lec5 motor control - lanka education and research …rohan/teaching/en2532/lectures/lec5...motor...

Motor ControlMotor Control

Prof. Rohan MunasingheDepartment of Electronic and Telecommunication EngineeringFaculty of Engineering

University of Moratuwa 10400

Motor Selection for the Control Board• Rated Voltage and Current

– A 6V motor isn’t happy to be powered by a 9V battery. A 12V motor runs too slowly when powered by a 9V supply. Rated voltage is to

be provided.

– Do not use 3V to 4.5V small DC motors found in many toy cars. – Do not use 3V to 4.5V small DC motors found in many toy cars. These inexpensive motors are extremely noisy and inefficient.

Frequent Board resetting will result.

• Testing a motor for compatibility with the control board

– Connect ohmmeter to the motor terminals, and gently rotate the motor shaft by hand until you obtain the smallest possible winding resistance Rmin., and calculate current I

max= V

rated/ Rmin, If motor

drives has a higher capacity > I motor can be interfaced the drives has a higher capacity > Imax

motor can be interfaced the

Board.

• Improving current handling capability (example: MIT Handy Board)

– use SN754410 quadruple half H driver, which is a plug-and-play replacement for the L293D, or stack two L293Ds by soldering

corresponding pins.

• Efficiency

– Mechanical friction, cause some electrical

energy to be wasted as heat

Small, cheap, reasonably efficient, easy to use ⇒⇒⇒⇒ ideal for small robotic applications

DC Motors

energy to be wasted as heat

– Toy motors 50%, Industrial-grade 90%

• Operating Voltage

– Is the recommended voltage you should use to power the motor

– Most motors will run fine at lower voltages, though they will be less powerful

– Can operate at higher voltages at expense of operating life

• Operating Current

– When there is no resistance to its motion, the motor draws the least amount

of current, and when there is so much resistance to cause the motor to stall,

it draws the maximum current

• Stall current: maximum current a motor can draw at its specified voltage

• Stall torque: torque at stall current

Brushed DC Motor Components

Basic Inductor Theory

dt

dILV =Ideal Inductor: Current rises at constant rate

Real Inductor: Current saturates as voltage drops

more across the resistor

Motor (Inductor) Switching OFF

• -ve high voltage builds up. Stored energy is dissipated by

producing an electric arc across the switching terminals

When a motor is running, the

armature acts as an inductor, and when the

current in the armature

changes, voltage spikes are

generated that might be of

higher voltage than the Vs higher voltage than the Vs

power supply or lower voltage

than ground.

Suppress Arc Using Diodes

• Circulating current dissipates stored energy slowly.

• Freewheeling diodesNo arc

Diodes connecting from each Diodes connecting from each

driver output to either Vs , or

ground perform the important

function of trapping and

shunting away inductive voltage

spikes that naturally occur as

part of any motor’s operation.part of any motor’s operation

Suppose all drive

transistors are “OFF”

suddenly, and as a result a

voltage greater than Vs is

One-Half of L293D/SN754410

Motor Driver Chip

Spike Cancellation

voltage greater than Vs is

generated at the motor on

the OUT–1 line. Then the

diode labeled D1

conducts, shunting this

voltage to the Vs power

supply. If the diodes were

not present, these not present, these

inductive voltage spikes

would damage sensitive

electronic components.

MOSFET Drivers

MOSFETs

• Smaller Bias Current compared to BJT

• Almost zero bias current under steady state condition (ON

or OFF)or OFF)

• Low ON Resistance

• Higher current

• Fast Switching speed (can be uses to control motor speed

via PWM)

H-Bridge Circuit

• Rotation of the DC motor can be controlled (CW or CCW)

• Mostly used 2 different channel MOS-FETs • 2 N-Channel MOS-FETs• 2 P-Channel MOS-FETs

CW Rotattion

CW

• Two MOS-FETs are ON and the others are OFF

• One MOS-FET Drive the positive supply, and the other one is driving the negative supply

CCW Rotattion

CCW

• MOS-FETs are working in the apposite condition fron the CCW rotation

• The polarity of the motor become the apposite from the CCW thus makes the DC motor turning in the apposite direction (CW)

• Two MOS-FETs are ON and the others two are OFF

• One MOS-FET Drive the positive supply, and the other one is driving the negative supply

MOSFET Driven through Optocouplers

• Optocouplers are used to separate the controller and motor ground signal

• The N-Channels are biased to +VSS

• The P-Channels are biased to -VSS

• Optocopuplers are supplied from the controller with different ground signal from the motor supply

