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Making sense of position sensors

Oct. 1, 2008
Sensor selection How does sensor selection contribute to productivity in today's motion-centric automation environment? Michelle BEI: Selecting the correct

Sensor selection

How does sensor selection contribute to productivity in today's motion-centric automation environment?

Michelle • BEI: Selecting the correct environmental, mechanical, and electrical sensor specifications for your application will provide the longest life for your position sensor and will also help reduce system downtime. It's important not to over-specify or under-specify the sensor. By over-specifying, you'll pay for more features and performance than you need, and risk giving false position information or overloading your controller with too much information. Under-specifying can also have negative effects, such as inadequate signal bandwidth, low accuracy, and poor environmental sealing, leading to unreliable operation.

Matt • MTS: Sensor selection plays a key role in motion control applications. Position sensor speed (response time or update rate), for example, is a primary driver of machine speed, which determines production rate. Repeatability and accuracy also play a role, achieving tighter control, a factor in product quality as well as cycle time.

Machine downtime is important, too, causing a big hit on productivity. Choosing sensors that are rugged and reliable will reduce downtime related to sensor failure, letting machines run a greater portion of the time. Diagnostics and cabling also impact uptime. Advanced diagnostics and reduced cabling, two benefits of sensor bus networks, minimize the time it takes to restore a machine to full operation. Another productivity-oriented feature, absolute output with multi-position programmability, reduces setup times during startup and lets a single sensor handle each motion axis.

Gregg • Omron: Rotary shaft encoders give designers a cost effective and reliable way to collect rotational data from devices such as servo and stepper motors, robotics, and pick-and-place machines. They're typically employed in closed-loop feedback systems that collect and process information, such as position, direction, and speed. Encoders are critical to compensate for mechanical elements of the motion system.

Fred • ASM: Accurate position sensors help automate and increase the efficiency of many processes. Engineers can choose from several different rotary position sensor types that can be grouped into two categories: contact and noncontact. The latest generation of rotary position sensors incorporates noncontact sensing technology in an industrial grade housing that allows the sensor to operate reliably in harsh environments. Inherently, noncontact position sensors are wear-free and boost productivity by requiring virtually no maintenance, which reduces the machine's overall downtime.

Rotary sensors have a variety of analog and digital output options. For certain applications, redundant outputs may be required, which can be accommodated in the same housing. The sensor housing needs to be as compact as possible so that it can be easily installed in tight locations. Likewise, the sensor enclosure is expected to protect its internal electronics from common environmental factors such as oil and water.

Sizing, specification

What's your best advice on specifying, sizing, and applying position sensors where productivity is the main goal?

Michelle • BEI: Consider these tips to get the most out of your sensors:

Don't forget to think about the operating environment. Wet or dirty environments need a properly sealed sensor. Environments with high vibration or shock require position sensors that can withstand these stresses.

Talk to the manufacturer about your application before making any decisions. This can head off unforeseen problems and compatibility issues that might require return or repair.

Find out how to properly install the sensor. By “winging it,” you may be overlooking some critical steps that can lead to mechanical failures. By following proper installation practices, you can avoid frequent replacements.

Consider keeping a spare sensor on the shelf, especially for types with long lead times. If your position sensor fails, you can quickly replace it and get your system up and running, instead of waiting weeks for a replacement to arrive.

Matt • MTS: Be as specific as possible on the application requirements, including repeatability, accuracy, response time, input power, housing style, temperature range, and interface type. Productivity issues are often the result of not matching the performance of the sensor with the application requirements.

Select the right sensor the first time. Coordinate with an applications engineer or systems integrator to get the right stroke and output type. Don't forget to match electrical interface options with the associated controller interface card. Digital sensors may have specific output settings, such as baud rate, which must be met to work properly.

If the sensor must operate in an extreme environment, contact the supplier for optional enclosures or vibration-resistant versions. Although magnetostrictive position sensors are inherently rugged, they may require special mounting considerations to protect them from excessive shock and vibration.

Sometimes the simplest solution is the best solution. It may not be necessary to set up a serial bus network such as Profibus when the goal is simply a single channel, high accuracy feedback signal that can be accomplished with an SSI type sensor.

