Analogue signals refer to the transfer of data through electrical signals by sensors to a data logging unit in the field of Structural Health Monitoring (SHM). The most commonly used analogue signals in SHM include -10V to 10V signals, 0V to 5V signals, and 4mA to 20mA current signals.
Voltage signals are a prevalent method of measuring sensor output in electronics. These signals are generated through analogue electronics, typically through changes in resistance, electrical fields, or other physical parameters. In SHM, voltage signals are commonly utilized as they are widely supported by sensors and the necessary analogue-to-digital converters (ADC) to measure the data are often the most cost-effective and standard option. However, voltage signals have a limited range for transmission via cables due to signal strength loss. As a general guideline, a voltage signal should not have a wire length exceeding 25m, as the resistance of the wire can cause a loss of voltage, resulting in a lower signal value. Voltage signals can be expressed in various ranges, such as a percentage of the incoming voltage or a 0V to 5V range, depending on the manufacturer. Measurements are typically taken through a parallel connection on the sensor, which includes a voltage input for power, a signal voltage connection, and a 0V signal connection.
Current signals, on the other hand, measure the current consumed by the sensor as opposed to the voltage, and the measured value is in amperes. Common measurement ranges include 0mA to 20mA or 4mA to 20mA. Measurements are taken through a current loop, which includes power input, signal output, and sometimes a 0V connection for other electronics in the sensor. One advantage of 4mA to 20mA current signals is their longer transmission range via cables compared to voltage signals. As the wire resistance increases, the voltage drops, but the current remains constant, allowing for wire lengths of up to 300m, as opposed to the 25m limit for voltage signals. Current signals are more prone to noise resistance, when using voltage signals additional measures may need to be taken to mitigate this.
To read analogue signals, an ADC is required, which is typically built into a data logger. ADCs convert analogue signals into digital numbers that can be stored and transferred. The precision of an ADC is described in bits, with a simple ADC capable of transforming an analogue signal into a 16-bit number, representing 65,636 different values within the sensor signal range. This results in a maximum precision of the signal range divided by 65,636. For a 4mA to 20mA signal, this equates to a precision of 0.0002mA or 0.000015% of the total signal range. However, it should be noted that this number refers to the resolution of the ADC and not the precision, which is typically lower. High-range ADCs have a resolution of 24 bits, representing 16,777,216 values, providing a higher precision.
Working with analogue signals also allows for the use of analogue filters to remove noise, spikes, and unwanted frequency ranges from the signal before it is converted to a digital output. To avoid aliasing issues, the signal should always be filtered with a low pass filter at least half the collection sampling rate. For example, if the measurement rate is 20 Hz, the low pass filter should be set at 10 Hz.