The document provides information about control systems, including: 1) It describes open and closed loop control systems, and how closed loop systems use feedback to reduce error between the actual and desired output. 2) It discusses analog control systems that use operational amplifiers as comparators to compare input signals. Proportional control systems are also mentioned. 3) Positional control systems are described, including how servo motors use feedback potentiometers and push-pull drivers to control motor position.

1. Dave’s Revision Notes
Programmable Systems
Gary Plimer 2004

2. Programmable Systems Outcome 1
Outcome 1 - Control Systems
The purpose of this unit is to introduce the operation of control systems.
When students have completed this unit they should be able to:
 Recognise and identify common control systems.
 Describe the operation of open and closed loop control systems.
 Make use of systems diagrams and systems technology.

3. Programmable Systems Outcome 1
Systems Diagrams
Most industrial product design is solved by the systems approach. This
approach involves studying the desired function of the product, and then
breaking this function down into a series of subsystems.
When applied to control systems, a Systems Diagram is a useful way of
visually representing the desired function of the system. The systems
diagram is a form of block diagram than contains all the subsystems within a
dashed box, called the systems boundary. The systems boundary indicates
the extent of the control system.
CONTROL OUTPUT
INPUT OUTPUT
DRIVER

4. Programmable Systems Outcome 1
CONTROL OUTPUT
INPUT OUTPUT
DRIVER
The "real world" input and output conditions of the system are
shown as arrows entering, and leaving, the systems diagram.

5. Programmable Systems Outcome 1
A simplified systems diagram of a washing
machine is shown below.
SET WASHING
CYCLE CONTROL
CLEAN COLD HEATER MOTOR PUMP HOT DIRTY
WATER WATER
DIRTY DRUM CLEAN
CLOTHES CLOTHES

6. Programmable Systems Outcome 1
Open Loop Control
At the simplest level a control system can process an input condition to
produce a specified output.
A good example of this type of system is a hand-
held electric hairdryer. The heating element and fan
motor are switched on when the appropriate
switches are held down.
SWITCH OUTPUT
CONTROL FAN & HEATER
HELD DRIVER SWITCHED ON
DOWN

7. Programmable Systems Outcome 1
SWITCH OUTPUT
CONTROL FAN & HEATER
HELD DRIVER SWITCHED ON
DOWN
This is an example of Open Loop Control, where an input is processed
to produce an output.
With the hairdryer example the heater and fan motor are held on until the
switches are released.
The air being blown out of the hairdryer is not temperature monitored or
adjusted - the air is simply just blown out at whatever temperature the
heater is capable of achieving.

8. Programmable Systems Outcome 1
DETECT SWITCH
CONTROL OUTPUT LAMP
LIGHT DRIVER
LEVEL ON/OFF
Another Open Loop System is
shown above. Vcc
By splitting the circuit shown
opposite into INPUT, PROCESS
& OUTPUT, describe the
operation of each subsystem
0V

9. Programmable Systems Outcome 1
OPEN LOOP
An open loop control system represents the
simplest and cheapest form of control.
However, although open loop control has
many application, the basic weakness in
this type of control lies in the lack of
capability to adjust to suit the changing
output requirements.

10. Programmable Systems Outcome 1
CLOSED LOOP
Closed loop control systems are capable of making decisions and
adjusting their performance to suit changing output conditions.
A personal cassette player is capable of detecting
the end of the tape and switching the motor
off, hence protecting the tape from snapping (or
the motor burning out).
END OF
TAPE
CONTROL OUTPUT
DRIVER

11. Programmable Systems Outcome 1
CLOSED LOOP
FEEDBACK
SENSING
CONTROL OUTPUT
INPUT OUTPUT
DRIVER
All closed loop control systems include a feedback sensing subsystem within
the systems diagram. The control subsystem will process the feedback signal by
making a 'decision' on whether the state of the output should change.

12. Programmable Systems Outcome 1
Pupil Assignment 1
 Explain what is meant by the term 'systems diagram'.
 Describe the differences between an open loop and closed loop
control system.
 Describe the purpose of the feedback sensing subsystem in a closed
loop control system.

