In industrial automation control, we often encounter various concepts such as digital quantities (switching quantities) and analog quantities. However, people can easily confuse these concepts in practical applications. So in the measurement of temperature, humidity, pressure, flow, etc., which are digital signals and which are analog signals? We only need to know the definitions of these two signals to distinguish them.
digital quantity
Digital quantities are a type of physical quantities, and their changes are discontinuous in time. It always occurs in a series of discrete moments. This type of physical quantity is called a digital quantity, that is, a discrete quantity, which refers to a quantity that is dispersed and has no intermediate value. Only the quantity of 01 will not change, which is the switching quantity.
Analog quantity
Analog quantity refers to the quantity of a variable that changes continuously within a certain range, that is, it can take any value (within the value range) within a certain range (definition domain). Digital quantities are discrete quantities, not continuously changing quantities. They can only take on a few discrete values. For example, a binary digital variable can only take on two values.
After understanding the definition, we can quickly distinguish that temperature, humidity, pressure, and flow are all analog signals, while normally open and normally closed are digital signals.
We found that in actual industry or industrial control engineering, common standard electrical signals are 4-20mA/0-10V/0-5V, etc., but the most widely used is to use 4-20mA current signals to transmit analog quantities. Why is this so?
1. At construction sites, electromagnetic interference is generally very serious, and voltage signals are more susceptible to on-site electromagnetic interference than current signals. In order to reduce interference and make transmission more accurate, current signals are used.
2. The minimum current of 4mA is chosen because the transmitter circuit cannot work without static operating current. The signal starting point current of 4mA.DC is the static operating current of the transmitter. At the same time, the electrical zero point of the instrument is 4mA.DC, which does not coincide with the mechanical zero point. , this “live zero point” is helpful for identifying faults such as power outages and disconnections.
3. The selection of the maximum current of 20mA is based on safety, practicality, power consumption and cost considerations. Safety spark meters can only use low voltage and low current. The spark potential energy caused by the on-off current signal of 20mA is not enough to ignite the explosion of combustible gas. point.
4. When the distance between the site and the control room is long and the resistance of the connecting wire is large, if the voltage signal is used for remote transmission, a larger error will occur due to the voltage division between the wire resistance and the input resistance of the receiving instrument. The signal is transmitted remotely. As long as there are no branches in the transmission loop, the current in the loop will not change with the length of the wire, thus ensuring the accuracy of transmission.