Installation considerations for pneumatically-extended, spring-retracted LVDTs, models GHSAR 750-A, GHSDR 750-A, GHXA 750, and GHXD 750

Prepared by Edward Herceg

Air-extended, spring-retracted LVDT sensors offer both users and systems integrators of factory automation equipment a good solution for making many position and/or dimensional measurements that would not be practical or cost-effective by any other means. They are especially attractive for applications in which it is desirable or important to have the LVDT probe shaft well out of the way when loading or unloading a test piece from a gaging jig or fixture. They also offer the advantage of setting the LVDT probe tip’s contact force by adjusting the applied air pressure, as well as purging the LVDT probe shaft bearing of possible contaminants by venting air through the bearing space. However, because these sensors are pneumatically operated, it is necessary to consider the installation in accordance with the following recommendations.

The air used to operate these sensors must be clean and dry. Typical filtration requirement is 2 micron maximum particle size. The moisture content of the air at room temperature should produce 10% or less relative humidity.

Under no circumstances should an air-extend spring-retract LVDT gaging probe be connected to a lubricated air supply such as is typically supplied to rotary air tools.

The minimum air pressure required to extend these probes varies with the model of the probe and with each installation, but is typically between 15 and 25 psig. Factors affecting it include the preload force and spring rate of the probe's retraction spring, the probe's orientation to the gravity vector, and the air flow venting through the probe's bearings. The actual operating air pressure is determined by the probe contact force desired and the air flow needed for the cycle rate required, as detailed in a later section. To adjust the operating air pressure, a low-pressure relieving regulator for 0 to 30 psig with an outlet pressure gauge is recommended.

Air flows through the probe’s bearings when the air pressure is applied. With the minimum operating pressure applied to a probe and no separate flow restriction, the purge air flow rate is low, typically much less than 1 cfm. As the pressure is increased for greater contact force or cycle rate, the purge air flow also increases. To conserve both air and energy, the lowest operating pressure and purge air flow that give consistent results should be used.

The operating cycle begins with the LVDT probe shaft fully retracted. Then air pressure is applied to extend the LVDT probe shaft, a measurement is taken, and the LVDT probe shaft is fully retracted again. The duration of this cycle is determined by the probe shaft velocity, which is proportional to the transient air flow while the probe is being extended and/or retracted, and to the dwell time while a measurement is being taken. If there is no external flow restriction, air flow through the probe is limited only by the bearing clearance, so the probe shaft can extend very quickly. The probe’s retraction velocity depends on whether the air pressure bleeds off only through the bearing space, or vents to atmosphere without restriction by another flow path as well.

The service life of the LVDT's probe is directly related to cycle rate. In continuous operation at a rate of 1 complete cycle per second, a probe will accumulate over a million cycles in 12 days, over 10 million cycles in 4 months, and over 30 million cycles in a year. For maximum service life, the longest cycle rate consistent with system throughput requirements should be chosen.

The effect of high LVDT probe tip velocity during extension is to impact the surface of the test piece like a hammer blow. This impact effect can not only damage the test piece but also causes the probe tip to bounce, which mandates a longer settling time before a measurement can be taken. During retraction, the same hammering effect occurs against the shaft stops within the probe itself, potentially impacting its life.

To prevent this probe impact effect, an adjustable flow restrictor such as a small needle valve or similar variable orifice should be installed between the probe’s air inlet connection and the air supply regulator. After the restrictor is installed, different restrictor valve settings and/or pressure regulator adjustments can be tried until a satisfactory probe shaft extension velocity is achieved at the desired tip contact force.

If a retraction velocity similar to the extension rate is desired, then the same flow restrictor could be used during the retraction portion of the operating cycle. However, the air flowing through the bearing will also affect the retraction velocity, so the retraction rate will always be faster than the extension rate.

There are two different air control paradigms to use with air-extend, spring-retract LVDTs. Both methods utilize a valve, typically an electrically operated solenoid valve, between the pressure regulator and the flow restrictor valve. The simplest method uses a 2-way valve, which is opened to extend the probe shaft and closed to retract it. While this method has the advantage of simplicity and uses a lower cost valve, the air pressure vents only through the bearing space, so the retraction velocity for any given probe and air pressure is fixed.

The preferred method is to use a 3-way solenoid valve with its supply port connected to the pressure regulator, its load port connected to one end of the flow restrictor, and its exhaust port vented to the air through a pneumatic silencer. The other end of the flow restrictor is connected to the probe’s air inlet fitting.

When the valve is energized, the air supply is connected to the load port and the LVDT probe shaft extends. The dwell time for measurement is determined by the on-time duration of the valve. When the valve is de-energized, the load port is vented to atmosphere, so the probe shaft retracts, with the air venting primarily through the valve. In either case, the internal orifice size of the solenoid valve should be significantly larger than that of the variable flow restrictor.

This method has the advantage of permitting adjustable retraction rates, unlike the simpler method described previously. If a faster retraction velocity is required than is permitted by this arrangement, it is possible to utilize a full flow check valve to bypass the extension flow restrictor during the retraction cycle.

In addition to the foregoing pneumatic installation considerations, there is one other important installation consideration applicable specifically to GHXA 750 and GHXD 750 series custom LVDTs. Because their short range dimensional measurements can be made properly only when their LVDT probe shafts are nearly fully extended, but those same probe shafts also retract more than 2 inches, it is necessary to notify their signal conditioning and/or data acquisition electronics to disregard any output from these LVDTs when their probe shafts are retracted.

An easy way to do this is to use a relay whose coil is connected in parallel with the coil of the solenoid valve used to operate a probe. When the relay is energized, its contact closures can be used to communicate that the LVDT’s probe shaft is extended so its output signal can be accepted. Alternatively, when the relay is de-energized, its contacts can communicate that the LVDT’s probe shaft is retracted, so any LVDT output signal should be ignored.

The application of air-extended, spring-retracted LVDT position or gaging sensors involves more care and consideration than is normally associated with a typical LVDT's installation. The guidelines and recommendations contained herein should make that task easier for the majority of applications of air-extended, spring-retracted LVDT sensors. For any additional applications assistance, please contact Macro Sensors at 856-662-8000.