Productivity Forum: Sensors and switches

April 1, 2002
It's all about feedback. Encoders, switches, and all species of sensors provide information about machine health and critical processes on a need to know basis

It’s all about feedback. Encoders, switches, and all species of sensors provide information about machine health and critical processes on a need to know basis — which happens to mean 24/7 reporting in most applications. If you’re looking for sensible advice on feedback, let your vision sensors and CPU (eyes and brain) process this industry roundtable.

Frances Richards, Senior Editor

What is “productivity” and how do feedback devices contribute to it?

Christine/Allegro: In the scope of automotive applications, productivity means better gas mileage, reduced tail pipe emissions, and greater passenger safety. For consumer and industrial products (cell phones, lap tops, PDAs, exercise equipment), productivity means higher reliability and increased capabilities.

Joe/Banner: Sensing productivity can be defined as the speed and capability of the sensor to accurately respond to a defined attribute. However, it also relates to the sensor’s advanced electronics that allows it to solve the application and makes it easy to set up and operate.

Brian/Baumer: Productivity is the act of producing a product or service in the quickest way with as little “scrap” as possible. Absolute encoders add to productivity by always knowing where the device (such as a conveyor belt) is even if power has been disconnected. These can be used for position in applications such as feedback for AGVs, large printing presses, packaging machines, or CAT Scan machines.

Proximity and photoelectric sensors tell presence or absence of an object prior to starting a process. They can be used to verify a part is in place or ejected from a mold so the next process does not happen, which might otherwise crack the mold. Prox switches in grippers can verify that a part is in the gripper hand as well as verify whether the gripper is open or closed. Photos can tell the location of objects on conveyors to increase or decrease the speed to prevent jams.

John/EPC: Productivity is a measure of performance and efficiency. An optical encoder is a key component used in many automation projects to improve productivity. Many engineers prefer optical encoders to other feedback devices because they provide high resolution and accuracy. Encoders produce standard digital output signals and can operate at very high speeds.

Mark/Hyde Park: A simple way to increase productivity is to decrease downtime. Easy setup and custom configuration can also boost productivity. For example, a simple pushbutton setup that programs the sensor to the application allows it to be up and running in a short period of time. Sensors also help productivity by performing tasks that are impossible or difficult for humans to do. It would be impossible, for example, for a human to accurately count containers on a conveyor moving at 2,000 cpm.

Dave/MTS: Productivity is not only a measure of the ability to produce, but to do so at a reasonable if not minimal cost. Obviously, the throughput and quality of the production process is crucial and this relates directly back to the responsiveness and accuracy of the sensors. In any precision or high-speed process, system performance will never be better than the capability of the sensors used. Looking at the cost of productivity, a major part of this is the installed system costs, but maintenance and repair are being scrutinized more than ever before. The availability of device level intelligence is a new value-added component of the productivity equation that can help reduce overall costs.

Dave/Omron: Productivity is a meticulously engineered system that is reduced to its simplest and most cost-effective process steps to mass produce a quality product at a competitive price. It is the discrete sensors and switches that signal the machine’s transitional operations and allow the machine to operate through its designed cycle of events. Feedback devices like analog sensors, laser measurement sensors and machine vision sensors offer information regarding size, distance, precision measurements, and flaw detection of products during their assembly process. These devices allow defects to be detected at the point of lowest value-added, lowering manufacturer’s costs with regard to rework and field defects.

What elements associated with sensors and switches most often limit machine productivity?

Brian/Baumer: The typical reason prox and photo sensors have to be replaced is physical damage. Either a person or another part damages them. They get damaged during maintenance or production when a part of the machine starts to sway because some component (such as a bearing) starts to wear out. As the bearings wear the machine can then slide over and hit the sensor. Sensors that sit in liquids may get an ingress of the solution over time that causes the sensor to fail. Encoders typically fail due to the bearings on the unit failing. This is the mechanical part of the device. The electrical part usually can still be used.

Dave/MTS: One of the biggest enemies of productivity is downtime due to environmental and durability issues. The most significant mechanisms influencing sensor durability include mechanical wear of sensing elements and extreme shock and vibration. Major environmental issues include temperature, ingress, and electromagnetic susceptibility.

Dave/Omron: Regarding sensors, their weak point is typically the cable or connector wiring that wears due to flexing over time. Specific points are the internal wiring connection and the point of entry into the sensor. Regarding limit switches, contacts wear over time, especially when using standard contacts to switch very low currents or micro-loads. This is easily solved, however, with gold plated contacts. For vision and other optical sensors, the most common causes of field failure are misapplications regarding environment, temperature, ambient lighting, electrical noise, vibration, and changes in the actual target regarding color, texture, glossiness, and degree of repeatability as the target is presented to the sensor.

