NEW ANALOG INDUCTIVE PROX TECHNOLOGY
monitors moving targets for economical process control
and automated positioning

by Craig Brockman, Product Manager, Balluff Inc.

Inductive proximity sensors are industry's feedback "ground troops," reliably soldiering amidst cutting fluid, metal chips, oil and other harmful media found in the trenches of automated manufacturing. Now these sensors are being given greater capabilities that make them cost-effective alternatives to encoders and laser sensors for position sensing -- not just presence sensing in manufacturing's toughest battlefields. New Ultralinear™ analog prox technology, developed by Balluff, combines prox sensor toughness with a highly linear analog feedback signal -- sans external switching device -- that is proportional to target distance from the sensor face and provides resolution down to 1.5 microns.

By providing more than a sample yes/no output as to the presence of a metal target, continuous-feedback Ultralinears allow a variety of difficult process control and positioning applications to be solved with tough, simple prox sensors. Possibilities include:

* measurement of distance, thickness, concentricity, circularity, wobble and deflection;

* part sorting by size, shape or material;

* monitor thermal expansion of ballscrews and shaft imbalance on rotating machinery;

* detection of object orientation;

* absolute linear positioning using direct metal target approach; and

* absolute rotary positioning using spiral eccentric metal cam.

Triple-setpoint Ultralinear models employ an onboard microprocessor that allows teach-in of up to three separate output "on-zones" to provide three-sensor functionality from a single prox. Users can setup detection zones to monitor parameters, such as component wear or bandsaw drift, using only one affordable sensor.

Ultralinear technology provides repeatability (less than 3%) and temperature drift (less than 1% between +15° to 55°C) due to Balluff-patented linearity/temperature-compensation and calibration ASICs. Rugged alternatives to encoders and laser sensors, self-contained Ultralinear proxes not only let the control know where the target is, but their continuous-feedback monitoring tells where the target's going.

Analog straight-shooter technology

All inductive proxes operate internally as analog devices, yet most are designed to produce a binary output signal. The classic switching prox design uses an oscillator coil assembly to generate an inductive eddy current field that extends in front of the sensing surface. When a conducting metal is introduced into this inductive field, the field is absorbed (collapses), reducing the voltage output in the circuit. At a predetermined low-voltage trigger level, the circuit opens (for normally closed circuits) or closes (for normally open circuits).

Ultralinears are based on this standard prox design, but instead generate a linear analog voltage (0 to 10VDC) or current (0 to 20mA or 4 to 20mA) proportional to the distance between sensor active surface and metal target. While standard proxes do operate as analog devices, their outputs are not linear. So in addition to standard sensing coil and oscillator in the front-end circuit, Ultralinears have a patented ASIC which linearizes the output signal and compensates for temperature changes. This allows the proxes to provide a resolution of 1.5 microns, with linearity of ±3% (±0.045mm) for a 1.5mm stroke, as measured by a flush-mounted M12-style sensor.

A short-lead mechanical thermal connection from the chip to the sensing coil ensures that the temperature at the coil is the same as the thermal sensing point at all times. This, along with ASIC temperature compensation, provides a temp drift spec of less than 1%, ensuring reliability and accuracy despite fluctuating environmental conditions.

Another patented front-end ASIC is used for post-production calibration to lock-in min/max output signal range for each sensor. Typical factory calibration for a prox circuit is done prior to installation and potting in the prox housing. Calibration entails positioning a target in front of the sensor and trimming the prox resistor with a laser until the correct spec window is reached. The sensor circuit is then installed and potted in the housing, which changes the original trimmed sensing distance. That error must be continuously monitored and the trimming operation adjusted accordingly. The Ultralinear teach-in calibration ASIC accounts for any variation due to housing material/shape and potting material by making prox calibration the final step of assembly.

All Ultralinear sensor specs are listed per IEC60947-5-2 test, which takes into account variations in supply voltage, temperature, warm-up time, etc. Therefore published specs reflect performance under "worst-case" conditions, so actual sensor performance in more precisely controlled field applications will likely be better.

An output driver provides signal generation (voltage or current, depending on version), and uses an integrated power supply converter to drop the 24VDC input to 10VDC to power the ASICs. Short circuit and reverse polarity sensor protection functions are also built into the output driver circuit.

Triple setpoint technology offers three-prox functionality

Ultralinear versions are available with a special microprocessor built into the output driver that allows teach-in of three separate sensing zones, effectively turning the single prox into three sensors. Threshold setup procedure is simple thanks to a special "connect-to-high" control wire and three zone-specific output LEDs.

