Using precision temperature sensing in industrial monitoring systemsTemperature sensing forms an integral part of any industrial monitoring system. It is a widely used parameter in any given process controlled environment. Some of the common sensors used for measurement of absolute temperature or change in temperature are resistance temperature detectors (RTDs), diodes, thermistors, and thermocouples.
In this article, we will go through the points to be taken care while implementing precision temperature measurement system using these sensors. Temperature sensing involves selection of right sensors as well as necessary signal conditioning and digitization in order to efficiently and accurately measure the temperature. Before we go to the temperature measurement system, let's examine the advantages and disadvantages of each of the commonly-used temperature sensor.
Thermocouples work on the principle that the junction between two metals generates a voltage that is a function of temperature. A thermocouple consists of two pieces of dissimilar metals joined at one end which is called the hot junction. The other end which is called cold junction is connected to a measuring circuit. The difference in temperature between the hot and cold junctions causes an electromagnetic force (EMF) to develop. This EMF can be measured by the measurement circuit. Figure 1 shows a basic thermocouple circuit.
The actual voltage generated by the thermocouple depends on the temperature and the type of dissimilar metals used in the thermocouple. The sensitivity and temperature range of a thermocouple is dependent upon the metals used. There are numerous thermocouples available on the market which can be classified/distinguished based on the metals used; for example, type B (Platinum / Rhodium), type J (Iron / Constantan), and type K (Chromel / Alumel). One can be chosen based on the application's requirements.
The primary advantages of thermocouples are their robustness, wide temperature range (-270 degree to 3000 degree), fast response, availability in several package types, and low cost. Looking at their limitations, they suffer from low accuracy and high noise.
Resistance Temperature Detectors
Resistance temperature detectors (RTD) works on the principle of change in electrical resistance in metals due to a change in temperature. Each metal has a specific and unique resistivity. The resistance of a metal is directly proportional to the length of the wire and inversely proportional to the cross-sectional area of the wire. The constant of proportionality is the resistivity co-efficient of the wire.
The selection of metal for the construction of the RTD is a critical consideration in being able to measure temperature accurately. Metals used for the construction of RTDs are platinum, nickel, or copper. Of the three materials used in RTDs, platinum RTDs are the most accurate and reliable. They are also less susceptible to environmental contamination, which results in long term stability as well as repeatability. High temperature range (-250 degrees to 900 degrees), high accuracy, and linearity are the major advantages of RTDs. Their limitations include higher cost and slow responsiveness.
Similar to RTDs, thermistors also work on the same principle that resistance changes with temperature. However, thermistors generally have a negative temperature coefficient. The major advantages of thermistors are their low cost and reasonable accuracy. Their disadvantages can be identified as low temperature range and non-linearity. However, given that many of today's microcontrollers have on-chip Flash, a lookup table can be maintained to mitigate linearity issues. If the temperature needs to be measured within -100 degrees to 300 degrees, thermistors can be used reliably for precision temperature measurement.
Temperature Measurement Systems
In any temperature monitoring system, a sensor must convert the temperature to an electrical signal which is then passed through a signal conditioning stage (the requirements vary based upon the sensor used), and onto an analog-to-digital converter (ADC). The system also needs communication peripherals to interface to other larger systems to provide feedback, as well as on-chip Flash memory for logging the values and some kind of display. Figure 2 shows the basic block diagram of a temperature measurement system.
Though Figure 2 shows signal conditioning before the ADC, there may be a need for post processing as well after converting the signal, depending upon whether the system is analog or digital. Overall accuracy is dependent upon the noise, offset, and gain error introduced in the pre-processing circuit and ADC. Data acquisition of temperature values from remote locations in real-time is a good feature in some of the applications like refineries, industries, and various automation. Serial communication protocols like UART, I2C are supported to transfer this data to a master system controller.
Improving Thermocouple Accuracy
Thermocouple-based systems are the most widely used for industrial control due to their huge temperature range. Here the EMF generated between junctions is measured to sense the temperature. It is assumed that the cold junction is at exactly zero degrees centigrade. However, maintaining a cold junction at this temperature is not practical in real applications. To achieve a precise measurement requires a technique known as cold junction compensation (CJC).
For CJC, a thermocouple-based precision temperature application is equipped with additional temperature sensor mounted at the tip of cold junction to measure the cold junction temperature. The most commonly-used sensor for cold junction temperature measurement is a thermistor due to its low cost and wide enough temperature range to cover cold junction temperatures for most applications. To measure the CJC voltage, first find the cold junction temperature and then check for the thermocouple EMF for that temperature. This is then added to the cold junction voltage, yielding the CJC voltage whose corresponding temperature is the actual value.
The EMF produced across a thermocouple measures only a few uVs, making it susceptible to noise. Also, before this signal is fed into the ADC, it needs to be amplified which also adds noise and offset. For precision measurement, this noise and offset should be removed. For the example system, let us see how to remove the offset and low frequency noise using correlated double sampling (CDS).
CDS reduces low frequency noise and offset in the signal processing stage. First the zero referenced offset is measured (to measure it, both inputs are shorted) and then the thermocouple voltage is measured. When the direct thermocouple signal is measured, it will include the actual thermocouple voltage, noise voltage, and offset (equation 1). The zero referenced reading includes noise and offset (equation 2).
The previous zero referenced sample with respect to the current zero referenced measurement is:
The difference, then, between the current thermocouple measurement and the previous zero referenced signal is:
Voffset is static, so its value on the current sample is the same as the value on
the previous sample. VN is not static as it is noise and so the drift term needs to be eliminated. Subtracting the previous noise term from the current sample will remove low frequency noise. Thus, CDS works as a high-pass filter.
The ADC itself has a low pass filter response removing high frequency noise. However, an IIR filter at the output of ADC will help to further attenuate the noise in the frequency range of transition or pass band of ADC. Mixed signal controllers available in markets are equipped with digital filters which can perform filtering in hardware instead of doing filtering in firmware and requiring CPU cycles. Figure 3 shows an implementation of a thermocouple-based temperature monitoring system using PSoC3 and PSoC5 devices from Cypress Semiconductors. These devices have an on-chip DelSig ADC with 20-bit resolution, built-in programmable gain buffer for signal amplification, and digital filter block (DFB) for filtering. It provides a highly integrated temperature measurement system. However, an additional gain stage may be needed based upon the thermocouple being used in the design. This gain can be given by an instrumentation amplifier which can be built using on-chip programmable gain amplifiers (PGA).