They can also be Normally Open and Normally Closed, depending on what type is used. Temperature Limit Switch PLC Basics Temperature Limit SwitchĪlso called Thermostat, the temperature limit switch is used to detect temperature changes in your system. There are also different types of limit switches which allow us to choose the physical quantity to limit in our control system design. These are actually useful in automation because you can set a limit (using the limit switch) where a specific process stops. Limit switches, as the name implies, change state when a predetermined limit is reached. This process actually selects a varying load for the motor in order to control its speed. The usual example is found in electric fans, where you can select a number that then dictates the speed of the fan’s motor. Selector switches are still manually operated switches, however instead of being normally open or closed, there are more than two contacts to select from. Simply put, the other one BREAKS contact while the other one MAKES, hence the name. One side of the switch is normally closed and one side is normally open, so when a button is pressed, one contact will be in the OFF position and one will be ON. These types of switches have BOTH the normally open and the normally closed in one construction. This means pushing the button will make its metal contacts touch with each other, closing the connection between the two connected terminals. This means that if you connect a normally closed push button to a circuit, the circuit will turn on immediately because there is electrical continuity already. In this scenario, current can already flow through the switch. Normally closed push buttons, when connected to the circuit, SHORTS the connected terminals. Normally closed (NC) Normally Closed Push button
![right handed input system flac3d right handed input system flac3d](http://docs.itascacg.com/flac3d700/_images/freefield-geometry.png)
Pushing the button will make its metal contacts touch with each other, closing the connection between the two connected terminals and hence allowing the current to pass. In this scenario, the current can NOT flow through the switch, as there is no electrical continuity.
![right handed input system flac3d right handed input system flac3d](https://ars.els-cdn.com/content/image/1-s2.0-S2405844019361559-gr3.jpg)
Normally open push buttons, when connected to the circuit, normally make an OPEN circuit. Normally open (NO) Normally Open Push button Hence, pushbuttons are divided into two categories: Normally open or Normally closed. Push buttons are the ones that you will usually see as START or STOP buttons in a PLC control system. It operates by either MAKING contact (Make) or by BREAKING contact (Break). Push buttons PLC Basics: Push buttons(Photo from Siemens) To name a few, there are push buttons, selector switch, and limit switches. In most PLC applications, there are a number of components that you will usually see. For this PLC application, ultrasonic sensors are most widely used.īy connecting the sensor and the lamp to the PLC, you now have an automated system that performs the same task! Common PLC Input Devices In that case, you may want to use a sensor that can detect the presence of people or objects. In the previous example, what was the feedback mechanism? The fact that there were no more people in front of your house. They are usually placed as a feedback mechanism-which they are! “How can this be applied to an automated system?”Īutomated systems use sensors-devices that output electrical signals based on environmental conditions. Because of this, you now turn off the lights. If the person ran away because you scared him off for some reason, you will see that there are no more people in front of your house (feedback). Turning the switch on made the light bulbs outside light up (output). If there was a person standing in front of your house and you saw it (input), you will turn on the switch (process). Let us take a deeper look at what really happened.
![right handed input system flac3d right handed input system flac3d](https://www.mdpi.com/applsci/applsci-10-02006/article_deploy/html/images/applsci-10-02006-g006.png)
“How did that become a closed loop system?” What would you do? You’d check outside whether there were people present, and then turn the lights on or off accordingly. Let’s say you wanted to turn on the lights whenever someone walks in front of your house (That sounded creepy). I discussed both types of systems at the beginning of the PLC Basics series, make sure to check it out if you haven’t already! Closed loop systemĪ closed-loop system is a type of control system where the input is varied depending on the feedback from the output.Īs an analogy, I’ll use an example from your very own home. 5 How basic PLC input devices are connectedįirst, let’s talk about the type of system that is the ONLY type applicable to Automation-Closed Loop Systems.4.9 Linear Variable Differential Transformer.4.5.1 Modes of Operation of Photoelectric Sensors.
![right handed input system flac3d right handed input system flac3d](http://docs.itascacg.com/flac3d700/_images/step01.png)
3.2 Two types of sensors: Analog and Discrete.