13. Programmable Systems Outcome 1
Pupil Assignment 2
The figures above show two types of electric fire.
The second electric fire is a more modern device
fitted with a thermostat.
a) Name the type of control system used in
each type of electric fire
b) Draw a system diagram for each type of
electric fire.
c) Name a type of electronic sensor that may
used for measuring temperature.

14. Programmable Systems Outcome 1
Pupil Assignment 3
LIMIT
SWITCH
SWITCH TRANSDUCER INPUT/OUTPUT
INVERTER LATCH
UNIT DRIVER UNIT
The systems diagram for a simple lift
control system is shown below.
MOTOR/
LIFT PLATFORM GEARBOX
PLATFORM POSITION
SENSOR
START CONTROL OUTPUT RAISE LIFT
LIFT DRIVER PLATFORM
Continued

15. Programmable Systems Outcome 1
Pupil Assignment 3
A limit switch at the top of the lift detects when the lift platform has reached the
top of the run. The block diagram below shows the system diagram sub-systems
broken down into smaller blocks.
LIMIT
SWITCH
START SWITCH TRANSDUCER
INVERTER LATCH
LIFT UNIT DRIVER
MOTOR
GEARBOX
RAISE LIFT
PLATFORM
Continued

16. Programmable Systems Outcome 1
Pupil Assignment 3 +V
Simulate the circuit shown.
a) Clearly explain S Q
how the system
operates.
R Q
0V
RELAY
(b) Explain the purpose of the relay,
transistor and diode within the relay
driver circuit.

17. Programmable Systems Outcome 1
Pupil Assignment 4
Build the circuit shown, using modular electronics system boards.
a) Draw a system diagram of the control system.
b) Draw a circuit diagram of the control system.
c) Explain the operation (function and effect) of the DPDT relay.
LIMIT
SWITCH 1
INPUT/OUTPUT TRANSDUCER INPUT/OUTPUT
INVERTER LATCH
UNIT DRIVER UNIT
MOTOR/
LIFT PLATFORM GEARBOX
+V
LIMIT
OV SWITCH 2

18. Programmable Systems Outcome 2
Outcome 2 - Analogue Control Systems
The purpose of this unit is to introduce the operation of analogue control
systems.
When students have completed this unit they should be able to:
 Correctly identify common applications of analogue control.
 Represent analogue control systems using control diagrams and
circuit diagrams.
 Select the appropriate configuration of op-amp in the design of
analogue closed loop control systems.
 select appropriate output drive subsystems for control
applications.

19. Programmable Systems Outcome 2
Analogue Closed Loop Control Systems
V1
Vo
V2
Many control systems involve processing analogue signals such as heat, light
and movement. Therefore analogue closed loop control systems require
analogue processing devices such as the operational amplifier (op-amp).
One of the most common control applications involves using the op-amp as a
comparator. In it's simplest form a comparator just compares two voltage
signals, V1 and V2. If V1 is higher than V2 the output, Vo, is 'low', if V1 is lower
than V2 the output is 'high'

20. Programmable Systems Outcome 2
Pupil Assignment 1
The block diagram below simulates an automatic heating system. When
the bulb heats up the temperature rise is detected by the bead thermistor,
which sends a feedback signal back to the comparator.
BEAD THERMISTOR
INPUT/OUTPUT TRANSDUCER BULB UNIT
COMPARATOR
UNIT DRIVER
Continued

21. Programmable Systems Outcome 2
Pupil Assignment 1
TEMPERATURE FEEDBACK SIGNAL
+VCC
R RT
The circuit for the heating
system is shown opposite
RV
RV
R
OV
-VCC
Continued

22. Programmable Systems Outcome 2
Pupil Assignment 1
The variable resistor, RV1 (connecting to the non-inverting input of the op-
amp) is used to set the reference level (or threshold). This sets the
desired temperature of the bulb.
Simulate the circuit shown. Calibrate the system to the following
performance criteria:
 When the bulb heats above the reference level the thermistor should
sense the temperature and send a signal to the comparator which will
switch the bulb off.
 When the bulb cools below the reference level the bulb should switch
on again. The system should operate continuously
a) Describe how the circuit operates.
b) Explain clearly how you calibrated the system.

