The use of electronic gauging probes for dimensional gauging of manufactured parts is a well-established quality assurance technique. Typical gauging probes are cylinders with diameters of 8 mm or 9.5 mm (5/16 in. or 3/8 in.) and lengths ranging from ~. 65 mm to 100 mm (2.5 in.-4 in.). They incorporate a non-contact inductive position sensor, either LVDT or half-bridge, which uses a spring-loaded movable armature coupled to a shaft that is supported in a high-precision linear bearing (see Figure 1). They generally have a maximum gauging range of ±0.25 mm to ±2.5 mm (±0.010 in. to ±0.100 in.), with resolutions of fractions of a micron.

Figure 1 As indicated in this cross-sectional view of a typical 8 mm or 9.5 mm dia. gauging probe, the cable is usually connected directly to the LVDT windings.

The probes are connected via an integral cable to separate support electronics that power the probes, as well as amplify and demodulate their output, which is then displayed on a suitable readout and/or input into a computer-based data acquisition system for statistical process ! control. This ability to transmit data to a remote computer has made the probes popular in quality assurance schemes.

The devices function quite well in the 1 relatively benign environment of a quality assurance laboratory, or in a protected inspection jig or fixture located away from machines and manufacturing processes. In the hostile environment of the shop floor, however, these probes can encounter serious problems. For example, the zero-clearance linear ball bearing must be protected from even the slightest contamination or it will jam up and fail. The small diameter and slenderness of the probe body make it susceptible to accidental damage. And if its cable is pulled out or severed, the entire probe must usually be replaced.

Many of the problems associated with dimensional gauging in hostile environments can be solved by the GHS series of rugged and robust LVDT-based gauging probes (see Figure 2). These units feature a 19 mm (3/4 in.) dia. hermetically sealed stainless steel probe with integral connector, a clearance-fit sleeve bearing, and gauging ranges up to ±25 mm (±1 in.). The unit's accuracy is satisfactory for virtually any shop floor gauging function. The sleeve bearing offers nearly the repeatability of a zero-clearance linear ball bearing, but is less susceptible to jamming. For operation in particularly dirty environments that might contaminate the sleeve bearing, some versions of the GHS series can be connected to a positive-pressure air stream to continuously purge the bearing space and prevent infiltration of contaminants. These units are known as pressure-extend / spring-retract (PESR) probes.

Figure 2 A cross-sectional view of a GHS Series gauging probe shows the hermetically sealed interior with space for a microelectronic signal conditioning module. PESR versions have an additional fitting for the purge pressure supply.

In addition to their mechanical merits, the probe bodies are large enough to accommodate built-in sensor support electronics. This feature can reduce overall system cost by simplifying the gauging apparatus.

SHAFT STRAIGHTENING

There are many applications for shaft straightening in manufactured products, among them electric motor shafts, golf clubs, gun barrels, submersible pump shafts, marine propeller drive shafts, and stock straighteners for bars processed on centerless grinders.

The ends of the shaft to be straightened are supported on centers or in V-blocks. The shaft is slowly rotated to determine the point of maximum deviation from center by means of one or more gauging probes contacting the shaft radially. The shaft is then straightened by an iterative process of applying a radial force supplied at that point by either a jackscrew or hydraulic actuator to deflect the shaft beyond center with just enough overstress to allow it to elastically return to true center (see Figure 3).

Figure 3. In the common type of shaft straightening rig shown here, the V-blocks can be moved closer together for shorter shafts. This rig requires the workpiece to be rotated manually, but many have a motor-driven workhead.

Straightening generally takes place on the manufacturing facility's shop floor in an environment of chips, dirt, grit, and fluid splash and spray, especially around the grinding machines. The straightening rigs are often subjected to severe shocks and vibration when the shafts are dropped into place; conventional gauging probes take a serious beating.

A long-range (25 mm) PESR version of the gauging probes is designed for such applications. When the rig is idle, the probe's shaft is completely retracted out of harm's way. The long range permits larger workpiece clearance in the fixture, facilitating the workpiece loading process while still providing excellent resolution and repeatability. When a deflection measurement is needed, air pressure is applied to extend the probe tip to the workpiece; the outward air flow through the probe's bearing assembly keeps out fluids, dirt, and grit.