Consider redundant sensors for applications where downtime or access to the machine is a major issue for productivity.

Tom • Omron: Be sure to carefully consider these five aspects of rotary encoders to select the best one for the job:

Encoder types — Two basic encoder types are available to match the application: incremental and absolute. Incremental encoders are essentially tachometers to track information and speed. They're a low-cost option for unidirectional applications that require only position or speed information. Absolute encoders have as many tracks as output bits. The number of tracks on the encoder's disk determines the resolution, commonly expressed as cycles per revolution (CPR). Absolute encoders are generally chosen for applications that need to keep track of position at all times, and provide it as soon as power is applied. This feature is particularly useful where equipment runs infrequently.

Application — Understanding the application requirements will lead to the right encoder, i.e. motor feedback, web tensioning, cut-to-length, registration mark timing, conveying, etc.

Environment — When choosing an encoder, it's necessary to account for environmental considerations such as temperature, humidity, shock, and cleanliness. Encoders are available in a variety of enclosure ratings and duty factors to match the environment.

Space constraints — Motion systems don't always have unlimited space for components. Encoders are available in a variety of shapes and sizes to match space constraints.

Resolution — Encoders are available from slow to very high rotational speeds to match the reading of absolute position on a turning shaft.

Sad sensor stories

What's the worst that can happen if a position sensor is not specified or installed correctly?

Scott • BEI: Though most sensor failures don't have catastrophic consequences, they nonetheless can be costly in terms of lost revenue, reduced productivity, or equipment damage. One way to minimize this threat is to look at the sensors in your plant and do some disaster planning. Ask yourself, “What's my risk if this particular sensor fails?” Then pick your highest risk sensors and make sure you have emergency replacements. By far the most common situation we deal with involves machines idled for lack of a spare sensor.

Matt • MTS: For “in-cylinder” applications, make sure the stroke length and overall sensor length are correct for the gun-drilled piston rod. If the sensor's overall length is too short, you run the risk of the sensor rod falling out of the cylinder when it's fully extended. Upon the next retract of the cylinder, the sensor rod may be crushed or coil up.

Tom • Omron: One ugly scenario we heard about involved a vertical form, fill, and seal bagger for snack food. One of the encoder's channels short-circuited, so the control electronics were incapable of confirming the direction of the film. Due to the film's elasticity, registration was no longer maintained, which in turn produced a significant scrap rate.

Fred • ASM: Selecting the wrong position sensor may bring down an entire production line. Potentiometer-based sensors require frequent maintenance and may lead to an unplanned shutdown. Rotary sensors based on wear-free, Hall effect technology, on the other hand, don't require routine maintenance. Another potential pitfall is related to the sensor's enclosure. Many applications operate in demanding environments. In particular, protection against water ingress is critical to prevent the sensor's circuit board from shorting out. Some sensors are now available with enclosures that are rated up to IP69K to withstand high-pressure water spraying.

Future sense

Describe the ideal position sensor.

Scott • BEI: Existing position sensors have plenty of resolution and bandwidth for most applications, so performance isn't a limitation. It's usually the practical considerations that are “make or break” when it comes to sensor use — factors like temperature, operating voltages, ingress protection, ease of installation, and resistance to shock and vibration. The ideal sensor of the future will be able to operate over a wide range of environments while still maintaining enough precision to do the job.

Matt • MTS: An ideal position sensor would endure even the harshest environments, generating fast, accurate measurements without a glitch. It would be fully customizable to fit any application, provide absolute position for any electrical interface, and could be delivered exactly when needed at an affordable price.

Gregg • Omron: How about a universal, incremental or absolute, encoder for the ultimate in flexibility (potential applications) and reduced inventory? Give it a wide voltage range, a bipolar NPN/PNP dc output, flexible mounting options, and a low price.

Industry expertise

Michelle Todd
Scott Orlosky

BEI Industrial Encoders
(800) 362-6337

Matt Hankinson
MTS Systems Corp.
Sensors Division
(800) 633-7609

Gregg Farnsworth
Tom Kahn

Omron Electronics LLC
(800) 556-6766

Fred Fiedler
ASM Sensors Inc.
(630) 832-3202

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