A big manufacturer of industrial monitoring equipment was looking to revamp its product line to improve response times and add features customers had requested. While the design team had multiple experts in control systems and measurement, when it came time to redesign the temperature measurement subsystem, they felt constrained by schedule. They needed to improve Electromagnetic Interference (EMI) immunity and measurement time while ensuring the new system would be backwards-compatible with their customers’ existing equipment utilizing 3-wire platinum resistance temperature detectors (RTD).
Chris, a design engineer on the measurement team knew that doing all this and maintaining the 0.5°C accuracy and 0.1°C resolution over the –200°C to +850°C range of the sensor, on a short deadline, was going to be a challenge. Luckily, Chris had been to a trade show recently where a demo was done showcasing the EMI immunity of a 3-wire RTD measured by the ADMW1001, a MeasureWare™-enabled industrial monitoring platform device. Asking around, he discovered ADI’s MeasureWare team had developed a patented Kelvin mode for sensing the 3-wire RTD—enabling the use of low cost RC filters. Filters are required to deal with the EMI in a typical plant—with motors, actuators, and heavy duty power supplies. “It was brilliant in retrospect, a clever solution to a difficult problem that many of my colleagues have struggled with over the years!” At the trade show Chris saw the ADMW1001 could also measure pressure and humidity, a feature he noted for later projects.
3-wire RTDs are common in industrial monitoring applications where the sensor may be far from the measurement system, and the weight and cost of the cabling are major concerns. 4-wire hookups allow for Kelvin-sensing the resistance element directly, but they have an extra wire in the cable and have historically been reserved for laboratory use.
Unfortunately, when using a 3-wire RTD, the cancellation of lead resistance so necessary for accuracy requires the sensor provider and the cabling hookup work in concert so that the leads are identical—any mismatch will cause an offset. This lead resistance mismatch is exacerbated by adding RC-type filters often used as cost-effective methods to reduce measurement noise from electrical interference, a common problem in industrial applications.
With the deadline to revise the system looming, Chris used MeasureWare Designer to find the correct kit to validate the solution to this difficult measurement problem.
A complete evaluation kit was then sent to Chris’ office; the kit included a MeasureWare EV-ProMW1001ARDZ Arduino R3-compatible shield, an ARM® MBED-enabled firmware development board, and a quality 3-wire RTD immersible probe. Once it arrived Chris was able to open the box where he found the link to download MeasureWare Lab, the easy to use desktop evaluation software for MeasureWare. After logging in to his MeasureWare account and downloading the example 3-wire RTD configuration he had built earlier in MeasureWare Designer, Chris connected the EV-ProMW1001ARDZ shield to the development board and to his laptop via USB. With the supplied probe connected, Chris immediately saw the room temperature displayed on the live graph of MeasureWare Lab. “It was awesome how quick I went from getting the kit in the mail to seeing a 3-wire RTD reading pop up on my screen!”
Encouraged, Chris then took his laptop and the EV-ProMW1001ARDZ kit to the RTD embedded in a laboratory test chamber in place of their old measurement board. Within minutes MeasureWare Lab confirmed the correct operation of the temperature stabilization system.
Chris wanted to do a longer-term test to be sure the system was working correctly, so he attached a compatible Wi-Fi shield with MBED support to the EV-ProMW1001ARDZ and configured it to use his office’s network. This way he could use MeasureWare Lab to capture data from his desktop and upload to his secure private MeasureWare cloud datastore. He logged in from his laptop and saw the data displayed in his dashboard.
Chris headed home for the weekend, checking in on the web to verify things were going smoothly. “It was great to see the data continually updated via my MeasureWare account. Having secure access from anywhere in the world was a huge confidence booster.”
On Monday he showed the collected data and the simple reference design of the MeasureWare EV-ProMW1001ARDZ to his design team lead, and the response was positive. “I really liked how flexible the platform can be with the universal analog inputs. I have a feeling we’ll be reusing this circuit on a couple of other designs shortly,” Ralph, the team lead, noted. The other teams were in as well. Once the firmware team saw the MBED-enabled example project could be easily ported to their IDE of choice, and the ADMW1001 library host interface made register maps obsolete, the decision was made. Going forward, they would use the ADMW1001 to handle industrial monitoring duties in the new design, eliminating the painful learning curve normally associated with designing a new precision measurement interface from scratch.