Consider the Raspberry Pi: a full Linux computer the size of a credit card, available for an incredible $35. While the manufacturer originally envisioned it as an educational tool, we've seen a wide range of implementations: from DIY arcade cabinets to robot controllers, photo booths, and everything in between. With the help of the Rhubarb Raspberry Pi interface board by 3ML, which adapts typical industrial power to the Pi's more sensitive GPIO pins, this dynamic system can now be at home in industrial applications as well.
As we can see from Rhubarb's product documentation, the device is compatible with nearly every Raspberry Pi available. Additionally, as an "open," or platform-agnostic, interface, users could theoretically pair it with other boards that bear the same 40-pin GPIO arrangement as the Pi. The Jetson Nano, which we featured here, comes to mind if your application calls for advanced processing power, though you'll need to come up with a cabling solution to accommodate for its larger physical size.
To understand how this device came about, I chatted with the Rhubarb's creator, John Davis. Davis specializes in high tech industrial motion control at his day job, and, as someone who has been using Linux since he was a teenager, bringing these two worlds together made perfect sense. The problem, however, is that the Raspberry Pi's GPIO much prefers carefully-regulated 3.3V signals to the higher, and often unpredictable, voltages you'll experience in the industrial control world.
Rhubarb takes care of civilizing these industrial signals:
- The board attaches to the Pi and provides a bank of isolated spring clamps to which users can run wires with a wide range of voltage levels.
- To power the Pi, the Rhubarb's setup calls for screw terminals and a buck converter.
- All an engineer or technician must do is attach it to an industrial controls panel and plug signal and power lines in.
And just like that, there's no fear of frying the voltage sensitive Raspberry Pi attached to the top.
Understanding Rhubarb: Raspberry Pi Industrial PLC
Let's break down the board's impressive specs:
- Supply Voltage: 10-30VDC
- Integrated real-time clock (Raspberry Pi does not have one — especially important as it may not always be connected to the internet)
- 12 isolated and filtered DC inputs
- Four analog inputs capable of industrial spec 4-20mA or 0-10VDC control
- Two relay outputs rated at 2A, 250VAC, 220VDC
- Two open collector (MOSFET) outputs rated to 50V, 300mA
- Physical Dimensions: 142mm x 65mm x 25mm height (with Raspberry Pi attached)
- RS232 Ethernet Output
- Built-in 2A power supply for Raspberry Pi
Davis is quick to note that while the Rhubarb/Raspberry Pi can perform some IO functions of a PLC (programmable logic controller — a typical industrial controls computer), the device isn't intended to replace such a device for larger machines, but instead act as a complementary piece of hardware.
For instance, while a PLC responds well to specific inputs and outputs without the overhead of an OS, if you need to process this information and put it on the internet or an internal network, things become much more prohibitive in terms of time and money. A Raspberry Pi, however, is very much at home processing and displaying data, and anyone competent in using these single-board computers could set up a simple web-based display in a few hours.
If you need a more localized display, the Pi can certainly handle this task as well. The Pi's HDMI port and audio jack are still exposed when installed with a Rhubarb. Considering the price of a Rhubarb (less than $500), a Pi, and a large video display, say 65 inches (~$500), you could have a plant metrics system up and running for around $1000. If you needed it on multiple displays, you can set up a second or third Pi to display this info on other screens via Wi-Fi or wired Ethernet, all without needing Rhubarb adapters. Considering the cost of a plant-wide metrics display just a ten years ago — Davis estimates somewhere in the $50,000 range to start — this type of setup represents a truly amazing bargain. As such, he's seen significant interest in this type of implementation, and it may be that this is where industrial Raspberry Pi usage really finds its niche.
When I asked Davis about what inspired this device, he said, "When I made the Rhubarb, I had an entirely untested hypothesis: as engineers and tech school students are graduating knowing how to use the Raspberry Pi, and other similar inexpensive 'maker' boards, we'll soon start to see this tech bleed into the industrial space. With these devices in their metaphorical (and actual) toolboxes, their instinct will be to use them in new and innovative ways."
The Rhubarb allows this kind of industrial implementation without reinventing the wheel (cobbling together their own adapter boards), thus providing a convenient platform with which to apply the Pi's processing power to the Industrial Internet of Things, or IIoT.
So far, it seems like Davis' hypothesis is proving correct, and as this maker and industrial hardware collision accelerates, we hope the Rhubarb continues to be at its forefront. With the Rhubarb available on Arrow.com, purchasing managers won't have to set up a new vendor to procure the product. For engineers under pressure to get a project done, with free overnight shipping, you could —theoretically — have your Pi-based data display up and running tomorrow!