Christine/Allegro: With sensors, the inability to compensate for thermal drift and installation offsets limits productivity, as does sensitivity to target eccentricities in speed sensing applications. The inability to tolerate dirt and other particle contamination is another limiting factor with certain sensors.

Mark/Hyde Park: Older technologies like contact switches were prone to wear out, even expected to after a certain amount of use. Most noncontact type sensors are solid state and will typically not wear out if used in the right environment.

John/EPC: Encoders have very few limitations and typically provide years of reliable operation when applied properly. The most common problems occur when the wrong model is selected for a particular environment — for example, using a non-sealed encoder in places where it is exposed to abrasive dust or liquid spray. A second problem is bearing damage caused from misalignment or overloading the encoder shaft.

What can sensor MANUFACTURERS do to alleviate component weaknesses and increase end-user productivity?

Dave/MTS: Focus on environmental and durability issues. Noncontact sensing, superior shock and vibration durability, and environmental and EMI immunity are a solid foundation. For example, in addition to encapsulation, each sensor module could be shock mounted within the application housing to provide superior shock and vibration immunity.

Brian/Baumer: Manufacturers can make sure sensors are properly sealed in the best way possible to prevent dirt and liquid ingress. Dirty lens alarms on photoelectrics or end-of-range alarms on prox sensors will warn if the sensor is about to fall out of proper operation. Making smaller sized packages with the longest ranges possible allows for less chance of the sensor being damaged in the field from swaying parts or machine repair. Also, easy replacement with features such as quick disconnect cables will increase productivity by reducing downtime due to faster replacement.

Dave/Omron: Many of the problems facing sensors and switches have already been solved by manufacturers, but designers have yet to be properly informed. Many designers choose a pre-wired sensor due to its lower initial cost, when a connector version is more robust and offers ease of maintenance during replacement. It’s always about expense. Designers are continually looking to cut costs and it is easy to apply a general-purpose sensor for $35.00 and think you have done well — until the first failure. For example, your material slightly changes in color and the sensor no longer detects the product at 100% accuracy. After hours of troubleshooting with maintenance, engineering, and purchasing involvement, not to mention lost production, scrap, or defective product, a $120.00 properly applied sensor is a bargain.

John/EPC: Exceptional technical support is important to ensure that designers select the correct encoder for the application. One of the most significant design improvements is the development of optical application specific integrated circuit (ASIC) sensors. This technology combines several electronic components into a single sensor increasing speed, reliability, and signal quality while reducing size and power consumption.

Christine/Allegro: Certain Hall-effect sensors have patented techniques that address offset, temperature drift, and target eccentricities by using a single chip solution. The inherent nature of the Hall-effect principle means that this technology is impervious to dirt and particle contamination.

Joe/Banner: Sophisticated motion control systems require sensors with on-board intelligence and advancing semiconductor technology has allowed microcomputers to become an integral part of photoelectric and ultrasonic devices. Many new photoelectric sensors use this added sensing intelligence to analyze received light signals and to monitor and adjust sensor performance. The greatest benefits for motion control applications include more reliable sensing of difficult to detect objects or materials, elimination of background interference, the ability to adjust the sensing field to the range of the target object, and the ability to precisely gauge the distance of the target from the sensor.

Mark/Hyde Park: Sensors fully filled with epoxy seal out any potential for moisture to accumulate on the electronic circuitry. Also, using high quality plastics enhances survivability in caustic chemical washdowns and other harsh environments. Another way to increase end user productivity is to produce a sensor that is easy to set up, which allows you to install it and forget it.

What can DESIGNERS do to optimize productivity?

Christine/Allegro: Integrate sensor design as early as possible in system development. Early optimization can prevent costly redesigns late in the design process, which can delay time to production.

Brian/Baumer: Designers should make sure they are using the sensor within the tolerance of the specified product. I recommend only using it up to 80% of the rated sensing distance. This leaves a buffer zone as the machine parts wear in and become “looser.” And the sensor is not always being pushed to its limits. Try to have the sensors mounted in a protected manner and not hanging out in the open where they can easily get hit. Mount them so the part will go past the face of the sensor and not directly towards the sensor. Realize the limits of the sensor and how it operates.

Mark/Hyde Park: Specify the correct sensor for the application. Temperature, size and shape of object, power supply quality, output characteristics, chemical compatibility, and other environmental conditions can all affect the sensor’s performance. Factors such as lighting, type of metal, and humidity are problems specific to certain types of sensors.

