Rotary and linear encoders are standard equipment in today's automated motion systems. Both types, generally, are optically based, relying on light passing through or reflecting off a scale and then interpreted by opto-electronics. For rotary sensing, this works well, but the same cannot be said of optical technology when it's used in linear motion applications.

One lingering problem stems from the inherent difficulty of sealing linear scales. Optical linear encoders, as a result, are susceptible to interference from dust, solvents, and debris, limiting the environments in which they can be used. Other constraints are imposed by the fragile nature of optical scales (especially over long distances), the speed at which they can measure position, and thermal mismatches between glass scales and the metal machine beds they often contact.

Although there is an alternative to optics — linear magnetic encoders — these too are hampered by limitations. Rather than glass scales, magnetic encoders use magnetically imprinted tapes, operating on a principle like that employed in VHS systems and floppy disks. The technology is generally less expensive, but also less accurate and less precise.

Assessing the challenge

Measuring linear motion in an industrial environment can be a difficult task. It requires, at minimum, the consideration of four factors — sealing, speed, connectivity, and environment.

Sealing: Dust, liquids, and debris are common in industrial environments, and they can wreak havoc on traditional optical linear sensors. Other dangers include corrosive solvents and lubricants, both of which can work their way into all but the most tightly sealed scales, obscuring critical light paths and introducing errors.

Methods to combat these threats include frequent cleaning and mounting schemes carefully arranged to protect the scale. Adding supplemental filtering for optics is another option. This takes care of minor anomalies such as dust, light oils, fingerprints, and scratches, but it cannot compensate for more significant interference. A more proactive approach, although it requires compressed air and a dedicated clean-air system, is to repel contaminants with positive air pressure. All these remedies, of course, add time and cost, creating a compromise between capability and expense.

Speed: Anyone who has been stuck behind a cement mixer on a long uphill grade knows that the slowest vehicle sets the pace. The same holds true in automation. Processes can run only as fast as the slowest device in the system, be it a sensor, actuator, or mechanical drive.

When traverse speeds increase, so does the potential for linear measurement errors. Here, physical limitations, particularly in the read head, are a common error source. The read head, which contains the sensing mechanism, usually mounts on some form of carriage that rides on bearings. This allows the sensor to move along the scale.

Optical and magnetic encoders require the gap between the scale and sensor to remain constant within a very tight tolerance. But as acceleration and traverse speeds increase, so do the forces exerted on the carriage and bearings, making it more difficult (and eventually impossible) to maintain this crucial gap. For some applications, this inability to hold the gap is the “cement mixer” slowing everything down.

Connectivity: Computer users often take for granted that their new hard drive or MP3 player will be able to “talk” to their PC instantaneously upon connecting the two together. This sort of connectivity is the result of a concerted effort and the setting of standards that electronics manufacturers universally follow. In industrial electronics, it's a different story.

The electrical interface between sensors and their host systems bears the mark of many manufacturers. Not surprisingly, there is no single “language” to be found among sensors, controllers, and other industrial electronic devices. This can make it difficult, if not impossible, to employ new technology with existing devices. It also places a premium on devices that are more universally compatible with others.

Environment: In automated manufacturing facilities, powerful machines expend huge amounts of energy, generating mechanical shock, vibration, and heat, which can quickly ruin overmatched sensors. Only the most rugged and dependable sensors can survive in today's high-productivity environments.

Inductive sensing

The shortcomings in traditional technology point to a need for a new type of linear position sensor, one better matched to the challenges posed by industrial environments. Years in the making, such a device now exists. It uses inductive sensing technology, applying electromagnetic fields and measuring the interaction with surrounding materials. By eliminating light transmission from the equation, inductive sensing solves many of the problems previously encountered when measuring linear motion.

The operating principle is simple in concept. It begins with a standard reference signal transmitted through a drive coil. Adjacent sensing coils monitor the resulting electromagnetic field, which is distorted in a predictable and repeatable way when exposed to certain metals. Think of it as a metal detector, except here, the metal is a scale with cyclic ferromagnetic properties and the detector is the read head.