Bias for CW Rotation

CW

1

10

0

Q4=on

Q2=on

Bias for CCW Rotation

CCW

0 1

0Q3=on

Q1=on

1

Speed Control for CW

SpeedCW

1

10

PWM

Q4=on

Q2 Pulsed

Speed Control for CCW

01

Speed CCW

PWM 1

0Q3=on

Q1Pulsed

1

DC Motor Braking

11 1

0

Q2=on

0

Q1=on

1

• We can short motor terminal by pulling all the motor terminal to the low side (-VSS) . It make the maximum current flow as the motor rotate

Prohibited Condition 1

ShortCircuit

0

Q2=on

0

Q1=on

0Q3=on

0

Q4=on


0

ShortCircuit

0

Q1=on

0

Q4=on


ShortCircuit

0

Q2=on

0Q3=on

0

Decoder

• Logic Decoder can be used to Protect H-Bridge from the Prohibited Conditions

Input A Input B Input C Input D Conditions

0 0 0 0 All Circuits short (Prohibited condition)

0 0 0 1 Short Circuit (Prohibited condition)




0 1 0 1 Motor Turn Clockwise

0 1 1 0 Brake (Upper Part is closed loop)

0 1 1 1 Circuit Open (Motor Off / Free running)


1 0 0 1 Brake (Lower Part is closed loop)1 0 0 1 Brake (Lower Part is closed loop)

1 0 1 0 Motor Turn Counter Clockwise





1 1 1 1 All Circuits Open (Motor Off / Free running)

Assignment 2

• Design the following combinational logic circuit, which will make sure that prohibited conditions won’t be generated.

Combinational

A

B Prohibited Determine ...Combinational

Logic Circuit

B

C

D

Prohibited Conditions won’t happen

required inputs

...

L298 Dual Full Bridge Driver (4A)

Motor

Control

PinsUp to 46V, 4A

Motor Terminals < 2A

5 - 7 V

Motor Control Pins

• To Drive inductive loads : solenoids, relays, DC / stepper

motors

L298 Dual Full Bridge Driver (4A)

External smoothing caps

Homework: Draw Truth Table for Out1 and Out2 as a functions of In1 and In2

• Power

– Product of the output shaft’s rotational velocity and torque

– Output power is zero when

Torque is zero: Motor is spinning freely with no load on the shaft. Rotational

velocity is at its highest, but the torque is zero, and it’s not driving any

Power and Speed of DC Motors

velocity is at its highest, but the torque is zero, and it’s not driving any

mechanism (Actually, the motor is doing some work to overcome internal

friction, but that is of no value as output power)

Rotational velocity is zero: Motor is stalled, it is producing its maximal

torque. But as there is no motion, no work is delivered onto the load

– In between two extremes, output power shows a parabolic relationship

Feedback Control• Example: Air conditioner

• Temperature sensing system (sensor or mechanism) feeds back room temperature, and compares it with the desired temperature.

Error is determined.

• Use the error signal (positive or negative) to adjust the cool airflow • Use the error signal (positive or negative) to adjust the cool airflow to the room.

• If error is +ve reduce cool air flow

• If error is –ve increase cool air flow

• There is a delay in air temperature measuring system. Delayed response can cause oscillations in room temperature.

27

Proportional Control of Motorscontrol signal is proportional to the amount of error: Generates a stronger

control signal when the present state is farther away from the goal state

At t=0, suppose that the setpoint =100 and actual position=0. Then

e(0)=100, so the motor turns at 100% power, driving the wheel toward the

desired position. As it moves, the error becomes progressively smaller.

When it is at position=50, the error is only 50, and the motor is given only

50% power. When it arrives at the intended position of 100, the error is zero,

and the motor turns off momentarily.

28

Proportional Gain (ratio between error and power): Instead of a one-to-

one ratio between error and motor %power, modify the controller so it

multiplies the error value by 5. The wheel should reach the set-point faster,

and it should resist being turned away from it much more aggressively.

Overshoot:

Response moves beyond set-point, and stops and turns back.

Oscillations:

Proportional Control of Motors

Oscillations:

After an overshoot, error becomes –ve, thus, proportional controller drives

the motor in the opposite direction. In the subsequent motion, motor might

“undershoot” as well. This overshoot and undershoot phenomena repeats

for a while, and gradually, the oscillation is expected to die out.

29

Controller Design:

Design to minimize both overshoot and oscillation, but let little overshoot to

improve system response to reach the set-point as quickly as possible.

Kp =10

Kp =10: Full-power is delivered to

motor as long as error>10. When

position falls below 10, power

command begin to fall off. Position

overshoots, and shows little oscillation

Steady State Error (Offset):

Proportional Gain Kp

Kp =20

Steady State Error (Offset):

System does not stabilize at the

set-point, but at 1. This generates

a power command of 10%, which

is too small to activate the motor

Kp =20: should ameliorate the

offset problem, since the same

30

offset problem, since the same

static error will result in a higher power command

Offset has been eliminated, but at

cost of oscillations before settling

Kp =30

Causes predominant oscillation

Kp =50

Proportional Gain Kp

Kp =30

Kp =50

Oscillation behavior has taken over.

