Hall Effect Sensors vs. Proxes
While doing some of the minor website maintenance tasks that you sometimes have to do when blogging, I came across a site called Alexa. I always wondered about search engine optimization (SEO) and how it works… this gave me a bit of a clue. Since I have two websites to maintain I typed both URLs into the site and surprise… the Automation Primer actually showed up! My other Automation Consulting, LLC site did not, but since its a work in process and its brand new I’m not too worried about that.
Anyway, one of the cool things about the Alexa site is that it tells you where your traffic comes from, what people were looking for when they found your site. I’ve always been curious as to how people got to my site if I didn’t tell them about it. Many of my hits come from Europe and Asia, for some reason especially India. (By the way, if you are reading this and you don’t know me personally, please comment or send me an e-mail telling me how you found the site… my curiousity is killing me!) Anyway, one of the biggest hits were from people searching for the following: “Is hall effect inductive or capacitive?” This seems like a somewhat misguided question unless you are just curious. But since this is one of the biggest hits on my site, who am I to question it? So here goes…
A Hall Effect sensor is really niether inductive or a capacitive, but it is closer to an inductive prox. For a great description of what the Hall effect is, just check out Wikipedia, which has a pretty good description. Essentially a hall effect sensor senses a magnetic field, whereas an inductive sensor creates its own magnetic field. Any current-carrying conductor creates a slight magnetic field of its own, transverse to the direction of current flow. The hall sensor creates a voltage difference based on the amount of magnetic field it senses, hence it is used to sense a magnetic object such as a magnet moving with a piston inside of a cylinder body. This makes it a great choice for end of cylinder stroke sensors since it can sense the magnet through a metal (typically aluminum) body.
An inductive prox uses an oscillator circuit to set up its own magnetic field. When a metal object large enough to absorb the field gets close enough, the oscillator stalls and the condition is sensed and used to switch a solid state device. So this is the fundamental difference between an inductive prox and a hall effect sensor. As far as a capacitive prox, this uses a plate that carries a charge. Any object with its own charge (which does not have to be metal at all…) will drive electrons off the plate and also switch a solid state device. Totally different application. Capacitive proxes are often used for sensing liquids through the side of a plastic tank or sensing a solid object such as your hand.
A better question might be when to use a Hall effect sensor or an inductive prox? Well, as I mentioned, since a hall effect senses the magnet inside of a metal body, it is a great choice for sensing end of stroke. Whenever possible however it is best to sense the actual tooling that is moving rather than the cylinder position; what if your cylinder linkage breaks? What if the magnet comes loose inside of the cylinder? This is actually somewhat common. When sensing a piece of metal tooling attached to an actuator the choice would be an inductive prox.
Asking the right question is often the most important part of engineering. Knowing what I have found about what people are searching for will hopefully allow me to tailor this site to search words. Other than the few comments I receive on this site I really have no idea how much meaningful traffic I get. This site does count the people who visit the site but it doesn’t say anything about who actually reads it. If you are reading this, please leave a comment just to let me know you read this far!