Measuring the long haul

Jan. 25, 2001
New techniques for gauging position target distances of about 75 ft or more.

Franz-Josef Hoffmann
Stegmann Inc.
Dayton, Ohio

Edited by Leland Teschler

Engineers who have tried to measure position information over long distances are probably familiar with the potential difficulties of the task. The determination of position over spans of 75 ft or more generally entails use of a metal-tape linear encoder system, a rotary absolute encoder, or laser reflectometer.

A relatively new technique is designed to overcome many of the drawbacks these methods entail. It employs a linear profile that is somewhat analogous to the stainless-steel tape used by some linear encoders. But where stainless tape is punched with a coded pattern and read by an optical sensor, the new method makes use of an extruded aluminum profile containing buried magnets. A special Hall-effect sensor detects the magnets and determines absolute position.

These position multiplexer encoders, or Pomux systems, get their name from the use of electronic multiplexing to check individual magnetic sensors in the read head. The technique works well on spans ranging from a few dozen feet to 1.7 km.

The first application for Pomux was on large overhead cranes. Some of the more common uses today include automated warehouse and car parking systems, floodgate adjustment, freight elevators, and cut-to-length systems.

Basic components
Pomux consists of two basic elements, profiles and a sensor head. The aluminum profiles (called Omega profiles) serve as the measurement scale. Designers construct a measurement system by first stringing a number of profile sections, each about 2 m long or less, end to end along the complete measurement path.

Each profile section carries its own individual identification number that tells installers in which order the sections should mount. The number of profiles used determines the actual measurement length of the system.

This configuration makes the measurement system easily expandable. Users simply mount additional profiles as necessary to extend it (no system recalibration is necessary). Profiles come in five standard lengths. Those of a given length are interchangeable, making replacement simple. If something happens to mar one of the profiles, only the damaged section need be replaced. (This contrasts sharply with optical tape systems, where the whole tape must be replaced, and laser systems that require recalibration when repaired or extended.) Maintenance is extremely low compared with other distance measuring systems. In fact, properly mounted profiles can double as steps, always a plus in industrial applications.

Profile sections contain hard ferrite magnets, each 20 mm long, installed in nonrepeating patterns along the profile length. In contrast to other measurement systems, the magnets are not spaced equidistant apart. Rather, the separation between each magnet is unique and never repeated. These unique separations are what determine the absolute value of each resolvable position over the complete measurement path.

Put another way, each profile section can be viewed as consisting of several equal segments. The segments are defined by fixed magnets. Two code magnets sit in each segment between each fixed magnet. Within any given profile section, the spacing both between any two code magnets and between any given code and fixed magnets is unique. Thus they provide a way of unambiguously identifying the segment. The system creates a segment address by reading the spacing between the fixed and code magnets.

The absolute addressing technique provides some benefits in applications that demand position information only in certain sections of the measurement path. Profiles need only be mounted where position information is required. This contrasts with optical tape systems which cannot be used in the same way without a special (and potentially expensive and wear-prone) rail system to guarantee perfect alignment.

Sensor head electronics
The sensors in the read head each contain four magnetically sensitive resistors connected as a Wheatstone bridge. The resistors are composed of Permalloy strips embedded into a flat metal conductor at a 45° angle (called a Barberpole configuration because of the resemblance). The resistive Permalloy area has a serpentine pattern.

The Permalloy changes resistance when it sees a magnetic field applied perpendicular to the resistor's long axis. This unbalances the Wheatstone bridge and produces a voltage that is a function of the field strength. Each sensor gauges four reference points that are 1.27 mm apart. The read head consists of 92 such sensors that are staggered apart to produce a coarse scale for length measurement. During a factory calibration process all reference points get compared to a precision reference scale. A correction value is calculated for each reference point and stored in EEPROM, producing a calibrated position value for each reference point.

After power-on or reset, a magnet-detection cycle takes place. Each sensor is switched on through the multiplexing process. The resulting signals each get transformed into a 10-bit value. A controller checks the data for the presence of magnetic fields. Once the system has detected a magnet and identified its position, only the sensors close to the magnet remain active.

The system gets enough information to calculate magnet position by selecting two sensors. It reads the calibrated geometrical position of the first sensor from the EEPROM. A special algorithm determines the location of the magnetic pole transition relative to spacing between the two sensors. Linear interpolation between the two sensor positions yields the correct absolute position value.

A point to note is that the sensor head moves over the profiles without touching them. It sits in a fully sealed aluminum alloy extrusion.

The final result is accuracy to ±1 mm or ±0.5 mm, depending on performance level specified, and resolutions of ±0.1 mm. One benefit of the system is that it provides this accuracy without the costs typically associated with precise construction. (As, for example, often required in precision rack-and-pinion arrangements used to allow absolute rotary encoders to measure linear position.)

In most real-world positioning applications, the key specification is repeatability rather than accuracy. Repeatability is defined as the ability of a feedback device to return to a specific location within a certain error limit. The repeatability of Pomux systems is 0.3 mm, more than enough for all but the most precise applications.

The read head sends out digital position information via the Synchronous Serial Interface. This standard employs three twisted pairs regardless of distance. A serial/parallel converter card can handle applications requiring a parallel output. This card typically mounts near the position controller (inside or close to the control panel), to take advantage of SSI transmission integrity. There are Interbus interfaces as well, and commercially available modules can provide SSI-to-Fieldbus connections for other widely used protocols.

Environmental effects
Pomux systems are unaffected by salt water or outside contaminants common to industrial environments. The sensor head is sealed to IP67 and can operate over an extremely wide temperature range (typically 20 to 85°C). In addition, the system has no exposed components that require protection, maintenance, or delicate handling.

One concern often voiced about Pomux is how stray magnetic fields might affect it. A point to note is that the permanent magnets in the profiles are quite strong (exceeding 150 mT at the profile surface). This lets read-head sensors be desensitized, greatly reducing susceptibility to stray magnetic fields. Nonetheless, recommendations are that Pomux profiles be mounted at least 100 mm away from ferrous materials and sources of magnetic fields such as high-current conductors. (Supplied mounting brackets help ensure this gap is set correctly. Similarly, a supplied mounting tool helps set the spacing between profile sections, which is critical.)

Equipment expansion and contraction in applications involving long distances can translate into large mechanical tolerances. Measurement systems must be able to handle these large deviations and, fortunately, Pomux systems do so.

Its noncontact method of operation gives Pomux a generous vertical tolerance between sensor and profile of ±10 mm around a 25-mm nominal distance, and an axial tolerance of ±10 mm around the center line. This extremely forgiving baseline lets the system serve in retrofit applications where machine tolerances have risen significantly since manufacture, or where temperatures fluctuate constantly.

Finally, there are special Pomux systems available to handle shorter runs. Most of the cost for short measurement lengths is in the sensor head. So the sensor head is kept short and the profiles are longer and hold a higher density of magnets.

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