Sensors 101 DATA FORMAT

Transcription

1 SENSORS 101

2 Sensors 101 A sensor is a device which provides information to a controller. The information provided can be in a variety of formats. Digital and Analog are two of the most common formats when dealing with sensors. Sensors are often referred to as input devices, as they are normally wired to the input side of a controller and provide signals/information to the controller. Much the same as a human senses, industrial sensors are capable of seeing, feeling, hearing, and in some cases tasting. Sensors have many different applications spanning anything from consumer goods to industrial machinery. They are able to detect temperature changes, object presence, and can even measure distance to an object. DATA FORMAT Digital sensor signals are often referred to as a discrete signals. A digital signal in its simplest of forms, is one piece or bit of information. This single piece of information (bit) has one of two values. It s either ON or OFF. This value is represented in a binary fashion by either a 1 (ON) or a 0 (OFF). The transition from ON to OFF, or OFF to ON, happens very abruptly. Bits are often combined into larger pieces; Nibbles, Bytes and Words, to communicate more complex information or data. The values increase/decrease in very specific abrupt steps. Discrete data format usually follows one of two electronic designs; PNP or NPN. This refers to the theory of operation of the electronics inside the sensor. Discrete signals can be controlled by either Normally Open or Normally Closed switching. PNP sensors provide an electrical source, a positive potential difference or positive voltage to the controller. The load is connected between the output of the sensor and 0 Vdc (-). NPN sensors provide an electrical sink, a negative potential difference or a negative voltage to the controller. The load is connected between the output of the sensor and Supply Vdc (+).

3 Normally Open/Normally Closed refers to the sensors state at rest, without detecting an object. Normally Open (N/O) the sensor acts like a Normally Open switch. When the unit is at rest, the contact is open and no signal passes from the sensor to the controller. When the sensor detects an object the switch closes and the signal is passed to the controller. Normally Closed (N/C) the sensor acts like a Normally Closed switch. When the unit is at rest the contact is closed. The signal is then passed from the sensor to the controller. When the sensor detects an object, the switch opens and the signal is blocked and prevented from passing to the controller. Analog sensor signals are continuous signals which vary in value. Analog signals vary either current or voltage. Current signals are typically varied between 0 (4) to 20 ma. Voltage signals typically range from 0 to 10 Vdc. These are the most common signal ranges, however, is important to understand that there are other analog signal ranges that are used as well. it SENSING MEDIUMS Optical sensors use light as the medium for detection. These sensors are categorized by the type of light (where the light is located on the spectrum) used for sensing. The three main categories of light are Red Light (visible), Infra-Red Light (invisible), and Laser (visible/invisible). There are three different principles of operation with which light can be used for detection, they are Diffuse Reflective, Retro-Reflective, and Through-Beam (Fork). Diffuse Reflective sensors have the transmitter (emitter) and receiver located in the same housing. Light is emitted from the sensor into the air. When an object is placed within the light beam, the light is reflected off the object, back to the receiver. This principle of operation relies on the object reflecting back a very small percentage <2% of light. Light coloured objects reflect more light than dark coloured objects. The varying degree of colored object reflectivity means that the sensitivity of the sensor needs to be considered. Setup is usually accomplished with a teach-in button, or a potentiometer adjustment.

4 Retro-Reflective uses an inverse principle to the Diffuse Reflective. Although the transmitter and receiver are still mounted together in the same housing, Retro-Reflective sensors require a larger percentage of light to be reflected back. A reflector is used to accomplish this. The sensor looks for an object which does not reflect as much light as the reflector. It is important to recognize that metallic objects, or other shiny materials, may reflect the light as well or better than the reflector. Through-Beam designs typically have the transmitter and the receiver mounted in separate housings. They are aligned to each other along the same axis creating a beam. Objects are detected by simply breaking the beam of light between the transmitter and the receiver. Fork or Slot Sensors use the Through-Beam principle but are mounted together in a specially designed housing. Capacitive refers to the type of detection where the electrical property known as capacitance, is used as the sensing medium. Capacitance is the ability of a material to hold electrical charge. Capacitive sensors belong to the category of sensors known as Proximity sensors (switches), more commonly referred to as Prox sensors. They are a non-contact object detector. Capacitive sensors are able to detect both metallic and non-metallic objects, provided that the non-metallic parts have a dielectric constant significantly greater than air. They are usually limited in their detection range to only a few centimeters. Typically, the larger the diameter of the sensor, the larger the detection range. The material which is being detected also plays a key factor in sensing range. Inductive sensors use the inverse electrical principle to capacitance, known as inductance. They also fall under the category of non-contact Prox sensor. An alternate inductive field is created around the sensor s active surface. Any metallic objects which come into range will disturb or dampen the field. Measuring this disruption enables the sensor to detect objects.

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