figure8.

A closed loop current control system providing

braking and acceleration independent of the sup-

ply voltage and the internal motor resistance is

easy to superimposeon the circuit(figure 11).

Similarly little difficulty is involved in modifying the

circuit in figure 10 to yield a switched bridge, be-

cause the process entails no more than convert-

ing the control error signal into a PWM output (fig-

ure12).

Figure10: Controlwith DC tachogenerator:a direct speedproportional DC voltage is generated.

APPLICATION NOTE

6/16

V-I Control (Internal Resistance Compensa-

tion)

V-I control is based in the principle that the volt-

age drop at the motor internal resistance IM, that

increases with load torque can be compensated

by increasing the motor voltage VM(figure 13).

However, compensationis less than complete be-

cause the winding resistance RMis heavily de-

pendent on the temperature, and brush resis-

tancemodulationmakes itself felt as an additional

interferencevariable.

In practice this means that the voltage drop is

slightlyunder compensatedand positive feedback

is reducedeven further as frequenciesget higher.

The control action result improves with the ratio of

EMF to IM.RM. A sample circuit in which the effect

of the positive feedback loop can clearly be seen

isshown in figure 14.

Figure11: In this circuit, accelerationand braking behavioris independentof thesupply voltageand the

motor’sinternal resistance.

Figure12: PWM conversion of the control error signal.

APPLICATION NOTE

7/16

The desired speed is set with the aid of R1 and

R2. The relationshipis expressed as:

EMF= VIN .R1/R2

The value selected for RSis one tenth of RMand

VRS is amplified by a factor of 10 in OP2 (R5 =

R4/10).

The output voltage of OP2 is then identical with

the voltage drop at RM. When R1 = R3, the inter-

nal resistance is compensated by 90%. Residual

control instabilitiescan be cancelledout by C1.

The circuit can also be extended to a bridge, al-

though this entails relocating resistor RS(figure

15). It is surprising that the V-I controller circuitry

is again simplified to a considerable extent if am-

plification is not needed. The V-I control concept

can be adapted for a PWM motor control system;

the functional layout is rather complex, however,

as figure 16 shows. Even so, it is worthwhile in

many instances because DC tacho-generators

areexpensive.

Figure13: The principle of V-I control.

Figure14: Examplecircuit in which the positivefeedback loop can clearly be seen.

APPLICATION NOTE

8/16

Figure15: Circuit as in figure14, expandedto include a bridge.

Figure16: The principle of V-I controltransferred to a PWM motor circuit: complexity is increasedsignifi-

cantly.

APPLICATION NOTE

9/16

EMF Sensing

The EMF can also be senseddirectly, rather than

be simulated as in the V-I controlsetup, when the

current IMis zero (EMF = VM-IM.RM). To achieve

this the motor current must be switched off as

quickly as possible. Motor inductance represents

an obstacle since the energy it stores must first

be dissipated before an EMF measurement can

be made at the motor terminals. This is the rea-

son why only coreless motors of bell armature or

pancake design are suitable. Figure 17 is a block

diagram showing how the EMF can be sensed.

In the major partial time t1 the motor carries cur-

rent.This is followed by a time window t2 in which

the motor is de-energized and the motor induc-

tancedischarges. There then follows a shortsam-

pling phase t3 in which the EMF is sensed and

stored in a capacitor until the next sampling

phase. The number of cycling cycles per second

depends on the dynamic behavior of the load

torque. The interval between any two EMF mea-

surements should be of a duration such that the

kinetic energy of the drive system bridges a load

change without a significant speed drop. Figure

18 illustrates a layout using a current-controlled

output stage that has a high impedance output

when the input is open.

The circuit for sensing EMF is particularly well

suitedto switchmode motor control schemes. The

monolithicswitching output stagesavailable today

already have an enable input for releasing the

motor, but the concept will usually accommodate

this option even if discrete output stages are

used. An example circuit is shown in simplified

formin figure 19.

Figure17: Principlebywhich the EMF can be sensed.

Figure18: Drivercircuitwith currentcontrolledoutputstagewith highimpedanceoutputwheninputisopen.

APPLICATION NOTE

10/16

AC Tachogenerator

Economic and with a signal that is easy to proc-

ess, the AC tachogenerator is the most fre-

quently used meansof sensingthe speed of a DC

motor. Problems arise, however, when the

tachogenerator frequency is low, due either to a

low speed or a lack of poles on the generator.

However, multiple pole tachogenerators are ex-

pensive regardless of whether they are magnetic

or optical. Most circuits convert the speed propor-

tional tacho frequencyback into a DC signalin an

f/V converter(Fig. 20).

However, some circuits make use of the propor-

tional relationshipbetween speed and AC voltage

amplitude when the tachogenerator is inductive

(figure 21). Accuracy is wanting to a certain ex-

tent in thisarrangement.

