By Andrew Waugh, AutomationDirect
A trip to the local coffee shop can serve to reveal the consequences of too much choice. Some patrons already understand the available possibilities from experience and quickly order exactly what they want. Others are confused or even intimidated by the variety of styles, flavors, sizes, and options — which may delay their selection — or even result in them buying something they didn’t quite want.
Most people appreciate the benefit of having many choices available to them, but it is possible to have too many options. For these situations, having a summary, guidelines, or some other assistance aid selection.
For designers of industrial automated equipment and systems, sensors are a crucial consideration because a control system operates most effectively with the proper feedback. Sensing the position of parts and product is fundamental in this regard, and there are many types of targets.
Compounding the challenge are the wide variety of available sensing technologies and performance levels. Let’s examine some commonly available position sensing methods, benefits, and costs, sort of an introduction for those newer to the subject and a refresher for those with more experience.
When it comes to industrial sensor selection, a basic balance must be achieved between cost and performance. The most straightforward cost is that of the sensor itself. More challenging to pin down are the initial design, installation, and configuration costs, along with required ongoing support and maintenance efforts.
Performance can be measured in many ways. The most important point is for a sensor to reliably sense the appropriate target with sufficient accuracy. The form factor needs to physically fit the application, and the sensor must survive the environmental conditions.
An inexpensive sensor that isn’t durable will require more frequent attention and replacement, significantly driving up costs over time. These costs are not just for servicing the problematic sensor, but also include downtime for the associated equipment.
Note that for this discussion we are simply looking at discrete sensors for detecting the presence or absence of a target. However, versions of some technologies can return an analog signal to indicate how far away a target is, instead of just an on-off signal.
Good engineering practice usually guides designers to follow a keep-it-simple approach, as long as the cost and performance needs are met. Therefore, the following sections are generally arranged to review sensing technologies in order of increasing cost and complexity.
The most basic sensing technology is the humble electromechanical limit switch. They are compact, easy to adjust, available with many actuator types, may be mounted in many ways, and can be quite accurate. Their biggest downfalls are related to their need to physically touch what they are sensing, and because they are mechanical with moving parts, they are more subject to wear and damage than other non-contact methods.
When the geometry or motion of equipment or parts can be arranged so that a rugged limit switch will work, this is a reliable and inexpensive approach.
Magnetic proximity switches
The most basic magnetic proximity switches were originally very sensitive mechanical reed limit switches that closed in the presence of magnets mounted on the target. The latest magnetic proximity switches are solid-state and much like inductive proximity switches (described in the next section) except they only detect magnets mounted on the targets, Figure 1.
This may seem like a specialized and limited application, but because it works through non-ferrous materials like aluminum, the widest industrial use for these switches is to detect the position of a magnetically equipped actuating rod within a pneumatic cylinder. Alternately, a magnetic target can be installed on an object to deliver reliable sensing without false trips, which could occur in milling or cutting applications with errant metal particles.
Because these sensors have a detection range of three to four times greater than inductive sensors, they allow for much better mounting flexibility. Also, the sensor can be mounted within an aluminum box for better protection while not impacting the detection ability.
Inductive proximity sensors
When the target to be sensed is metal, and if a small sensing range is acceptable, then an inductive proximity sensor will usually be the best option, Figure 2. Inductive proximity sensors are durable and extremely reliable solid-state technology. Their use of electromagnetic detection fields enables them to sense metal objects repeatably, but not with extreme accuracy. Note that they work best with ferrous materials, and for non-ferrous metals the sensing range is reduced.
Standard and extended sensing distances are measured in millimeters, which is usually sufficient for detecting equipment positions, especially at end-of-travel. Because of the high performance, non-contact longevity, and low price, it often makes sense to configure equipment as needed, including adding dedicated targets, to take advantage of inductive proximity switch benefits.
Capacitive proximity sensors
Capacitive proximity sensors look much like the inductive type but operate on the principle of detecting capacitance differences in the sensing zone when a target is present or absent. These sensors can be a great solution in many applications, although they are increasingly being passed over for newer technologies.
Because capacitive sensors will sense just about any type of material, they can be used in a wide range of applications. Their ability to detect liquids and other non-metal products is a very useful feature, although they can be affected by buildups of such products and may provide false readings due to ambient materials in the area such as wood chips or coolant.
An additional feature is the sensors can be adjusted or “tuned” to ignore non-ferrous materials between the sensor and the target. This means the sensor can be used to detect “through” a window or sight glass, for example when used as a non-contact point level switch for a vessel or tank containing dry or liquid contents.
Ultrasonic sensors are another solution for presence sensing. As indicated by the name, it is possible for an electronic device to use sound, much like a bat, to determine the presence of an object. A sound impulse is generated, and the device operates by sensing an echo from the target. This method is not extremely precise or fast but offers a better solution than optical sensors in some applications.
Much like capacitance technology, ultrasonic sensors can detect most materials. Since they are not optical in nature, they are not affected by variations in color, transparency, reflectivity, or ambient lighting conditions, making them a good solution for detecting clear targets.
The most common variation is a diffuse “reflective” style, which is trained to recognize a consistent background and can therefore identify any object exceeding the trained baseline response. Another style, referred to as through-beam, uses a transmitter/receiver pair with a comparatively short sensing range of 300mm or so, but improved accuracy. A down-looking configuration can be useful for non-contact liquid level sensing in tanks.
Photoelectric sensors, sometimes called optoelectronics, are some of the least expensive and most commonly used technologies. There are many variations and technologies in this family of sensors, but the basic concept is the same for all types.
Light is transmitted out of one lens and received into another lens. The amount of light received at any given time is used to evaluate whether a target is present or not, Figure 3. Due to the many available types and applications, photoelectric sensors require some design effort for correct application.
The light transmitted can be visible red, infrared, or laser light, with each offering specific benefits and improvements at an increasing cost. The beams can be very tight for detection of small objects, or broader for uneven surfaces or large detection areas.
There are four popular arrangements:
• Diffuse: Only needs to be mounted on one side of a target but are the least accurate and require more setup effort.
• Reflective: The sender/receiver is on one side of a target, with a passive reflector on the other side, and offer more positive sensing than diffuse.
• Through-beam: A sender is installed on one side of a target and a receiver on the other. This is a more costly option for the sensor and installation, but it provides the most accurate and reliable sensing over a long range.
• Background Suppression: This style is useful when there are reflective backgrounds as it detects anything other than the trained background, but they are the most expensive and require the most setup time.
Photoelectric sensors offer many features and good versatility relative to cost, as long as a suitable configuration can be selected for the target and the environment.
Getting the right sensor
Presence sensing is a fundamental industrial automation need, complicated by the countless combinations of targets and environments, but with a spectrum of technologies available to support these diverse conditions. As end users approach the design for a given application, they can use the fundamentals discussed in this article in conjunction with vendor resources to help them make the best technical and commercial choices.