What is Sensor Calibration- Definition and It’s Application

We use different systems and types of equipment for measuring various physical quantities. The accuracy of the measurement depends upon various factors. The equipment used for measurements can lose their precision when used at higher temperatures, high moisture or humidity conditions, subjected to degradation, subjected to external shocks, etc…This can be observed as the error in the measurement. To tackle this error and make necessary changes to the equipment calibration methods are used. Today sensors are being used for making various measurements. There are sensors to measure temperature, color, humidity, etc…Sensor Calibration plays a crucial role in removing the errors in sensor measurements.

What is Sensor Calibration?

Sensors are electronic devices. They are sensitive to the changes in their working environment. Undesirable and sudden changes in the working environments of the sensors give undesired output values. Thus, the expected output differs from the measured output. This comparison between the Expected output and measured output is called Sensor Calibration.

Sensor calibration plays a crucial role in increasing the performance of the sensor. It is used to measure the Structural errors caused by sensors. The difference between the expected value and the measured value of the sensor is known as the Structural Error.

Working Principle

Sensor calibration helps in improving the performance and accuracy of the sensors. There are two well-known processes in which sensor calibration is done by industries. In the first method companies add an In-house calibration process to their manufacturing unit to perform individual calibration of the sensors. Here the company also adds necessary hardware to their design for sensor output correction. By this process, the sensor calibration can be changed to match the application-specific requirements. But this process increases the time to market.

The alternative of this In-house calibration process, several manufacturing companies provides sensor packages with a high-quality automotive-grade MEMS sensor along with complete system-level calibration. In this process, the companies include an onboard digital circuitry and software to help designers to improve the functionality and performance of the sensors. To reduce the product design time and component count, digital circuitry such as voltage regulation and Analog signal filtering techniques are included. To improve the overall performance and functionality, the onboard processor is provided with sophisticated sensor fusion algorithms. Some of the sophisticated onboard signal processing algorithms also help in reducing the manufacturing time enabling the faster time to market.

Standard Reference Method

Here the sensor output is compared with a standard physical reference to know the error in some sensors. Examples of sensor calibration are rulers and meter sticks, For temperature sensors- Boiling water at 100C, Triple point of water, For Accelerometers- ”gravity is constant 1G on the surface of the earth”.

Calibration Methods

There are three standard calibration methods used for sensors. They are-

• One point calibration.
• Two-point calibration.
• Multi-Point Curve Fitting.

Before knowing these methods we have to know the concept of Characteristic curve. Every sensor has a characteristic curve that shows the response of the senor to the given input value. In the calibration process, this characteristic curve of the sensor is compared with its ideal linear response.

Some of the terms used with the characteristic curve are-

• Offset – This value tells us whether the sensor output is higher or lower than the ideal linear response.
• Sensitivity or Slope – This gives the rate of change of sensor output. A difference in slope shows that the sensor output changes at a different rate than the ideal response.
• Linearity – Not all sensors have a linear characteristic curve over the given measurement range.

One point calibration is used to correct the sensor offset errors when accurate measurement of only a single level is required and the sensor is linear. Temperature sensors are usually one point calibrated.

Two-point calibration is used to correct both slope and off-set errors. This calibration is used in the cases when the sensor we know that the sensor output is reasonably linear over a measurement range. Here two reference values are needed- reference High, reference Low.

Multi-point Curve fitting is used for sensors that are not linear over the measurement range and require some curve-fitting to get the accurate measurements. Multi-point curve fitting is usually done for thermocouples when used in extremely hot or extremely cold conditions.

For all the above calibration process, the characteristic curves of the sensors are drawn and compared with the linear response and error is known.

Applications of Sensor Calibration

Sensor Calibration in simple terms can be defined as the comparison between the desired output and the measured output. These errors can be caused by various reasons. Some of the errors seen in sensors are errors due to improper zero-reference, errors due to shift’s in sensor range, error due to mechanical damage, etc…Calibration is not similar to adjustment.

The calibration process includes placing the DUT-‘Device Under Test’ into configurations whose inertial input stimuli for the sensor are known, which helps us to determine the actual errors in the measurements.

The calibration process helps us to determine the following results-

• No error noted on the DUT.
• An error is noted and no adjustment is made.
• An adjustment is made to remove the error and the error is corrected to the desired level.

For sensor calibration sensor models are used. Sensor calibration is applied in Control systems to monitor and adjust the control processes. Automatic systems also apply te sensor calibration to get error-free results.

Use of Sensor Calibration

The calibration process is used to increase the performance and functionality of the system. It helps in reducing errors in the system. A calibrated sensor provides accurate results and can be used as a reference reading for comparison.

With the increase in the embedded technology and low size of sensors, many sensors are integrated over a single chip. Undetected errors in one sensor can cause the whole system to degrade. It is important to calibrate the sensor to get the accurate performance of the automated systems. What are the standard references used for the calibration of the temperature sensors?