Since the output signal of an AC tachogenerator

contains no information concerning the direction

of rotation, the control loop functions in only one

quadrant. For the same reason it is common

practice to controlthe reference in a single quad-

rant. A separate digital signal determines the di-

rection of rotation. Figure 22 shows a typical

Figure19: Circuit as in figure18, but with PWMoutput stage.

Figure20: The tachogeneratorfrequencycan beconvertedback intoa DC signalin an f/V converter.

APPLICATION NOTE

11/16

PWMcircuit.

Comparator 1 converts the sinusoidal

tachogeneratorsignal into a squarewavevoltage

that triggers the monostable.The ON time is con-

stant, which means that the DC average in-

creases proportionally as the tachogeneratorfre-

quency increases. The error amplifier OP1 also

functions as an integrator (C1) and compares the

DC reference with the DC average of the monost-

able output. A DC signalsuperimposed by a trian-

gular wave AC voltage component can be de-

tected at the output of OP1.

An analog power operational amplifier can also

be used instead of the switchmode output stage.

In an arrangementlike this, the output of the error

amplifier OP1 drives the VIN input of the output

stage as shown in figure 3.

Figure 21: Alternatively,the proportionalitybe-

tweenspeed and AC amplitudecan

be used if the tachnogeneratoris in-

ductive.

Figure22: In this PWM circuit the comparator1 converts the sinusoidaltachogeneratorsignal intoa

squarewave.

APPLICATION NOTE

12/16

COMMUTATION Sensing

Commutation sensing is a process that exploits

the inherent ripple of the EMF of the motor cur-

rent as an AC tachogenerator.However, onlymo-

tors with few poles yield an adequate signal-to-

noise margin. Three-pole motors with an AC

component equal to approx 30% of the DC value

are most suitable(figure 23).

The rapid current reversal is differentiated and

used as an equivalent tachogeneratorsignal (fig-

ure 24). The rest of the circuit follows the pattern

shown in figure 22, although only one output

stage of the type shown in figure 3 is used. A

switchmode output stage would interfere with the

ripple sensing so is not recommended.One draw-

back of commutation sensing is the exceptionally

low tachometerfrequency.A three pole motor,for

example, produces a frequency of 200Hz at a

speedof 2000 rpm.

Since the AC component of the OP1 error ampli-

fier output signal (figure 22) should not be more

than 10% of the DC component at rated speed

and nominal load torque, the integrator time con-

stant C1R1 is very large. Control response is

sluggish and no longer suitable for rapid load

changes.

Assistance can be obtained by superimposing V-I

control which has high-speed response to relieve

the tachogenerator control loop and accelerate

transient response by a considerablemargin. Fig-

ure25 showsa samplecircuit for a bridge.

Superimposed V-I control canalso be used with a

real AC tachogenerator to improve the transient

load response.

Figure23: Principleof commutationsensing.

Figure24: The fastest current reversal is commutatedand used as a substitutetachogeneratorsignal.

APPLICATION NOTE

13/16

Processingthe TachogeneratorSignal

The control principle (figure 26) applied in proc-

essing the speed feedback and reference signals

in a controlled system depends on a number of

factors(table page 6).

Figure25: Examplecircuitfor abridge.

Figure26: P, I, PI and PID controllers.

APPLICATION NOTE

14/16

The criteriagoverning the selection of a P control-

ler, an I controller, a PI controller or a PIDcontrol-

ler are as follows: stability of the control loop, re-

action time, transient response, load behavior,

speed range and control factor. For example, if

the reference signal is a frequencyit would make

sense to use an AC tachogeneratoras the feed-

back value sensor and process both signals on a

purely digital level. Powerful microcontrollers or

digital signal processors are used.

In special cases that demand a control error of

zero -- for example, when two drive shaftshave to

be phase synchronized as well as running at the

same speed -- PLL control is the only option. A

system of this nature compares reference and

feedbackvalue for phase as well as frequency.In

turn, of course, the AC tachogeneratormust meet

extreme requirements regarding phase stability

since any jitter would be interpreted as a control

error, producing a spurious response in the sys-

tem.

PLL speed control systems are used in video re-

corders, floppy and hard disk drives and in a

number of industrial drive systems. Figure 27

shows a typical PLL speed control circuit. The fre-

quency comparator is phase comparator 2 of the

HCF4046 CMOS PLL circuit.

Figure27: TypicalPLL circuitfor controllingspeed.

APPLICATION NOTE

15/16

Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the

consequences of use of such information nor for any infringement of patents or other rights of third parties which may resultfrom itsuse. No

license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications men-

tioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied.

SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without ex-

press written approval of SGS-THOMSON Microelectronics.

1995 SGS-THOMSON Microelectronics - All RightsReserved

SGS-THOMSON Microelectronics GROUP OF COMPANIES

Australia - Brazil- France - Germany -Hong Kong - Italy - Japan - Korea -Malaysia - Malta - Morocco - The Netherlands - Singapore-

Spain - Sweden - Switzerland - Taiwan - Thaliand - United Kingdom - U.S.A.

APPLICATION NOTE

16/16