Wireless oscilloscope sends data to a tablet

-October 25, 2013

Aubrey Kagan, engineering manager at Emphatec had a measurement problem. He needed a low-speed oscilloscope to monitor voltages and current inside a welding machine controller panel. For operator safety, the machine's controlling equipment had to be enclosed, making the electronics inaccessible through probes. Therefore, using a conventional oscilloscope was impractical because there was no way monitor when the panel was operational or to move the probes around to different locations in the machine. What to do?

At around the same time, Kagan won an iPad and thought "If only I could put a USB oscilloscope inside the enclosure and connect it the iPad." Unfortunately, that wasn't practical. Next, he thought about using a wireless connection, and that was the impetus for his BluDAQ device. Kagan entered the device in a contest and received honorable mention in the electronics category. I recently spoke to Kagan about the design and how he made it work with an Android tablet, not an iPad, and why.

The measurement electronics needed to connect to several voltages and currents in a welding machine used in the automotive industry. Kagan wanted to measure signals on two differential low-voltages, several high voltages (about 30V), and AC/DC currents. Thus, the measurement system would need a multiplexer. Figure 1 is a block diagram of the system. (Click on the image to open a larger image in PDF format.

Figure 1. A block diagram of the BLuDAQ shows the I/O channels and communications link. Click on the diagram to view a larger image in PDF format.

Initally, the measurement system was intended as a development tool, but, as Kagan explained, "If we could get the cost low enough, we could incorporate one into every machine's control panel." With a control panel cost of several thousand dollars, a low-cost monitoring tool would be worth adding.

Instead of trying to buy a data-acquisition system, Kagan designed one based around a Cypress Semiconductor PSoC5, which Kagan described as "a wonderful chip." Kagan needed only design the signal-conditioning and multiplexer circuits for his board. The rest is done in the microcontroller. During development, Kagan had some triggering issues that he was able to solve just by reconfiguring the logic circuits in the controller. Figure 2 shows both sides of the board.

Figure 2. The BluDAQ board has several I/O connectors and an RS-232 port for delivering data to a Bluetooth adapater, where the data is displayed on an Android tablet.

Using the microcontroller, Kagan added additional I/O: three digital I/O lines and two analog outputs that can be used for diagnostics and control if necessary. The microcontroller's 10-bit DAC provides sufficient resolution. There's also a PWM output. The PSoC5 also handles DMA transfers from its ADCs.

Getting the board to connect to and digitize the analog signals was one thing, but getting the data out of the closed metal box was another. The PSoC5 has an RS-232 link that Kagan could use to bring data out of the enclosure through a D-9 connector on the welding-machine's panel, which then connects to a Bluetooth module (Figure 3).

Figure 3. The BluDAQ board is housed in a plastic enclosure that mounts on a rail. The Serial-port cable connect to the Bluetooth module, which is mounted outside the welding machine's enclosure.

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