A sensor is a device that
sends the physical condition or status of a CNC machine part to the PLC in the
form of an electrical signal. CNC machines use various sensors, which are
typically connected to an Input Module of the PLC. Therefore, all the sensors
used with the machine are considered the 'Input Elements' of the PLC. The PLC
recognizes the physical state of a machine part based on the status of a sensor
(activate/deactivate). A push-button switch is a simple example of a sensor. When
the PLC detects whether the push-button switch contact is open or closed, a 24V
DC line connects to the PLC through the switching contacts. Thus, the PLC
monitors the 24V DC voltage to identify the status of a sensor.
Usually, a sensor or input
element has two states: ON or OFF, representing the activate or deactivate states
of the sensor. The PLC considers the ON state of a sensor or input element as
'Logic-1' or 'Logic High,' while the OFF state is considered 'Logic-0' or
'Logic Low.' When a sensor activates and its status becomes Logic-1, a 24V DC
voltage is present at the input module of the PLC from that input element.
Conversely, when a sensor deactivates and its status becomes Logic-0, no
voltage is available from that input device to the PLC.
Types of sensors commonly
used with CNC machines:
Sensors are primarily classified into two groups: Contact Sensors and Non-contact Sensors. A contact-type sensor requires physical touch between the measuring device and the sensor, whereas a non-contact sensor does not require a physical connection. A non-contact sensor is also known as a Proximity sensor. Sensors are further divided into two categories based on the type of signal they receive: Digital sensors and Analog sensors. Digital sensors detect two different states of a measuring device: presence or absence. The state or status is represented by a digital signal: 1 or 0. Continuous variable signals like temperature, pressure, and acceleration are typically conveyed by an Analog Sensor, which provides a linear analog voltage or current as its output signal. The signal received from an analog sensor does not directly interface with the CNC controller and requires a separate converter unit to make it comprehensible to the controller. In some cases, modern CNC systems employ Micro-Electro-Mechanical Systems (MEMS) as distinct sensor technology, which transfers a substantial signal for a specific device. Here is a concise list of sensors commonly used in different mechatronics systems.
|
Digital contact sensor |
Digital non-contact sensor |
Analog sensor |
Advanced sensor |
|
Limit switch Pressure switch Float switch Flow switch |
Proximity sensor Magnetic sensor Infrared sensor Tilt sensor Sound sensor Smoke sensor Ultrasonic sensor PIR sensor |
Temperature sensor Light sensor Touch sensor Humidity sensor Strain sensor LVDT Flex sensor Hall sensor Potentiometer |
MEMS sensor Barcode sensor QR code sensor Electromagnetic Identification Transponder |
How does a digital
sensor connected with a PLC input module:
The following picture illustrates different input devices, such as switches and sensors, connected to the input module terminals of a PLC.
The PLC's input module has different input addresses that correspond to individual input terminals on the terminal strip. By monitoring the conditions or states of the sensors, the PLC recognizes the 'Status' of those input addresses, which can be either zero or one. If the status of an input address is one, the PLC understands that the sensor is activated, whereas a status of zero indicates that the corresponding sensor is deactivated. To provide a visual indication of the sensing status, different LEDs are used for each input address within the terminal strip of the PLC's input module. When an LED is glowing or ON, it represents that the corresponding sensor is activated, while OFF indicates that the sensor is deactivated. Referring to the previous image, the input I 0.3 was activated, while inputs I 0.1 and I 0.6 (where sensors are connected) were deactivated. Depending on the machine requirements, two, three, or more input modules can interface with the PLC, and they may differ in terms of the number of input devices that can be connected to the terminal strip, whether electrical isolation is required, the current rating of the sensors, etc.