The scale itself is practically indestructible, a 0.6-in. diameter stainless steel tube loaded with a column of precision 0.5-in. diameter nickel-chrome ball bearings. Each scale is calibrated at the time of manufacture using a laser interferometer as the standard. In addition, the pre-load on the column of balls is precisely adjusted to lock in an appropriate scale factor, and the tube is then welded shut.

The read head, which works in tandem with the scale, incorporates drive as well as sensing coils. The drive coil is a single winding that wraps around the sensing coil. The sensing coil is a bit more involved. It consists of six sets of windings, each of which is one ball diameter in length and is further divided into four identical coils. The four coils, designated A, B, C, and D, are evenly spaced in a repeating sequence so that, at any given instant, each phase coil aligns with the same point on each of the six balls within the read head's range.

In essence, the coils and balls in the inductive sensor form a magnetic circuit. The drive coil conducts ac current, generating an alternating magnetic field whose flux path is completed through the scale's steel balls. What's actually changing as the read head moves relative to the column of balls is the magnetic reluctance of the flux path. In other words, the varying reluctance presented by the balls moving through the flux path modulates the amplitude of the voltages induced on the pick-up coils.

The magnitude of this modulation effect is a function of the position of each coil relative to the steel balls in the scale. The result is a unique ratio of signal levels among the A, B, C, and D coil waveforms with respect to the reference or drive current waveform. Analyzing and comparing these signals gives the exact position of the read head over a particular ball. By tracking the number of balls traversed and adding the position of the ball currently in the flux path, the inductive sensor can get an accurate and repeatable position measurement.

Making the grade

An apples to apples comparison shows how inductive sensing measures up to other linear sensing technologies in industrial applications.

Sealing: Because the read head in an inductive sensor does not have to contact or physically “see” marks on a scale, but senses them through a stainless steel tube wall, the scale can be completely environmentally sealed to a rating of IP67. What's more, the read head itself can be filled with an epoxy or other encapsulant, resulting in a solid block, impervious to outside elements. Inductive sensors can even operate while submerged because there are no “lines-of-sight” that could become obscured by dirt or debris.

Speed: Inductive linear encoders are not subject to the speed limits imposed on their optical counterparts. Instead of a bearing-supported carriage riding on a track, inductive encoders rely on read heads that completely surround the scale. This eliminates the possibility of critical gaps varying with traverse speeds. With appropriate electronics, inductive encoders can measure position accurately at traverse rates of up to 20 m/sec.

Connectivity: To process their many complex signals, inductive encoders include onboard digital signal processors. These speedy chips analyze amplitude differences and calculate the read head's absolute position. They also translate position data into a series of digital square waves representing total movement. Programmable logic controllers, data acquisition systems, and other devices can easily read these digital waves to precisely monitor and control linear position.

Inductive encoders are not limited to the basic 5 Vdc output signal. Equipped with either line-driver or open-collector outputs, they can comfortably operate at voltages anywhere between 5 and 28 Vdc in industry standard quadrature format.

Environment: A sensor can't do much better than enclose itself in stainless steel, which is the case with inductive encoders. These hard-headed sensors can stand up to vibration, shock, temperature extremes, and chemical exposure better than their optical and magnetic tape cousins. Using steel scales instead of glass also reduces breakage and lost productivity. What's more, the temperature coefficient of a stainless steel scale closely matches that of most machine components and workpieces.

Repeatibility: Highly uniform ball bearings in the scale and ample a/d processing power make inductive encoders repeatable to within ±1 count. And with error mapping and appropriate correction factors or “teach points,” this repeatability can be leveraged to achieve system accuracies greater than the encoder's out-of-box accuracy.

For additional information, contact BEI at (800) 362-6337, visit www.beiied.com, or write the editor at ctelling@penton.com