Common sensors in industrial automation

1.Why do we need to learn sensor knowledge?

Modern factories have developed toward automation, so sensors have important observation and detection functions for electrical systems just like human eyes. Sensors are divided into many types, each of which has different usage, functions and precautions. , So whether you can choose a suitable sensor or whether you can distinguish it after using an inappropriate sensor is a very important skill, and learning different sensors is also very helpful for learning PLC programming. The sensor is electrical. An important point of knowledge, so it should also be focused on learning.

 So let's take a look at the common sensors in the factory and how to select them; according to different applications, we can roughly classify the common sensors.

(1). Measure position or distance; proximity switch, photoelectric switch, contact position sensors, encoder, laser distance meter, ultrasonic distance measuring sensor.

(2). Measuring medium; various forms of flow meters, pressure sensors, liquid level gauges  current transformers and voltage transformers, vibration meters, sensors for measuring gas content and concentration.

(3). Temperature measurement; thermal resistance and various forms of thermocouples.

(4). Measure weight; load cell.

(5). Other sensors.

2.How to choose the right sensor?

(1). Determine the type according to the measurement object and output conditions

To carry out a specific measurement work, we must first consider what kind of sensor principle is used, which can only be determined after analyzing many factors. Because, using flow meters for example, there are electromagnetic flow meters, vortex flow meters, and ultrasonic flow meters. We need to select flow meters for specific goals. In addition, we need to refer to which output mode we need to use, for example, 2 The wire system is still a four-wire current signal, 0-20ma, 4-20ma, 0-10v voltage signal or some kind of protocol communication. 

(2). Selection based on sensitivity 

Generally, within the linear range of the sensor, it is hoped that the higher the sensitivity of the sensor, the better. Because only when the sensitivity is high, the value of the output signal corresponding to the measured change is relatively large, which is conducive to signal processing. However, it should be noted that the sensitivity of the sensor is high, and external noise that has nothing to do with the measurement is also easy to mix in, and it will also be amplified by the amplification system, which affects the measurement accuracy. Therefore, it is required that the sensor itself should have a higher signal-to-noise ratio and minimize the interference signal introduced from the outside world.

The sensitivity of the sensor is directional. When the measured is a single vector, and its directivity requirements are high, you should choose a sensor with low sensitivity in other directions; if the measured is a multi-dimensional vector, the smaller the cross sensitivity of the sensor, the better.

(3). Judge the frequency response characteristics 

The frequency response characteristic of the sensor determines the frequency range to be measured, and it must remain undistorted within the allowable frequency range. In fact, the response of the sensor always has a fixed delay, and it is hoped that the delay time is as short as possible. The higher the frequency response of the sensor, the wider the measurable signal frequency range. In dynamic measurement, the response characteristics should be based on the characteristics of the signal (steady state, transient state, random, etc.) to avoid excessive errors.

 (4). According to the stability of the sensor

The ability of a sensor to keep its performance unchanged after a period of use is called stability. In addition to the structure of the sensor itself, the factors that affect the long-term stability of the sensor are mainly the environment in which the sensor is used. Therefore, in order to make the sensor have good stability, the sensor must have strong environmental adaptability. Before selecting a sensor, investigate its use environment, and select the appropriate sensor according to the specific use environment, or take appropriate measures to reduce the impact of the environment.

(5). The range and accuracy of the sensor are the most difficult pair of enemies to coordinate.

Accuracy is an important performance index of the sensor, and it is an important link related to the measurement accuracy of the entire measurement system. However, the accuracy of the sensor is restricted by the range. Generally, the larger the range, the lower the accuracy, but the high-precision sensor is likely to have insufficient range, which results in the high-precision and large-range sensors being very expensive. So when choosing, you need to adjust their relationship appropriately.

In this article, we have a general understanding of the types of sensors commonly used in factories. In future articles, we will explain specific precautions for the use of a certain sensor and some cases of application onPLC.