Designing for the Digital Age

Online Editor

Michael Mende discusses the emergence of a new generation of calibration systems created for the digital sensor age

Most of the sensors that are used for measurements of dynamic quantities in laboratories or in the field today still have an analogue output. A calibration system for such sensors is required to take care of the signal conditioning and AD conversion of the analogue signals. In turn, a calibration software is comparing the output signal of the device under test (DUT) to the output of a reference sensor to determine a calibration result. Thus, the DUT sensor is simply a signal source while the signal processing is handled by the calibration system.

Meanwhile, an increasing amount of sensors on the market are all-digital sensors. Examples include accelerometers, angular rate sensors and force sensors used at automotive crash test sites that come with a serial bus interface called DTI. Vibration sensors for machine diagnostics also have a digital bus interface such as the CAN-MD. Some of these vibration sensors as well as geophones used for the monitoring of building sites even store their measurement results directly in a cloud application. Being involved in the testing and adjustment of MEMS sensors on high volume production sites, product managers and developers at Spektra have observed this transition from analogue sensors to digital sensors already some years ago. Thus, the teams decided to design the new CS Q-Leap generation of sensor calibration systems, which is capable of calibrating analogue as well as digital sensors.

When the project started, it was immediately clear that both the measurement electronics as well as the software of the calibration system had to be prepared to handle digital sensors. The new Hero measurement system and vibration controller had to support very different types of interfaces such as SPI, I2C, I3C, CAN, PSI5, and many more. For this purpose, Spektra has integrated a digital input card that was already approved in the internal production test system besides the analogue input channels. Based on a FPGA, this digital input card now makes it possible to configure nearly any digital interface on the market. Additionally, there is a programmable power supply for the DUT on the card that allows users to supply up to four sensors independently.  

Other Design Factors

But besides the physical sensor interface, developers had to take care of the logical communication between the DUT and the calibration system. Since signal conditioning and AD conversion of the signal are now integrated in the sensor, many sensors can be configured by software commands to switch between measurement ranges or filter characteristics. Last but not least, the digital output has to be read from a defined sensor register. Spektra decided to use a microcontroller on the digital input card that can handle such low-level communication tasks in real-time and that translates the low-level communication into a standardised high-level protocol for the calibration software.

Although the company developers had to write a short ‘microcontroller software driver’ for each specific type of sensor for the low-level communication, the PC software does not have to be adapted to new sensor types. It just needs to load the suitable driver for the DUT into the microcontroller before starting measurement. Thus, the system can be easily adapted to new sensor types by programming a new FPGA logic for new hardware interfaces and new ‘drivers’ for the logical communication. An integrated sensor database in the calibration software helps system operators load the correct driver for the DUT and only displays configuration options that fit the DUT.

With the CS Q-Leap family, Spektra has developed a new generation of calibration systems that are ready and fitted for the digital sensor age, without neglecting ‘older’ sensors.

Michael mende is with Spektra

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