23. Programmable Systems Outcome 2
Control Diagrams
TEMPERATURE
SENSOR
SET CONSTANT
CONTROL OUTPUT
TEMPERATURE OUTPUT
DRIVER
LEVEL TEMPERATURE
The circuit constructed in the last activity, can be shown as a Control
Diagram, (see above).
However, when we use an Op-Amp, the circuit shown on the next slide is used.

24. Programmable Systems Outcome 2
Control Diagrams
TEMPERATURE
SENSOR
SET OPERATIONAL OUTPUT CONSTANT
TEMPERATURE AMPLIFIER DRIVER OUTPUT
LEVEL TEMPERATURE
The error-detection
The control diagram symbol is also used to
breaks the generalised indicate that the control
'control' subsystem of the involves two signals. The
systems diagram into feedback signal is connected
more specific blocks. to the negative symbol to
indicate the use of negative
feedback

25. Programmable Systems Outcome 2
Negative Feedback
The purpose of closed loop control is to ensure that the output is
maintained, as closely as possible, to the desired output level. In the case
of a central heating system, a graph of the temperature in a room might
appear as in the graph below.
ACTUAL TEMPERATURE
As can be seen from the
graph, the control system is
TEMPERATURE
constantly trying to pull the
SET TEMPERATURE temperature of the room
back towards the set
temperature level by
reducing the error. This type
of control uses negative
feedback to reduce the
error.
TIME

26. Programmable Systems Outcome 2
Positive Feedback
The opposite effect can be created by reinforcing the error, as can
sometimes happen with public address systems when the microphone is
held too close to the speakers. A sound is picked up by the
microphone, amplified, and then output through the speaker. The
amplified sound is then picked up, re-amplified and so on. The net result
is a high pitch sound, which can be represented by the graph below
ACTUAL SOUND
SOUND SIGNAL
SIGNAL
REQUIRED SOUND
SIGNAL
TIME

27. Programmable Systems Outcome 2
Pupil Assignment 2
Explain the following terms when applied to control systems:
open loop, closed loop, negative feedback, positive
feedback, error detector

28. Programmable Systems Outcome 2
Positional Control
A good example of positional control is the model servo motor commonly
found in radio-control cars and aeroplanes.
The servo contains a motor, gearbox, feedback
potentiometer and electronic control circuit on a
small printed circuit board.
The feedback potentiometer is directly connected
to the output shaft. Therefore, when the motor
spins, the output shaft is turned slowly by the
gearbox, which in turn moves the potentiometer.
Naturally movement of the output shaft is
restricted to 180º as the potentiometer cannot
spin continuously.

29. Programmable Systems Outcome 2
Positional Control
The control diagram for the servo is shown below.
FEEDBACK
POTENTIOMETER
SET OPERATIONAL OUTPUT CONSTANT
POSITION AMPLIFIER DRIVER POSITION
Note how the feedback signal from the feedback potentiometer is fed into the
negative symbol of the control diagram, indicating negative feedback.

30. Programmable Systems Outcome 2
Positional Control
Push-Pull Follower Analogue Driver
An important element in producing +Vcc
positional control, such as that required
in the servo, is a dual rail push-pull NPN
follower analogue driver circuit, that
allows the motor to spin in both OP-AMP LOAD
SIGNAL
directions. The op-amp cannot source
sufficient current to drive most output PNP 0V
components, and so the dual rail push-
pull follower is required to drive the -Vcc
output components.
The dual rail push-pull follower is based
around a two transistor circuit as shown
opposite.

31. Programmable Systems Outcome 2
Positional Control
When the signal from the op-amp is positive the NPN transistor will switch
on, and current will flow through the load from the positive supply
rail, +Vcc, to the ground rail, 0V.
+Vcc
I
I
I
OP-AMP +V LOAD
SIGNAL
I
0V
-Vcc

32. Programmable Systems Outcome 2
Positional Control
When the signal from the op-amp is negative the PNP transistor will switch on
and current will flow through the load from the ground rail, 0V, to the negative
supply rail -Vcc.
+Vcc
I
OP-AMP -V LOAD
SIGNAL I
I
0V
I
-Vcc

