Proximity sensing

July 1, 2007


Alex • Baumer: Proximity sensors, through their specifications, have a significant effect on the overall design of a machine. For example, in a multi-axis system, their switching speed affects how rapidly the machine can be positioned, as these sensors are frequently used to provide rough position information. The overall safety of the machine is also influenced by the inherent reliability of the sensor. If a sensor fails, the machine may stop working properly or shut down altogether. Sensor size (housing diameter and length), likewise, can impact a design due to space considerations.

Last but not least is the effect of accuracy and the amount of deviation in sensing distance. With greater deviation and error, system designers must incorporate greater tolerances into the design, affecting the overall machine precision. If sensors are manufactured to provide more consistent operation in the field, this overall error is significantly reduced, resulting in a stronger price-performance ratio for the complete assembly.

Pat • Balluff: As more motion is applied to machines, the need for proximity switches has diminished for individual machine actions as it relates to an individual machine cycle. In other words, proximities are used less often to determine machine position within a machine cycle because closed loop motion control systems are doing this now.

However, when performing discrete or intermittent actions such as assembly automation, the need still exists. The best applications for proximity switches include error proofing, missing component detection, and determining positioning for non-continuous machine actions.

Jim • Omron: Many sensor types are used to help control motion-based systems. These include photoelectric and ultrasonic sensors, limit switches, vision and camera-based systems, as well as other options. Sensor choice depends on the nature of the particular system and its intended purpose.

Inductive proximity types are a likely choice for motion-centric automation systems due to simplicity of operation and wiring, survival in harsh environments, and significant cost savings. Because of the nature of the inductive sensing principle and the sensing problems it solves, this type of sensor is ideal for motion-based systems. Typically, these are simple devices, used to sense the presence of a metal target or flag. This flag is used for system positioning and to signal the PLC or motion controller and send important position information, such as “home,” “origin position,” “slow down,” or “end of travel.” Use of proximities in motion systems is not limited to axis position, but can be expanded to detect the presence of metal product and robotic gripper position, and to confirm correct parts position before additional operations take place.

Using encoders — both absolute and incremental types — is a common way to detect position in motion systems. Absolute encoders provide precise positioning of the system without the use of other reference sensors. This allows accuracy, even if power is lost and restarted. However, the initial cost of using an absolute encoder is much higher than that of an incremental type. Lower-cost incremental encoders can team up with proximity sensors to provide the same information. As many as three sensors per axis are generally used. If power is lost and restarted, sensors enable the system to find its “home” or “origin position” in a more robust package.


Alex • Baumer: Mounting of sensors often presents a challenge. For example, how can a sensor be mounted close enough to the application to do its job, yet far enough to prevent interference? New bracket designs allow the production team flexibility in mounting, granting the ability to get at tough positions without presenting problems in overall machine design.

Operating temperature range is another limitation in designs, as systems that operate at extremes must naturally take into consideration the temperature range of all components. Variations from sensor to sensor (of the same type) can be a big headache for system designers as well because disparities can mean different performance within the same application on several spots on a machine.

Pat • Balluff: Technology has continued to evolve around proximity switches chiefly because they are extremely reliable and compact. They are continually becoming smaller — in some cases, the size of a pencil eraser — offering greater flexibility in complex mechanical production environments.

One major challenge, however, has to do with cabling and mounting, and the best solution is having ample options and choices.

Jim • Omron: The main challenges of using proximity sensors for motion applications involve sensor size, accuracy, repeatability, response time, and reliability. These are common themes throughout industry as demands increase for lower production cost and higher quality.

Sensor size and sensing accuracy are directly proportional. As the size of the sensing face increases, so does the sensing distance. This affects accuracy and repeatability in an adverse way, as the repeatability tolerance is a factor expressed as a percentage of sensing distance. Sensing distance is reduced as the sensing face gets smaller. However, repeatability and accuracy go up. A balance must be reached to provide more accurate information vs. sensing distance.

Sensor response time, on the other hand, translates to operating frequency. The sensor's ability to respond quickly can affect the system's processing time, speed of operation, and thus production capacity. Advances in dc-powered sensor types allow higher operating frequencies and lower response times for on and off operations.

Reliability and the choice of the proximity sensor used are often dictated by two factors — the environment it's used in and the costs to build equipment. Applying the right sensor for the motion application can be balanced by taking into account what the sensor may be exposed to. If operating temperatures are constant, stable, and within the operating specification of the sensor, its lifespan will be longer. Using a lower cost proximity product can reduce the overall price of the equipment and maintenance. This would be the best-case scenario.

