What tools are available for calibrating sensors?

Tools used for calibrating sensors range from basic multimeters and multi-product calibrators to application-specific tools, like pressure calibrators, calibration baths, and infrared blackbody calibrators, depending on the type of sensor and the measurement requirements. As sensors get more sophisticated and precise, so do the tools used to calibrate them.

Sensors require calibration to deliver consistent accuracy. That’s important to ensure the correct operation of industrial process controls, meet regulatory requirements for medical and other devices, and even satisfy the expectations of insurance companies for the continued performance of advanced driver assistance systems (ADAS) after repairs following a vehicle crash.

Calibration can help compensate for offset errors, where the output is consistently higher or lower than the actual value; slope errors, where the output changes at a different rate than the actual value; and linearity errors, where the sensor output does not change linearly with changes in the stimulus. Three common calibration methodologies include:

One-point calibration, which corrects for offset errors by using a known reference value.
Two-point calibration uses two known reference values to correct for offset and slope errors. For example, a pressure sensor might be calibrated by exposure to a vacuum (0 Pa) and a known pressure. A temperature sensor can be calibrated using an ice bath (0°C) and boiling water (100°C) as reference points.
Multi-point calibration is the most accurate method, utilizing multiple reference values to correct for offset, slope, and non-linearity errors.

Calibrating temperature sensors

Temperature is the most sensed and measured quantity. It’s crucial for monitoring and controlling temperature in industrial processes, consumer appliances, medical devices, aerospace systems, and more. Accurate calibration of temperature sensors, such as standard platinum resistance thermometers (SPRTs), resistance temperature detectors (RTDs), thermistors, and thermocouples, can be implemented using a dry well and support tools (Figure 1).

Figure 1. Examples of a metrology well calibrator (left) and an expandable and configurable thermometer readout (right) for calibrating SPRTs, RTDs, thermistors, and thermocouples. (Image: Fluke)

Three common techniques for calibrating temperature sensing systems are (in order of improved calibration accuracy):

Using a simulator to calibrate only the electronics.
Using a dry well to calibrate both the electronics and the sensor.
Using a dry well and a reference thermometer to calibrate both the electronics and the sensor.

ADAS recalibration after an accident

Sensor calibration for advanced driver assistance systems (ADAS), such as automatic emergency braking (AEB), lane keeping, and blind spot monitoring, is a new skill set being utilized in automotive body repair shops.

Damage from vehicle crashes is measured in inches, and ADAS sensor alignment is measured in millimeters, with tolerances of ±3-5 mm being common. Accurate sensor alignment is necessary for sensors like radar that are designed to look down the road.

If a small amount tweaks the radar sensor alignment, it won’t be looking where the system expects it to be looking. That could mean it doesn’t see obstructions as far away as it should. For example, suppose the misalignment limits the system’s ability to identify and classify obstructions to only 50 meters, rather than 250 meters. In that case, it reduces the system’s reaction time to only one second at 50 miles per hour, potentially compromising safety.

Two-step ADAS calibration

ADAS calibration at automotive body shops is typically a two-step process. The first step involves static alignment of the sensors using predefined targets. Wheel targets can be used on all four wheels to ensure proper suspension alignment. Camera and radar targets are positioned on a test stand some distance away from the vehicle to test sensor alignment and perform initial calibration (Figure 2). The more precise the target setup, the better the calibration.

Figure 2. Example of sensor calibration targets used for static ADAS calibration. (Image: Body Shop Business)

Once the static calibration is completed, dynamic calibration is used to fine-tune the system. Dynamic calibration utilizes a scan tool connected to the vehicle’s computer and a technician operating under specific conditions. Dynamic calibration mimics real-world driving conditions and is especially useful for features like lane departure warning, AEB, or adaptive cruise control.

Summary

Sensor calibration for modern electronics applications typically relies on sophisticated tools and test methodologies. Dry wells and calibrated reference thermometers are often used to calibrate temperature sensors like SPRTs, RTDs, thermistors, and thermocouples. Recalibrating an ADAS system in an automotive requires a combination of static and dynamic calibration.