33. Programmable Systems Outcome 2
Pupil Assignment 4
A proportional control system is
used to regulate the flow rate of
coal onto a conveyer belt. The POWER
SUPPLY
system should sense the weight of
coal on the conveyer belt and RACK
AND CONTROL
automatically adjust the gate height PINION UNIT
to ensure that a constant flow of
coal is supplied.
VARIABLE HEIGHT
GATE
P
CONVEYOR
FRAME PIVOTED
HERE CONSTANT SPEED
DRIVE MOTOR
LOAD SENSOR

34. Programmable Systems Outcome 2
Pupil Assignment 4
• Draw a systems & control diagram of the flow rate control system
• Explain the term 'proportional control‘
• Name the configuration of op-amp used in proportional control systems
POWER
SUPPLY
• Name a suitable
output driver circuit RACK
AND CONTROL
which could be used PINION UNIT
with the control
system
VARIABLE HEIGHT
GATE
• Draw a circuit
diagram of the driver
P
circuit. CONVEYOR
FRAME PIVOTED
HERE CONSTANT SPEED
DRIVE MOTOR
LOAD SENSOR

35. Programmable Systems Outcome 2
Pupil Assignment 5
The figure shows the layout of an audio mixing desk.
TACHOGENERATOR
AUDIO MIXING
The tape is fed through the audio HEAD
mixing head by being pulled on to a
recording reel driven by a dc motor. As
the tape builds up on the recording
reel, the tape speed through the audio
mixing head will increase.
To prevent this from happening, the dc RECORDING REEL DRIVEN BY DC MOTOR
motor is fitted to a closed loop control
system.
A tachogenerator connected to a pulley
senses the tape feed rate and sends an
error signal to the control system.

36. Programmable Systems Outcome 2
Pupil Assignment 5
• Name the type of closed loop TACHOGENERATOR
control used in this AUDIO MIXING
HEAD
application
• Name the amplifier used in
this type of closed loop
control
• Draw a control diagram of RECORDING REEL DRIVEN BY DC MOTOR
the system
• Draw a circuit diagram of the
control system and explain
the function of each
part of the circuit.

37. Programmable Systems Outcome 2
Pupil Assignment 6
The figure illustrates a system for controlling the wing mirrors on a car by
adjusting remote dials on the dash.
DIALS
A control diagram of the system for
rotational movement in the X-axis (one
mirror) is shown in the figure below.
Similar systems are used for the Y-axis
and for the other mirror.
FEEDBACK
SENSOR
REMOTE OPERATIONAL OUTPUT WING MIRROR
MOTOR
DIAL AMPLIFIER DRIVER POSITION

38. Programmable Systems Outcome 2
Pupil Assignment 6
FEEDBACK
SENSOR
REMOTE OPERATIONAL OUTPUT WING MIRROR
MOTOR
DIAL AMPLIFIER DRIVER POSITION
• With reference to the control diagram, explain clearly how the
system operates
• Name the type of control used in this system
• Name the configuration of op-amp required
• State two reasons why the op-amp cannot be used to drive the
motor directly
• Name a suitable output driver which could be used with this
system.

39. Programmable Systems Outcome 2
Sequential Control Systems
Sequential control is used to control systems whose outputs are required
to follow a fixed cycle of events (i.e. a sequence).
An interesting example of sequential control was used in music boxes.
Actuators on the drum are arranged to produce notes, as the rum rotates
the notes are played in the correct sequence to produce a tune.
A more modern example of sequential
control is a robot arm used to weld cars
together on a large production line.

40. Programmable Systems Outcome 2
Common Input Transducers used in Computer Control
 Potentiometers are used to provide feedback on position. This
might be a simple potentiometer or a combined system such as a
servo motor.
 Tachogenerators are used to provide feedback on speed.
 Light Dependent Resistors are used to provide feedback on lux
level changes.
 Thermistors are used to provide feedback on temperature changes.
However, there are other methods available to us, Slotted Discs, Binary
Coded Discs, Grey Scale Discs.

41. Programmable Systems Outcome 2
Binary Coded Disc Slotted Disc
A problem with Binary
discs is that more than
1 bit can change at a
time which may cause
errors. A gray disc
Gray Code changes only 1 bit at
each stage.

42. Programmable Systems Outcome 2
We need to be aware of the types of signal produced from these encoders
are and how a computer could use them to control its output.
Consider each of the encoders or input transducers, state
for each the type of signal produced and how this could
be used by the microcontroller.