The 4-20 mA Current Loop

The I/O capabilities built into most microcontrollers make it easy to measure the analog world. Say you want to build a data logger for temperature. All you need to do is get some kind of sensor that has a linear voltage output that represents the temperature range you need to monitor — zero to five volts representing 0° to 100°C, perhaps. Hook the sensor up to and analog input, whip up a little code, and you’re done. Easy stuff.

Now put a twist on it: you need to mount the sensor far from the microcontroller. The longer your wires, the bigger the voltage drop will be, until eventually your five-volt swing representing a 100° range is more like a one-volt swing. Plus your long sensor leads will act like a nice antenna to pick up all kinds of noise that’ll make digging a usable voltage signal off the line all the harder.

Luckily, industrial process engineers figured out how to deal with these problems a long time ago by using current loops for sensing and control. The most common standard is the 4-mA-to-20-mA current loop, and here we’ll take a look at how it came to be, how it works, and how you can leverage this basic process control technique for your microcontroller projects.

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Life-Sized Pinball Drop Targets

[Bob] wanted to build a pinball-drop-style resetting target that he could use while practicing with his pistol. His first idea was to make the targets sturdy enough for use with 9 mm ammunition, and he planned to use 1/2” thick steel for the targets and 11-gauge steel tubing for the frame. However, the targets weighed 50 pounds together and that was more weight than the pneumatic actuators could lift. He ended up using 1/4” steel and thereby halving weight. The downside was that [Bob] had to switch out the nine for a .22.

Controlling everything is a 555 circuit. When triggered, it opens up a relay for one second, which trips the solenoid valve controlling the pneumatic actuators. Originally he wanted to have switches under each target, and only by dropping all four would the reset circuit be triggered. However, he built a simpler solution: a bulletproof button off to one side–effectively a fifth target–that when triggered resets the targets.

HaD have some pretty good shots in our number but we’d probably end up hitting the pneumatic actuators at least once. [Bob] did add 16-gauge steel sheeting to protect the air lines and wires from bullet splatter, which in his experience is more of a threat than a direct hit.

 

 

Soft-legged Robot Handles Rough Terrain with Ease

Whether it’s wheels, tracks, feet, or even a roly-poly body like BB-8, most robots have to deal with an essential problem: dirt and grit can get into the moving bits and cause problems. Some researchers from UCSD have come up with a clever way around this: pneumatically actuated soft-legged robots that adapt to rough terrain.

At a top speed of 20 mm per second, [Michael Tolley]’s squishy little robot won’t set any land speed records. But for applications like search and rescue or placing sensors in inhospitable or inaccessible locations, slow and steady might just win the race. The quadrupedal robot’s running gear can be completely 3D-printed on any commercial printer capable of using a soft filament. The legs each contain three parallel air chambers within a bellowed outer skin; alternating how the chambers are inflated controls how they move. The soft legs adapt to unstructured terrain and are completely sealed, eliminating intrusion problems. The video below shows how the bot gets around just fine over rocks and sand.

The legs remind us a little of our [Joshua Vazquez]’s tentacle mechanism, but with fewer parts. Right now, the soft robot is tethered to its air supply, but the team is working on a miniaturized pump to make the whole thing mobile. At which point we bet it’ll even be able to swim.

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A Robot Arm With The Tender Grip Of An Octopus

If you’ve ever experimented with a robot gripper, you’ll know that while it is easy to make an analogue of the human ability to grip between thumb and forefinger, it is extremely difficult to capture the nuances of grip with the benefit of touch feedback to supply only just enough of the force required to grip and hold an object. You as a human can pick up a delicate eggshell without breaking it using the same hand you might use to pick up a baseball or a cricket ball, but making your robot do the same thing is something of an engineering challenge.

The robot gripper is something that has exercised the minds of the folks at Festo, and the solution they have arrived at is as beautiful as it is novel. They have produced a gripper based upon the action of an octopus tentacle,  though unlike the muscle of the real thing they’ve created a silicone tube which bends inwards when inflated. Its inner surface is covered with octopus-like suckers, some of which can be activated by a vacuum. The result is a very capable and versatile gripper which due to its soft construction is ideal for use in environments in which robots and humans interact.

