There are two general applications to use LVDTs with pneumatic cylinders described here:

One, which is the larger of the two, is to instrument an air cylinder in a way that makes it into a remotely programmable positioner. Right now a lot of air cylinders have some kind of limit switch on them. A technician has to physically go to the installation site, move the switch around and check it out to see if it works correctly or not. Typically it's set up that way for two or three weeks, and then manufacturing has to change the line over and the tech has to come down and do the same thing again. The ideal thing would to be able to program the cylinder ram position independently by picking up the position sensor's output, and when the cylinder ram moves the desired distance, that becomes the set/trip point. Ideally, it could be done using a laptop computer on-site, or maybe even by programming the cylinder stop point from the tech office by means of an ethernet connection. So a large application is to measure the position of the cylinder ram, typically but not exclusively, for the purpose of having the equivalent of a programmable limit switch where a user can set a position and then when the cylinder gets to that position, the output of a PLC or a set point controller shuts off the air supply and the cylinder ram stops moving and stays put.

The other way to use LVDTs with pneumatic cylinders, in which Macro Sensors has shown an interest, is making a long range (several inches) type of air-extended gauging probe. In such applications, the probe (ram) moves out, and the measurement is made only near the very end of the probe movement, i.e., when it is almost all the way extended, so as to be able to use a short range LVDT to get high resolution and then have the probe out of the way so it doesn't get damaged when it's retracted. The idea is to make a gauging probe using a "0.050 inch or a "0.125 inch ("1.25-3.0 mm) range LVDT out at the tip area to make measurements when the cylinder is fully extended. But when not making a measurement, the cylinder is retracted to get the ram and the gauging probe tip way out the way for manually loading parts in and out, for parts coming from transfer lines, for heavy parts that get banged and bounced in a foundry, etc.

Macro Sensors thought this gauging application held a lot of promise and did an application note which was published in a design engineering magazine. A lot of respondents said that this style of long range gauging probe is just what they need. They wanted something that will only measure a short distance: "I don't have to measure the total range of this; I just need to get the probe out of the way". We showed a one- or two- inch (25-50 mm)stroke cylinder pulling the probe back, but really measuring a nominal plus or minus one millimeter or so, at full probe extension and then getting the probe out of the way.
There is a minor issue that requires being careful because the pneumatics is not a simple instrumentation thing; it depends not only on what pressure is used but more so, how much air flow is permitted. With an excessive air flow, the cylinder ram moves out too fast and the result is banging the probe tip or beating the workpiece to death. Thus, there is a risk of damaging by impact the parts being measured. Typically this requires flow control valves in the cylinder ports to adjust the rate of motion and the momentum applied to the probe tip.

Some air cylinder manufacturers do buy potentiometers, that go inside of their cylinders. But it's not a big market for them at this point. It's more like a customer convenience as opposed to a real significant market. However, air cylinder makers show in their catalog that they do have this built-in proportional output sensor. Being built in means it is contained within the cylinder. LVDTs, on the other hand, are not easily contained within a cylinder, because the ram diameter is typically too small for even miniature LVDTs. Thus, LVDTs are usually connected in tandem with the cylinder ram. There are two ways to have a tandem linear sensor installed on a pneumatic cylinder. One way is to have one unit in front of the other, which is "in-line tandem", also called "back-to-back" tandem mount. The other installation approach is where both cylinder ram and sensor axes are parallel, with both units mounted side-by-side and with their moving parts connected together. The choice of one method over the other is really determined by the space available and the mechanical skills of the installer.