1. Electronic Control Systems Week 3 – Switches and Sensors
EET273
Electronic Control Systems
Week 3 – Switches and Sensors

2. Lab 1 Recap
The PWM board produces a PWM signal proportional to the motor speed
The tact board produces a set of square waves that are proportional to the motor speed
What is the purpose of each of these boards? Why do we need both?
If a load was added to the motor, would the tact board signals or the PWM board signal change?

3. Switches and Sensors NO / NC switches Sinking / Sources switches
Read 9.1, 9,2, 9.4, 9.7, and 9.9 on Switches in Lessons In Industrial Instrumentation
Read first page or more of sections 9.3, 9.5, 9.6, and 9.8 in Lessons In Industrial Instrumentation.

4. “Normal” status of a switch
Out of the box, de-energized state
Not necessarily the state the switch spends most of it’s lifetime in

5. Hand Switch (push button switches)
May be NO, NC, or have NO AND NC contacts, like the switch below

6. Form A, B, C Form A switch – NO Form B switch – NC
Form C switch – combination of Form A and Form B

7. Stackable Push Button Switches
Add as many sets of connectors as needed

8. Limit Switch Detects physical motion of some object by direct contact
Can be NO or NC depending on application
Mechanical in nature – wears out over time
Very useful in robotic system to determine location of robot
Home position, maximum excursion, etc.

9. Proximity Switches Non-contact sensor
Normal state is when no object is nearby
Serve similar purpose as limit switches, but do not wear out due to no actual physical contact with objects
Typically “active” – require power to operate

10. Proximity Switches
Because they are active, they often have LED indicators to tell if switch is tripped
May be either NO or NC type
Typically have a fixed detection range

11. Proximity Switches
Inductive – sense metallic objects using a high frequency magnetic field
Capacitive – sense non-metallic objects using a high frequency electric field
Optical – detect the interruption of a light beam by an object
Ultrasonic – detect presence of dense matter by reflection of ultrasonic sound waves

12. Proximity Switches – Inductive & Capacitive
Inductive Switches
Sense metallic objects using a high frequency magnetic field
Contains an induction loop
Presence of metal increases current flowing in loop, causing switch to flip
Capacitive Switches
Sense non-metallic objects using a high frequency electric field
Contain an oscillator circuit with the capacitor as the sensing device
Nearby objects change the capacitance of a capacitor in the oscillator circuit
Most modern touch screens are capacitor sensors

13. Proximity Switches
Because proximity switches are active, they typically do not have simple switch terminals like a passive switch would. Instead they either source or sink current
Sinking – sensor sinks current from circuit, NPN type, “low-side”
Source – sensor sourcing current to circuit, PNP type, “high-side”
Notice that emitter is always connected to power rail – common emitter config

14. Pressure Switches Detects the presence of fluid pressure
Typically uses a diaphragm to detect pressure
“Normal” state of a pressure switch is when it is at minimum pressure
Can be used to detect high or low pressure
High/Low pressure alarm system:
Low pressure switch – NC switch that opens
when pressure is in acceptable range
High pressure switch – NO switch that closes
when pressure is outside acceptable range

15. Level Switches Detects the level of a liquid or solid in a vessel
“Normal” status is when sensor is at minimum level

16. Temperature Switches Detects the temperature of a substance
“Normal” status is minimum temperature

17. Flow Switches Detects flow of some fluid through a pipe
Often use “paddles” to detect flow, which activates switch contacts
“Normal” status is when minimal flow is detected

18. Switching Example
LED in this circuit will turn on if the liquid level rises above 14 inches AND the pressure falls below 22 PSI AND either the flow is less than 3 gallons per minute OR the temperature is greater than 125°F.
22 psi sensor (NC) is detecting a LOW pressure state, which is the normal state, but not the desired (or typical) state
Similarly, the 3 GPM sensor (also NC) is detecting LOW flow

19. Deadband “On” threshold is different from “Off” threshold
Ex. A temperature sensor switches at 300°C when rising, but switches off at 295°C when falling.
Typically this is a byproduct of the mechanics of the device
Can actually be useful, because it can prevent a system from “bouncing” around the setpoint, creating multiple alarms and excessive switching.

20. Hall effect sensor
A magnet in close proximity to a flowing current will attract or repel charges, and induce a “Hall Voltage” in the material where the current is flowing
Hall effect sensors can be made to respond to either the N field, S field, or both.
Can also be made to “latch” to either a N or S field

21. Hall effect sensors In Lab 2 Used to measure motor speed
2 Hall effect sensors – A & B
90° apart
A & B each produce a square wave output
Together A & B provide 4 distinct output states
These states can be used to measure location and direction of the motor

22. Lab 2 – Switches & Sensors
Inductive sensor
Hall effect sensor – used in motor encoder

23. Lab 2 – Switches & Sensors – Motor Encoder