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

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?

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.

“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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

Lab 2 – Switches & Sensors – Motor Encoder