The triple setpoint microprocessor provides three normally open outputs and one program wire. The program wire is used to mark setpoints manually with a user-supplied dry contact pushbutton or remotely via dedicated PLC output. To set switching thresholds, the control wire is connected to the power positive for a couple of seconds, putting the prox into program mode. After the first output LED begins blinking (or if connected to an input, the input responds), the wires are disconnected. With the target positioned at the first zone threshold position, the control wire is again connected to the power positive for a couple of seconds. When the wires are disconnected, the setpoint is recorded. The microprocessor then automatically switches to the next output LED and the routine repeats for the second and third setpoints.

For maximum application flexibility, machine control can be programmed to look at one, two or three zones, or just at the analog output like a standard Ultralinear. Since each setpoint is independently programmable, the sensing zones can be set up in any order as the target approaches the sensor's face. The setpoints can be programmed as close to one another as the band of repeatability, which is ± 0.12mm for the M18 version. If two setpoints are programmed closer than ±0.12mm together, a change in temperature or supply voltage could invert these setpoints.

Through maximizing sensing distance of 80 x 80 x 40mm "pancake style" Ultralinear is 50mm, the tubular proxes lack the standoff of most laser sensors. However, reliable operation and resistance to oil, dirt, vibration and impact make the IP67-rated proxes suitable for continuous feedback metal-target positioning applications in hazardous industrial environments.

Balluff Inc.

PRE-ENGINEERED CLEAR-SPAN TENSION MEMBRANE BUILDING
expands storage facilities for bus parts manufacturer

Located in the heart of the Canadian prairies is one of the largest bus manufacturing centers in North America. Winnipeg, Manitoba is a city of 700,000 people and the home of several companies that manufacture and supply product to some of North America's largest bus companies. One of these companies, Russel Metals, has been a part of the central hub of bus parts manufacturing for more than two decades. With approximately 1900 employees and operations throughout North America, the company is able to manufacture, store and ship product to their customers in an efficient and timely basis.

In the fall of 2002, the Winnipeg operation expanded their storage facilities with a heated 7200 square foot addition. What is unique about this expansion is the type of building Russel Metals selected: a pre-engineered clear-span tension membrane building manufactured by Cover-All Building Systems. "We needed a building that would enable us to utilize some of the dormant land we had available," says Doug Howell, branch manager. "Our management team did a thorough analysis of the Cover-All product and was impressed with the engineering qualities and design. The building is basically a roof truss system from the ground up that creates a large clear span area. The versatility of the building allows us to make changes five years down the road if we require," says Howell.

Neil Lapeire, plant manager for Russel Metals echoed similar thoughts. "The fact that we are able to re-locate this building makes the building a tangible asset. If the building is no longer required we could sell it and relocate it. If we need additional covered storage we can easily add onto it," says Lapeire. "The building is also classified as a temporary permanent building so the taxation is lower than a conventional structure."

The building is used to store about 700 bins of various bus parts that are manufactured by Russel Metals. "We can function more efficiently in the Cover-All than in our conventional storage building," says Howell. "The clear span working area and height of the Cover-All building allows us to easily stack bins 18 feet in the air."

The 60 foot wide by 120 foot long Cover-All TITAN building was engineered to meet the heavy wind and snow loads of Winnipeg. To insulate the building, an inner liner was installed to the bottom of the interior truss to create a 3.5 foot dead air space. The building is heated by radiant heating hung from the building trusses.

By insulating the building with an inner liner, Russel Metals employees enjoy the benefits of working in a naturally lit climate controlled environment. "Winnipeg is known for its extreme weather changes," says Lapeire. "It's common to have minus 30 (Celsius) in the winter and extreme hot and windy temperatures in the summer. The building provides a great work environment. It's cooler in the summer than our main plant and it's very comfortable to work in during the winter months."

When asked as how the building is performing, Howell was quick to comment. "Everyone is impressed when they walk into the building. We have our personnel working in this building year round. At first some of our employees had concerns working in this building during the winter but once they stepped inside, their concerns were quickly dispelled," says Howell. "The amount of natural light we have in this building is conductive to an excellent working environment. The reflective nature of the fabric makes the lighting requirements extremely efficient. The building has met our expectations. All in all, it's been a good investment," says Howell.

Cover-All Building Systems Inc.

Canadian Industrial Equipment News December 2003

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