by Harold Schaevitz, Macro Sensors

Explore hermetically sealed gauging probes for hostile environments like saw mills and wood processing plants.

Electronic gauging probes are commonly used in the dimensional gauging of manufactured parts. Gauging probes are typically 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 to 4-in). They incorporate a non-contact inductive position sensor, either an 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). Most gauging probes 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. A cross-sectional view of a typical 8-mm or 9.5-mm diameter gauging probe. The cable is usually connected directly to the LVDT windings.

Probes are connected by integral cable to separate support electronics that power the probes, as well as amplify and demodulate their output. Output is then displayed on a suitable readout and/or inputted 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 well in the 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 and fail. The small diameter and slenderness of the probe body make it susceptible to accidental damage. If its cable is pulled out or severed, the entire probe must usually be replaced.

Rugged and robust probes, such as hermetically sealed gauging probes, can solve many of the problems associated with dimensional gauging in hostile environments. Figure 2 shows an example of hermetically sealed probes featuring a ¾-in diameter hermetically sealed stainless steel probe with integral connector, a clearance-fit sleeve bearing and gauging ranges up to 2-in.

The sleeve bearing offers nearly the repeatability of a zero-clearance linear ball bearing, but is less susceptible to jamming. An outflow of air through the probe's bearing inhibits the ingress of grit, dirt, dust, oil, solvents or other contaminants, increasing reliability and probe life. Known as pressure-extend/spring-retract probes, these units can retract during loading/unloading operations in a gauging fixture or inspection jig, but extend immediately when pressurized by air.

Figure 2. Cross-sectional view of a hermetically sealed gauging probe shows the interior with space for a microelectronic signal-conditioning module.

Dimensional Measurements of Wood Products

Hermetically sealed gauging probes are ideal for use in dimensional and position measurement applications within sawmills and wood product processing plants. 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 to optimize lumber yield. In one installation, a servo system is used to coarsely position the saw blade carriage against a stop. Then the pressure-extend/spring-retract (PESR) gauging probe shaft is extended to touch the (nonrotating) blade (see Figure 3). The probe's position signal is 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.

In addition to exposure to flying sawdust and chunks of timber or wood, the saw carriage experiences 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 3. A typical heavy-duty automated sawmill uses two independent positioning servos. The 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 4). 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. Hermetically sealed, air-extend, spring-retract 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.