By Andrew Waugh, AutomationDirect
Marketing specialists are familiar with the idea of offering good, better and best products to give the consumer options, and possibly upsell them in the process. Yet many times, this concept is imprecise. We each weigh features differently, and sometimes the better product doesn’t simply have more features, but instead has other attributes. Industrial end users looking to select the right position sensors for their equipment face this very dilemma.
There are basic general-purpose versions and enhanced mission-specific styles of industrial sensors. From an engineering standpoint, it is tempting to select the most full-featured, optimized and easy-to-use product every time — but wise engineers understand this can add to costs. Business owners or purchasing agents, on the other hand, are likely to look for the best value, and sometimes both parties can realize their goals with the same device.
Very often, the availability of more choices is better for the end user, even if they end up using just one or a few of the options. This article will examine some criteria that users should evaluate when deciding on the best position sensors for their applications, but the discussion is also valid for investigating any type of sensor.
General-purpose versus mission-specific
For most position sensors, the three fundamental technical performance criteria are: reliable sensing distance, environmental suitability and mounting requirements. All three are related to each other, and how the sensor is designed to achieve each of these goals is partly what classifies it. A simple physical limit switch may suffice — but also be susceptible to mechanical damage — while a more advanced non-contact proximity switch has barely enough sensing distance, but can survive the environment.
Defining the use of the terms general-purpose and mission-specific is an important first step, because there is no industry standard for this terminology and products may not be labeled as such. General-purpose is largely associated with a basic part or product, applicable to most common applications. Mission-specific, on the other hand, implies a more highly-engineered product with additional features and unique characteristics. These enhanced elements help the product provide key functionality not available with general-purpose products.
Most observers would say that any mission-specific product would make general-purpose seem inadequate or inferior. However, a thorough analysis must be based on fact-based performance and cost evaluations, with an attempt to consider unintended consequences.
Weighing the features
With these thoughts in mind, why would anyone specify general-purpose for anything? The most important and obvious reason is cost. General-purpose implies a simpler and more standard device, likely mass produced to take best advantage of economies of scale. Usually, any enhancements that push a sensor toward being mission-specific mean added costs, and users who don’t need those benefits would end up overpaying.
However, mission-specific isn’t always better when all impacts are considered. A lesser known side effect is entirely possible whereby a mission-specific sensor could be counter-productive in a basic application due to its extra features. Perhaps it is more difficult to adjust, or less reliable at certain types of detection, so it causes difficulties for technicians and operators. If it ever needs replacement, its more specialized nature might mean less commercial availability. These collateral impacts may not be noticed on the first day of operation but can add up over time.
Another key user consideration is standardization. If end users could successfully select one sensor type for all applications, their personnel would become experts with that model, and they would only need to stock the one type. Many sites would gladly pay a little more for such simplicity.
As a practical example, let’s consider photoelectric sensors. This product group is extremely large and ever-growing with respect to both underlying technologies and potential applications. There are three general forms of sensing technology: diffuse, retro-reflective and through-beam.
All three types share a common philosophy of sending and receiving light, although the source may be red LED, infrared or even laser. Each technology type and light type has different advantages.
For a basic application like sensing a cardboard box or other opaque object on a conveyor, any sensor type would work — and in fact a general-purpose retro-reflective red sensor is an excellent and reliable choice, Figure 1.
Residing on the other end of the degree of difficulty spectrum would be an application for sensing a clear bottle moving down an infeed conveyor on a filling line. A general-purpose red LED sensor would likely sense on/off/on as the bottle moves through the sensing path because it will only trigger on the denser view through the leading and trailing edge sidewalls. This false reading could provide two pulses as a bottle moves by, instead of just the desired single pulse.
In this case, the application demands a technology specifically targeted for clear glass detection, such as an optimized sensor. This type of sensor would probably work in many other applications, but it would be more expensive and suffer other constraints, such as a limited range of sensing distances or mounting options.
Since many sites have varying requirements just like these examples, it is most likely that more than one sensor type must be selected to address all applications.
Focus on the environment and the technology
Many designers might find a cost-effective approach is using general-purpose wherever possible, and only moving to mission-specific when the environment or technology demand it.
Environmental specifics are easy to understand. Industry uses IP ratings or NEMA ratings to define how well a device can withstand ingress of solids like dust and liquids like water. Higher ratings can withstand pressure washing or even submersion. Furthermore, the materials of construction must be suitable for whatever chemicals might be in the environment.
Basic sensors and accessories may use plastic or plated metal housings, and only be rated to IP65 or lower, which is suitable for warehouse or shop environments, Figure 2. However, food or pharmaceutical applications subject to chemicals and pressure washing will drive the need for stainless steel housings and a rating of IP68 or higher, Figure 3. Of course, undesirable consequences of encapsulating a sensor in SS are usually a reduced sensing range and increased cost.
Other challenging environments are welding applications and tight locations, which might require specially coated sensors to resist slag buildup, or slimline right-angle style housings instead of a regular tubular type. Both add cost and are only needed for those specific cases.
Technology specifics can be much more subtle. The photoelectric example is a case where the best sensor might detect the angle of the light reflection instead of the amount of light, a technology called background suppression. This is great for ignoring the color of an object and is important when needed, but it adds cost when not required.
Finally, consider the case of proximity type position sensors for which cost versus distance evaluation is important. This is a situation where more advanced factor 1 sensors, also known as k-factor=1 or K1, cost little more than traditional sensors but offer a consistent sensing range for any type of target metal. In this circumstance, the technology offers a performance advantage for new and retrofit applications, and it could easily enable an end user to consolidate stock, a valuable benefit.
The right sensor for the job
If designers can choose a general-purpose sensor that survives in the environment and gets the job done, there is no reason for them to feel they are getting anything less as compared to a mission-specific sensor. For a new and untested application, the right approach might be to start with a standard model to see if it can pass muster.
For those situations truly requiring more advanced products, a myriad of options is available. Careful end users will educate themselves on the available technologies and characteristics, and they will lean on the device suppliers as required. Sensor manufacturers and distributors usually can offer some advice based on feedback of how their products are used throughout a wide range of applications.
The wide range of products available to the end user is a great benefit. With some careful evaluation, designers can target just the right models necessary to deliver the right performance and value for all their applications.