How close is it? That's the golden question proximity sensors were born to answer. Today, however, another question is when is a proximity sensor a better choice than a machine vision system – a line that continues to blur as sensors get smarter. In this forum, Motion System Design editors speak with industry experts about this as well as other proximity sensing trends.

HOW DOES THE CHOICE OF PROXIMITY SENSORS AFFECT THE DESIGN AND OPERATION OF MOTION-CENTRIC AUTOMATION SYSTEMS?

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 camerabased 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.