What you better know about Analog Sensors
Posted on 08 June, 2022
With rising demand for smart control and Internet of Thing (IoT), plenty of sensors have been developed to explore and detect a wide variety of physical quantities. Just like human sensory organ, physical sensors play a significant role in modern industry application. During embedded system development, different types of sensors play the roles to open the door for interaction with external nature. Sensor with analog output is a big branch in whole sensor family, providing continuous signal parallel to measured value. The outputs of analog sensors are normally current or voltage, which will change mostly liner with the variation of the measured value. The sensor output is calibrated over a range corresponding to a specific value range of engineering units, such as pressure, temperature, distance, load, et al. Most sensors analog output are DC (direct current) signal, and also have some AC (alternating current) signal sensors for AC performance monitor. Common voltage output ranges include 0~5V, 0~10 V, and -5~5 V. Common current output ranges include 0~20 mA and 4~20 mA. The supply voltage of analogy sensors are normally 24V DC in industry application, and many embedded type module will need 5V or 3.3V, in which 3.3V is quite common. To choose a proper analog sensor and then process the signal, several factors as following may be in consideration.
Current sensor
The current output range of 4~20mA is quite robust sensor signal standard, and its operation loop is straight forward with 4mA representing the Zero signal output and 20mA representing the full-scale signal output. There are also sensors having range of 0~20mA, but not that popular as the 4~20mA, which of its unique merits.
Current loops are ideal for data transmission, and all the signal current flows through all components, and the same current flows even if the wire terminations are less than perfect. All the components in the loop drop voltage due to the signal current flowing through them. The signal current is not affected by voltage drops through all components along the loop, as long as the power supply voltage is greater than the sum of the voltage drops at the maximum signal current of 20 mA. Another advantage of using a current loop is a current loop’s inherent insensitivity to electrical noise. Its main advantage over digital and VW is its suitability for dynamic monitoring.
The signal better starts at 4mA instead of 0mA to make a distinction between a Zero signal and a malfunction. A reading of 0mA is essentially “reserved” to signal a fault, maybe caused by an open circuit or a lost feed, in the system, while the reading of 4mA to 20mA would indicate a properly working circuit. The resistor Rm can be used to adjust the voltage input to the ADC (analog-to-digital converter) of the embedded system, maintaining safe voltage range.
Current output would be better choice if the signal has to travel through medium length cables before being processed. Sending a signal over longer distances produces voltage losses in the system, however the magnitude of the current in the loop is not affected by voltage drops in the system or electric noise from adjacent devices.
Voltage sensor
There are various voltage output ranges in analog sensor, such as 0~5V, 0~10V, -5~5V, -10~10V, among which, 0~10V is the most common ‘straight forward’ signal used in most controllers, PLC, and data loggers worldwide. Similar as the current output, the starting point and end point and the measured quantity is linearly distributed between the voltage signal range.
Two types of methods are normally employed to conduct voltage output sensor measurement, namely, differential measurement and single-ended measurement. As to differential measurement, the measurement is taken as the voltage difference between two signal wires, which are high signal and low signal. These two signals will all be received and processed by ADC, and then voltage difference can be obtained. This method is able to reject and filter the noise added to both wires. As to single-ended measurement, there is one signal wires, the other wire is directly connected to ground. This is usually done to reduce the number of channels between sensors and ADC. To take which measurement method will also depend on the data acquisition system or the cabling. For noisy environment, shielded twisted-pair cable would be a better candidate to effectively keep the signal clean. In a typical single-ended measurement loop, the resistor Rm is used to adjust the signal voltage to safe input voltage of the ADC, and resistor Rs is for voltage dividing.
Sensor of voltage output is of the advantage of easy troubleshoot, wildly accepted, and common and straightforward signal. Its downsides are also quite obvious. Voltage signals are susceptible to electrical interference. Surrounding devices, such as motors, relays, and power supplies will induce voltages variation onto signal lines and then degrade the voltage signal. The wire resistance will lead to voltage drop to the signal, especially over long cable runs. Therefore, sensors of voltage output are not well-fit for long distance signal transfer.
Starting from 0V makes it is unable to identify the malfunction and the Zero signal. On account of the application, a threshold starting point can be set manually for signal processing, such as starting from 0.5V or 1V. In this way, the faulty status or pure noise signal can be differentiated from starting point. For the sensor with negative starting point, such as -5V or -1V, a bias voltage can be added to adjust the voltage range to the testing range of the ADC.
A better feature of voltage output sensor is cheaper price, compared with current output sensor. In some cases, budget can be more flexible by selecting some more costly sensors. With cost reduction in components and fabrication, the cost difference is becoming smaller in many sensor types.