Time： 2019-11-16 10:07:55
There are many types of output signals of displacement sensor, such as voltage signal, current signal and digital signal. Voltage signal output type and current signal output type are more common in automation industry because of low production cost and acquisition and processing cost.
The internal resistance of voltage signal is relatively large, the load resistance is relatively large, and the transmission current is relatively small. When a high resistance input operational amplifier is connected to the receiving terminal, the signal can be received, and the power of the whole circuit is very small. However, the internal resistance of voltage signal is large and it is easy to introduce noise interference. The long-distance transmission signal attenuation is more serious, and it is easy to be affected by transmission cable to produce hysteresis and distortion. Therefore, the voltage output type of displacement sensor shall be selected, which requires better site conditions, transmission distance within 10m, and cable resistance less than 1 Ω.
The internal resistance of the current signal is very small. Because of the characteristics of the current, the current signal can not be directly detected by simple methods. It is necessary to insert the load resistance RL in series at the receiving end to receive the signal by detecting the voltage drop of the load resistance. Due to the small internal resistance of current signal, it is difficult to form a larger voltage for noise signal, so the current output type of displacement sensor has a relatively strong anti-interference ability, compared with the voltage output type, which has the characteristics of strong anti-interference ability, no attenuation, no hysteresis, long transmission distance, etc. However, in order to protect the output circuit, it is necessary to have a maximum load resistance limit and a large power.
The voltage signal is easy to detect. The current signal transmitted to the receiver needs to flow through a standard resistance (accurate value, no additional capacitance, or the capacitance is small enough to be ignored), which is converted into a voltage signal to measure the voltage at both ends of the standard resistance. In addition, the current of the voltage signal transmission circuit is small, usually multiple groups of signals are grounded together, and the current signal circuit current is large, usually each group of signals is one circuit, not grounded together.
Both voltage signal and current signal are called analog signal in the field of industrial automation. Analog signal is widely used in industrial control system of automation industry. Since the appearance of fieldbus, many people think that analog signal transmission will be eliminated by fieldbus technology. However, over the past decades, the product development of analog signal is more and more abundant, and the cost and signal transmission stability are also better than fieldbus technology.
Both voltage output displacement sensor and current output displacement sensor have their own characteristics and advantages. Users can choose freely according to their needs and site conditions. However, in order to reduce the error of displacement measurement and control system, the input impedance of voltage signal detection circuit should be as large as possible, and that of current signal detection circuit should be as small as possible. For the numerical control system, whether it is voltage signal, current signal or digital signal, it is the level that enters the processor to judge. If current signal is selected, it is also used after loading appropriate load resistance to convert it into voltage signal. It is suggested that the current output displacement sensor should be given priority in the special situation of complex field environment, serious interference or long-distance transmission.