System is too sensitive (responsive). A

slight error causes huge power delivered

through the motor. Subsequent slight

errors in +ve and –ve directions causes

Kp =50

31

sustainable swing around the set point .

While the position error is small on the

graph, the power swings vigorously.

• Proportional controller drives the wheel to the set point faster, but

results in overshoots and oscillations at higher gains

• Observation: At larger errors, velocity is small, and at smaller errors

Proportional-Derivative Control

velocity is large (as motor swings around the set point)

Use derivative control

• What if we reduce motor power by a quantity proportional to speed.

• No effect at larger errors

32

• No effect at larger errors

• Reduce power near set point (reduce oscillations)

This looks like what we need to improve response while reducing

overshoot

Kp=4, KD=1:

Overshoot is minimized, and there is

no any significant oscillatory behavior.

Kp=4, KD=1

Proportional-Derivative Control

Kp=10, KD=5:

KD is too large. Controller puts on the

brakes too aggressively and the

system stops too early before reaching

Kp=10, KD=5

33

the set-point. When the velocity hits

zero, the proportional gain kicks in

again and the system corrects

Servo Motors

Servo Motors

• The servo motor is actually an assembly of four parts:

– DC motor

– Gear reduction unit

– Position sensor– Position sensor

– Feedback control circuit

• Three wires of a servo: power, ground, control.

– The power source must be constantly applied

• Servo shaft typically does not rotate 3600 like a DC motor, but can only rotate ±1000 from the centre position.

• servo has its own power electronics, so very little power flows over the

Servo Motors cntd..

• servo has its own power electronics, so very little power flows over the control signal.

• Servo motors require a 5~6 V DC power supply. This can be taken from the control board power if the servo's aren't doing too much work. Otherwise, a separate power supply is recommended

• If available DC Voltage is higher, 1N4001 diodes can be used to drop the voltage down to around 6V.

• Servo will hold the position and resists disturbances to deflect from the • Servo will hold the position and resists disturbances to deflect from the command position.

• Servo needs a consistent (repetitive) voltage pulse to hold the position.

• Turn rate : is the time it takes to move between the two extreme positions. It is about few seconds in high torque motors

Servo Motor Control cntd..• Servo PWM method is different from the speed control PWM

– Speed control PWM: overall duty cycle (% on-time) determines the

power/speed of the motor

– Servo PWM: length of the pulse is used as the shaft position command

• Futaba S148 : 920ms (full counterclockwise), 1520ms (center), 2120ms (full • Futaba S148 : 920ms (full counterclockwise), 1520ms (center), 2120ms (full clockwise)

– The servo control pulse modulated frequency is 50Hz (20 ms period), which

means that you can command the servo in every 20ms “go here”

20ms

Servo Motor Specifications

★★★

★★

★

★★

Servo Motors on MIT Handy Board

control

control

servo 16V

servo 25V

9.6V

Winch Servo

• Winch servo rotates continuously

– Can be used for robot’s main drive motors

• Conversion

– Potentiometer is replaced by a pair of fixed resistors, and the position – Potentiometer is replaced by a pair of fixed resistors, and the position feedback is taken from the center (feedback signal becomes a

constant, and is referred to center position)

– Therefore, motor continues to turn due to error (reference - center).

– This methods allows both speed and direction control. the farther the

control signal is away from the center position, the faster the motor turns

Stepper MotorsCoil identification

• Unipolar motors (easy to drive type) have four coils and are likely to have 5 or 6 wires attached.

• Use an ohmmeter to identify the connections– A-E1, B-E1,C-E2,D-E2 �R

– A-B, C-D �2R– A-B, C-D �2R

– A-C, A-D, B-C, B-D, E1-E2 �∞∞∞∞

• To drive a stepper motor, you need to excite the coils in a particular sequence. there are two sequences

that will work. You can use either drive sequence, but find the most reliable one for the application.

unipolar stepper motor coil configurations

E1 E2

• Sequence 1: Energizes just one coil at a time (4step/cycle). As the current is switched from coil to coil, the motor moves

by one step (say 1.80). Reverse the sequence to reverse the direction of rotation

Stepper Motor Control

• Sequence 2: Energizes two coils at a time. There are still four steps to this cycle. Twice as much current is drawn in this sequence,

and about 1.4 times more torque than in sequence 1.

Stepper Motor Control cntd..

• Half stepping

Motor Controls

• Direction Control, Speed control, braking

• Open loop, or Closed loop control

Gearbox

Shaft

encoder

Break-Beam Sensor• Digital IR Sensors

Question : How to determine the direction of motion, and

speed of motion?

Position Feedback: Slotted Wheel

• Eg: 40:1 gearbox, 50 slots

slot/shaft18.05040

360 o

o

=

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