The probes' larger size and ruggedness reduce the chance of physical damage from shock or accident. The hermetically sealed housing prevents solvents or coolants from damaging the sensor or any built-in electronics. Should a probe cable be damaged, as occasionally happens, it can be disconnected and
repaired or replaced.

When a hydraulic actuator is used to straighten a shaft, the ram of the cylinder may be controlled by an electrohydraulic servo system that derives position feedback information from the gauging probe(s). This feature can enhance the efficacy and accuracy of the process. Computer-based technology can further augment the straightening process by using an algorithm that takes into account the shaft's dimensions, geometry, and material and calculates the proper overcenter deflection.

FUEL ROD GAUGING

The dimensions of the casings of fuel rods used in nuclear power plants must be checked on occasion to verify that they have not been bulged by internal gas pressure or warped by overheating or hot-spot damage. Because these rods must be stored in water that becomes irradiated, the gauging hardware must be both submersible and able to withstand the effects of moderate nuclear radiation.

This application makes good use of the gauging probes' hermetically sealed case that prevents "hot" water from attacking the LVDT sensor. The LVDT windings and probe body are constructed of materials unaffected by mild radiation. The probes' mechanical ruggedness reduces the chance of damage during handling or actual operation. LVDTs' reliability makes the probes particularly attractive because once exposed to radiation, the gauging fixture and probes are difficult and expensive to repair or replace.

PESR-style probes are arrayed radially in a ring fixture to simultaneously measure several points around the rod at a given axial position (see Figure 4). With the probe shafts retracted, a fuel rod is placed in the center of the ring fixture. The probes are pressurized (with water instead of air) to extend their shafts, and the fuel rod is moved in axial increments through the ring fixture. Dimensional measurements taken from each axial position are transmitted to a remote data logger, where a diametral profile of the fuel rod casing is developed.

Figure 4. A ring array of gauging probes is incorporated in this fuel rod gauging fixture . The probe fixture
is normally rigidly mounted in the irradiated water pond, and the fuel rods being measured are pulled through the fixture in known increments.

DIMENSIONAL MEASUREMENTS OF WOOD PRODUCTS

Sawmills and wood products processing plants offer quite a range of applications for dimension and position measurement. Two examples are fine positioning of saw blades and in- process thickness measurements of sheet products.

In modern sawmills, computer-controlled systems determine the sawing pattern for a given log or timber segment and thus optimize lumber yield. In one installation, a servo system is used to coarsely position the saw blade carriage against a stop. Then the PESR gauging probe shaft is extended to touch the (nonrotating) blade (see Figure 5). The probe's position signal is then fed back to another servo system for final blade positioning. When the blade position is set, the carriage is clamped tight and the air pressure to the probe is reduced enough to allow the probe shaft to retract. The saw carriage environment is one of flying sawdust and chunks of timber or wood; high levels of vibration during a cut; and severe shocks as heavy logs are dropped into the carriers. Ruggedness and air-purged bearings are key to probe survival.

Figure 5. A typical heavy-duty automated sawmill uses two independent positioning servos inaccurately and optimally slice logs into rough lumber. The GHS gauging probe senses final blade position.

For measuring the thickness of sheet lumber products, gauging probes are placed in pairs, one above and one below some point on the sheet, to control lamination presses and planer cutter or roller position (see Figure 6). The thickness at that point on the sheet is the difference between the dimensions as indicated by each probe of the pair. Typically, many probe pairs are used to sample the sheet thickness over some statistically valid area. A computer-based data acquisition system evaluates the output from all the probes to provide the thickness data that controls the process. Although the environment in such processes is less harsh than that of the sawmill, glues and resins are present. PESR-style GHS series gauging probes minimize downtime and probe failure by keeping the probe shafts out of the way during loading and unloading of the sheets. The air purge stream helps prevent contaminants from plugging up the probe bearing.

Figure 6. In a plywood laminating machine, pairs of pressure-extend / spring-retract GHS gauging probes are used to measure thickness. The large "mushroom" probe tips average out the effects of a rough surface on the sheets being measured.

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Edward E. Herceg is a consultant to Macro Sensors Div. of Howard A. Schaevitz Technologies, Inc. For more information on the GHS series of gauging probes, contact Richard L. Anderson, Macro Sensors, 600 Main St., Riverton, NJ 08077-1419; 609662-8000,fa x 609-786-1939.