Not fully considering these factors may allow for a lower initial price of the equipment, but will result in high maintenance costs and lower end-user satisfaction since equipment reliability affects production. Choosing a higher cost, more robust sensor may be a better long-term solution. It adds value to the equipment, increases reliability, and thus boosts end-user satisfaction.

New developments in construction materials protect these sensors from the degradation that would allow contaminants into the electronics. This is a leading cause of proximity sensor failure. Better materials also offer protection from another significant cause of failure. This occurs when the environmental temperature cycles, or changes rapidly. This eventually causes a breach of the protective enclosure, allowing the inner electronics to degrade if exposed to contaminants.

New materials allow certain sensors to operate reliably at higher temperatures and survive when exposed to impact, abrasion, moisture, coolants, cutting fluids, and corrosive chemical baths. Much of this has come about due to new modular production methods that help decrease the overall costs associated with using high quality, durable, and chemically resistant materials.


Alex • Baumer: As sensors gain intelligence, sensor designers are adding features that remove the need for monitoring tasks on the system side. For example, programmable output linear sensors are available today that allow users to simply set a specified sensing range or output slope, therefore removing the need to program this in the software. This results in decreased setup time and cost, and greater flexibility in the application.

Pat • Balluff: Reliability of these devices is already extraordinarily high and about to get higher with the introduction of I/O schemes that work in conjunction with technologies such as DeviceNet and Profibus. The technology is now at the point where users can download information to sensors and change their functions to better fit the given application. This will allow for multiple trip points and other operating features that let machines change functions via recipe-driven setups.

Using proximity sensors in conjunction with new vision technology allows for easier and more complete error proofing and missing component verification. The leverage in this area will come from camera technology merging with sensor application expertise, not by trying to use expensive vision systems, where the additional sophistication is unnecessary.

Proximity sensing is different than vision sensing. Where discrete parts or products are in exact position, it will remain a viable basic technology. When parts are not always in exact repeatable position is where sensor technology meets vision solutions. In general, vision sensors can make multiple decisions from a single image, while individual proximity sensors make one decision at a time.

Jim • Omron: New opportunities emerge as proximity sensors become “smarter” and easier to set up. Remote amplifier versions have helped to reduce size, increase sensing distance, and improve resolution. Linearity, repeatability, and accuracy also benefit from microprocessor-based signal processing.

Even higher resolution, analog output, calculation units, and programming are available by keypad or connection to a PC and can be accomplished by using, for example, a “smart” proximity displacement sensor. Advances in programmability, combined with a digital display, one button “teach” or teach by wire (with external input models), mutual interference prevention, and dual set point or “area” functions are also becoming more common, along with “zero reset” and interchangeable sensing heads.

These features allow new measurement opportunities requiring higher levels of precision, previously achieved with more expensive optical sensors. With recent advances in electronics, inductive proximity sensors have evolved into precision measurement devices that can calculate the displacement or distance of a work piece or metal target down to the micron level.


Alex • Baumer: As we move into the future, sensor designers are pushing to create even more features at a reduced cost to end-users. Wireless communication will allow sensors to be mounted in difficult locations, without the need for cable runs back to a controller.

New technologies are continually being developed that allow sensor designers the latitude to decrease the number of components in their sensors, increasing reliability and MTBF (mean time between failures), and to simplify the design, further decreasing costs while increasing performance.

All of these advantages will allow sensors designers to significantly expand sensing ranges beyond old industry standards. Also, the more simplified the designs become, the greater the consistency between sensors. This will mean that system designers will ultimately get closer and closer to using an absolutely identical sensor, time after time, in each application, on each machine, on each project, year after year.

Pat • Balluff: Proximity sensors will continue to get smaller, have greater range, and provide better accuracy. More devices will become wireless and possibly have built-in long term power supplies much like active RFID tags. When wireless becomes a reality, we will be able to embed proximity switches deep into machining assemblies and still communicate with them via wireless technologies.

Jim • Omron: Advances in inductive proximity sensor technology will drive down costs in commonly used industrial automation products. These devices will take on time-proven form factors, like the standard metrically threaded cylindrical body, and ever smaller versions will have ever longer sensing distances.

These will be available in traditional materials to keep costs low, but will also be offered with improved plastics. Materials once considered highly specialized will become more commonplace. Ceramics and carbon fiber will be incorporated into the construction, allowing even higher standards to be met.

Miniaturization will continue with micromachining, and full DeviceNet and CompoNet capabilities will make possible status, diagnostics, troubleshooting, and programming by wire over networks. An increased level of data processing functions will be available, as well as remote, wireless, and autonomous capabilities. Power requirements will be further reduced as well.

Meet the experts

Alex Gasser
Baumer Ltd.

Pat Helm
Balluff Inc.

Jim Pankiewicz
Omron Electronics LLC

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