You’ve probably noticed that rotating objects are a rather standard component in the countless machines and devices that have come to define the modern age. It’s not particularly difficult to make things turn, but precision control requires knowledge of the turning thing—i.e., feedback.
In a previous article I discussed a Hall-effect-based rotational sensor IC. In that arrangement a magnet is physically coupled to the rotating object, and the IC tracks rotational changes via the Hall effect. As usual, this non-mechanical approach has benefits. However, mechanical solutions are by no means obsolete.
I for one appreciate the simplicity of mechanical solutions: Sometimes it’s nice to just push a button, instead of worrying about capacitive-touch-sense algorithms. Sometimes it’s nice to control your power supply via a physical switch or jumper instead of a complicated power-source-detection circuit. And sometimes it’s nice to measure rotational characteristics without stressing about proper magnet-to-IC alignment.
The internal functionality of the A1337 Hall-effect-based angular position sensor. Compare this to the diagram in the next section, and you’ll see that the non-mechanical approach involves a bit more complexity. Diagram taken from the A1337 datasheet.
In general, you can’t compete with the simplicity offered by mechanical solutions.
Diagram taken from the PHS04 datasheet.
As you can see, this particular sensor is basically one long resistor. You apply voltage across pins 1 and 3, and pin 2 gives you a voltage corresponding to the angle. There’s one thing you have to be careful about, though: the PHS devices don’t actually provide measurements for the full 360°. The specs for the PHS04 series give “360°” as the “total rotational angle,” but the diagram above clearly shows a range of angles in which measurements are not possible. The “electrical angle” (presumably this refers to the actual angular measurement range) is 280° for PHS04 devices. For the PHS07 and PHS11 series, the maximum electrical angle is 310° and 341°, respectively.
This simplicity comes with a price. The primary complaint against mechanical solutions, in my experience at least, is reliability. The laws of nature seem to conspire against macroscopic moving objects in such a way that they will stop moving (or maybe just stop moving properly) long before a comparable microscopic device experiences equivalent degradation.
If you’re working on a DIY project that will be stashed away and forgotten in a few weeks, you don’t have to worry much about reliability. I imagine that reliability is also a low priority for various consumer products that emphasize low cost over durability. But there are many, many applications in which reliability is serious business—automotive, military, avionics, industrial. Are mechanical solutions an option in systems such as these?
Well, I think they certainly are, but you have to choose your components carefully.
The PHS Series
TT Electronics apparently believes that mechanical sensors can be perfectly adequate, even when conditions are adverse and high reliability is required. They describe the PHS series of rotary position sensors as “robust and reliable” and as offering high precision for “harsh environments” and “critical applications.”
How exactly TT achieves this performance is information that I certainly don’t have. They mentioned “dust-proof construction,” and it makes sense that a major aspect of high reliability with mechanical devices is a sealed enclosure that protects the internal components from troublemakers such as dust, moisture, and harsh chemicals.
A PHS04 rotary position sensor. The slot in the center is used to couple the sensor to the rotating object. Image taken from the PHS04 datasheet.
The most straightforward metric for reliability is “number of cycles” (or something to that effect)—e.g., how many times can you push a pushbutton before it stops doing the button thing?
The PHS sensors are rated for many, many cycles—over a million, except for the PHS04 series, which offers a 300,000-cycle option and a 500,000-cycle option. I don’t know exactly how “number of cycles” applies to a rotary device. My best guess is that one cycle corresponds to when the load stops rotating and then starts rotating again.
Another thing I’m not sure about: The product information from TT Electronics indicates that PHS devices convert angular position to an “analogue or digital electrical signal,” but it seems to me that all three series are strictly analog.
Do you have any advice on using mechanical vs. non-mechanical parts? Feel free to share your thoughts in the comments section.