Multi-Sensor Data Acquisition System

Here’s another three-company cooperative project. For our first, under Digital Projects see E-Beam Sweeper Control. Unlike the first, this project was spread across the country: our client on the west coast, us in Florida and the programmer in New York State. We made good use of Zoom and email.

Our client, Cutsforth, Inc, is a manufacturer of advanced generator shaft grounding systems They needed a system to monitor temperature and humidity in multiple control equipment cabinets. Cutsforth did the system design, SRQ Technology designed the electronics, and the programmer provided the firmware.

A diagram of a data acquisition system for up to 30 remote sensors.

Cutsforth selected a National Instruments data acquisition system (DAQ) and a temperature/humidity sensor from Sensirion. The system needed to accommodate up to 32 sensor pairs on a daisy-chained field network. Each pair would be mounted on a small circuit card inside an enclosure designed by Cutsforth. A separate circuit board would interface between the DAQ and the network and provide power to the sensor cards.

Our job was to design the sensor cards plus the interface between the National Instruments DAQ and the field network. The system involved three different communication protocols. The DAQ used RS232, the field communication needed to be an addressable multi-point RS485 Modbus network and the sensors themselves used I2C communication. RS232 to RS485 translation needed only hardware, level-shifting circuitry using standard ICs, but the RS485 to I2C conversion required simple, low-cost microcontrollers.

The field network needed to be galvanically isolated from the National Instruments system and from 24Vdc system power. Further, because the application involved monitoring high voltage power generators, we also needed to include surge suppression to 4,000 volts. The sensor was rated to 80OC: the circuitry needed to be the same.

We designed the little sensor cards and the interface & power board. We didn’t do the programing: that was done by the third-company programmer. They had been contracted to program the National Instruments DAQ system including communications, so it made sense for them to program the communication functions in the microcontrollers as well.

A final note: although this system used only temperature/humidity sensors, many other sensors also communicate on I2C. The system could easily be modified to work with a variety of sensors.