Dave/MTS: Designers need to be aware of the value of specific products to their system beyond nominal performance vs. installed cost. For most production processes, it doesn’t take much time to make up the price of an entire sensor due to down time as a result of inferior durability and immunity, or increased maintenance and setup efforts.

John/EPC: Contact technical support to make sure the best model is selected for the application. Special attention should be given to the encoder’s mounting style and electrical connection. For example, certain thru-bore encoders mount easily over the end of a shaft and the flex mount design provides automatic alignment for long life. These thru-bore encoders eliminate costly adapter plates and couplings.

What can END USERS do to increase productivity?

Brian/Baumer:Pay attention to sensor location when climbing around on the machine during repair. Routine preventative maintenance and replacement of older parts such as bearings will prevent the machine from damaging the sensor itself. Routine replacement of the bearing part of a sensor (if possible) or changing the whole encoder as part of preventative maintenance will prevent unexpected failure. Make sure the machine operator understands what to expect from the sensor. Knowing how to read the warning indicators like a dirty lens alarm will allow the operator to wipe off the lens before the machine shuts down. A little education can go a long way.

Christine/Allegro: Again, integrate sensor design as early as possible in system development.

Dave/MTS: End users are often more sensitive to overall productivity than the companies who provide their production machinery. They’re the ones that have to live with the installed system. Their issues — setup, maintenance, repair — are equally if not more important than process quality and throughput. In most cases, the voice of the end user eventually makes its influence on the machine manufacturers.

John/EPC: End users should contact technical support to make sure they have selected the best model for their application. Mounting style and electrical connection should be a key consideration. Selecting an encoder supplier with good support and delivery is also very important. Many end users can not get a replacement part in a timely manner; often these encoders are made offshore, so stocking a spare could make a big difference in lost productivity. End-user advice

What’s new and promising for the future of feedback devices?

Joe/Banner: New sensor technology includes patented “teachable” sensors that can learn to detect objects that were previously impossible to sense. These new single pushbutton sensors use an advanced microprocessor that differentiates between two received light levels for the most precise sensitivity adjustment. This offers increased reliability for sensing transparent materials, and is ideal for tough applications such as colormark detection on a continuous web and sensing clear bottles or wafer cassettes on a moving conveyor line. Time-of-flight laser sensing technology is an extremely capable new technology that allows very long range yet accurate gauging of distances. A short electrical pulse drives a semiconductor laser diode to emit a pulse of light. The emitted light is collimated through a lens, which produces a very narrow laser beam. The laser beam bounces off the target, scattering some of its light through the sensor’s receiving lens to a photodiode, which creates an electrical pulse. The time interval between the two electrical pulses (transmitting and receiving the beam) is used to calculate the distance to the target, using the speed of light as a constant.

Continued development of smaller packages, more on-chip diagnostic capabilities, and alternative output protocols.

Brian/Baumer: Production rates for sensors are being increased while they are starting to have more features incorporated. Absolute multiturn encoders that only were available using gears are now available using electronic methods of counting the revolutions. These devices are learning to talk to more and more bus systems. Before, a separate encoder would have to be purchased for every bus system and now the mechanical assembly only has to be purchased and the original electronics can be used. This is a double bonus, because the mechanical bearings can be replaced at a much lower cost while the same electronics are still used. Sensors are being manufactured faster with new designs such as putting more of the intelligence onto the chips and reducing internal components.

Dave/Omron: Over the next five years you’ll see wireless technology, programmable sensors, trends towards miniaturization, easy teach capability, self-diagnostics, common mounting practices, and stable to downward pricing trends.

Dave/MTS: The future of factory automation is definitely toward more distributed system control with the availability of process data and diagnostic information being a primary element. The use of intelligent sensors will provide more critical process information in “real time” and reduce installation and setup costs. Setup time and process changes will be accommodated quickly through the use of easily modified software recipes stored within these devices.

Sponsored Recommendations

How BASF turns data into savings

May 7, 2024
BASF continuously monitors the health of 63 substation assets — with Schneider’s Service Bureau and EcoStruxure™ Asset Advisor. ►Learn More: https://www.schn...

Agile design thinking: A key to operation-level digital transformation acceleration

May 7, 2024
Digital transformation, aided by agile design thinking, can reduce obstacles to change. Learn about 3 steps that can guide success.

Can new digital medium voltage circuit breakers help facilities reduce their carbon footprint?

May 7, 2024
Find out how facility managers can easily monitor energy usage to create a sustainable, decarbonized environment using digital MV circuit breakers.

The Digital Thread: End-to-End Data-Driven Manufacturing

May 1, 2024
Creating a Digital Thread by harnessing end-to-end manufacturing data is providing unprecedented opportunities to create efficiencies in the world of manufacturing.

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!