They’ve put up a slick video showing the device in action, which we’ve put below the break. Tasks such as gripping a rolled-up magazine or a plastic bottle that would tax more conventional grippers are performed faultlessly.

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Scissor Lift Shoes May Be OSHA Compliant

It’s been said that necessity is the mother of all invention. This was probably the fundamental principle behind the show “Inspector Gadget”, a story about a police agent who has literally any technology at his grasp whenever he needs it. Although the Inspector’s gadgets get him into trouble more often than not (his niece Penny usually solves the actual crimes), the Inspector-inspired shoes that [Make it Extreme] built are a little bit more useful than whatever the Inspector happens to have up his sleeve (or pant leg, as the case may be).

If a fabrication tour de force, [Make it Extreme] built their own “Go Go Gadget Legs”, a set of pneumatically controlled stilts that allow the wearer to increase their height significantly at the push of a button. We often see drywall contractors wearing stilts of a similar height, but haven’t seen any that are able to raise and lower the wearer at will. The team built the legs from scratch, machining almost every component (including the air pistons) from stock metal. After some controls were added and some testing was done, the team found that raising one foot at a time was the safer route, although both can be raised for a more impressive-looking demonstration that is likely to throw the wearer off balance.

The quality of this build and the polish of the final product are incredibly high. If you have your own machine shop at home this sort of project might be within your reach (pun intended). If all you have on hand is a welder, though, you might be able to put together one of [Make it Extreme]’s other famous builds: a beer gun.

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Pneumatic Launcher Gets Ham Antennas Hanging High

Amateur radio is an eclectic hobby, to say the least. RF propagation, electrical engineering, antenna theory – those are the basics for the Ham skillset. But pneumatics? Even that could come in handy for hanging up antennas, which is what this compressed-air cannon is designed to do.

[KA8VIT]’s build will be familiar to any air cannon aficionado. Built from 2″ Schedule 40 PVC, the reservoir is connected to the short barrel by a quarter-turn ball valve. Charging is accomplished through a Schrader valve with a cheap little tire inflator, and the projectile is a tennis ball weighted with a handful of pennies stuffed through a slit. Lofting an antenna with this rig is as simple as attaching a fishing line to the ball and using that to pull successively larger lines until you can pull the antenna itself. [KA8VIT] could only muster about 55 PSI and a 70′ throw for the first attempt shown below, but a later attempt with a bigger compressor got him over 100 feet. We’d guess that a bigger ball valve might get even more bang for the buck by dumping as much air as quickly as possible into the chamber.

Looking to launch a tennis ball for non-Ham reasons? We’ve got you covered whether you want to power it with butane or carbon dioxide.

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Keeping Humanity Safe from Robots at Disney

Almost every big corporation has a research and development organization, so it came as no surprise when we found a tip about Disney Research in the Hackaday Tip Line. And that the project in question turned out to involve human-safe haptic telepresence robots makes perfect sense, especially when your business is keeping the Happiest Place on Earth running smoothly.

That Disney wants to make sure their Animatronics are safe is good news, but the Disney project is about more than keeping guests healthy. The video after the break and the accompanying paper (PDF link) describe a telepresence robot with a unique hydrostatic transmission coupling it to the operator. The actuators are based on a rolling-diaphragm design that limits hydraulic pressure. In a human-safe system that’s exactly what you want.

The system is a hybrid hydraulic-pneumatic design; two actuators, one powered by water pressure and the other with air, oppose each other in each joint. The air-charged actuators behave like a mass-efficient spring that preloads the hydraulic actuator. This increases safety by allowing the system to be de-energized instantly by venting the air lines. What’s more, the whole system presents very low mechanical impedance, allowing haptic feedback to the operator through the system fluid. This provides enough sensitivity to handle an egg, thread a needle — or even bop a kid’s face with impunity.

There are some great ideas here for robotics hackers, and you’ve got to admire the engineering that went into these actuators. For more research from the House of Mouse, check out this slightly creepy touch-sensitive smart watch, or this air-cannon haptic feedback generator.

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