Quiz 8-3: Basic Sensor Concepts @🏠
- Due No due date
- Points 20
- Questions 4
- Time Limit None
- Allowed Attempts 2
Instructions
What are Sensors?
A device that detects or measures a physical property and indicates or otherwise responds to it.
In industrial control systems, it is most common for a sensor to be connected to a PLC and act as an input in a PLC program.
Sensors come in two broad types: Discrete and Analog.
Discrete Sensors
A sensor that provides information in the form of a contact closure of some kind. It is either ON (Closed) or it is OFF (Open). There are no other states. Just like a button, a discrete sensor is either Normally Open or Normally Closed.
Examples:
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- A normally open (NO) proximity sensor detects a box close by and closes a contact. A PLC responds to that input by turning on a second conveyor that whisks that box off in a different direction.
- A normally closed (NC) temperature switch opens when it detects a temperature of 150°F. A PLC responds to that by shutting down a motor.
- A normally closed (NC) pressure switch opens when it detects a high pressure. A PLC responds to that by shutting off an air compressor.
Analog Sensors
A sensor that provides information in the form of a number. In order to effectively control a system, it is often not enough to have a contact closure. Analog sensors relay information to a PLC via a continuous voltage (0-10V) or current (4-20mA).
Examples:
- A bag filling machine needs to run an auger until the bag weighs 50lb. One of the most common types of analog sensors that provides a continuous weight to the PLC is a "load cell".
- A room must be kept in a certain temperature range. When the room reaches 75°F, the PLC will turn on an exhaust fan. If the room reaches 80°F, an alarm is set to alert the operator. One type of analog sensor that provides a continuous temperature reading to the PLC is a "thermocouple".
- As a box moves on a conveyor belt, the belt must be slowed down in a certain zone so that a label can be applied. One kind of analog sensor that provides a continuous distance reading to a PLC is a laser distance sensor.
Here is a copy of the above notes in PDF format.