Getting Started with Smart Sensors
Sensors are a diverse family of devices. A simple description: a sensor is used to detect the presence of an object. They are very common throughout industrial automation, process control, and in most manufacturing facilities. With the right design and configuration you can turn your regular sensors into smart sensors.
What Sensor Should I Use?
This post will help you answer the FAQ “What sensor should I use?” Of course, there is no one magic sensor. The answer will depend on your application and your machines. But let’s started with a look at the common types of sensors and how they do their job:
- Photo Optic: Use light or the absence of light to sense
- Ultrasonic: Use sound waves to sense
- Capacitive: Sensing items with an electric field
- Inductive Proximity: Sensing metal with magnetism
- Limit Switches: Function via mechanically touching an object
We will get into further detail on each of these types below. Right now, consider that the first four items listed do not have to physically contact anything to sense an object. This equates to longer life, and higher speeds. In a production environment, speed and component life are critical metrics.
It’s important to pick a sensor based on (1) what you are detecting and (2) the environment/application you are working with. From there, you may have requirements for speed. All of these are important considerations when you are answering the question “What sensor should I use?”
Photo Optic Sensors – Three Basic Options
Photo Optic Sensors, commonly called just “Photos” in the industrial automation space, allow you to see a long distance, offer fast response time, and give you many options. The most reliable way to use a photo is to set up a light beam to be broken by a part. There are three types of device that can do this:
1.1. TRANSMITTED BEAM. Two devices, a transmitter and a receiver. This is the best for a slightly dusty environment, because even if the lenses get slightly dirty, the light can still make its way through. Considerations: Light can only make its way through to a certain point. Both devices must be aligned and protected from impact. If misalignment and/or impacts, i.e. bumps, will be an issue in your application, consider…
1.2. RETRO-REFLECTIVE. Uses a reflector to create a beam. This is one device with both send and receive capability, bouncing light off a reflector. Considerations: You must be careful that the object(s) to be sensed are not shiny, or the object may bounce the light back just like the reflector. Not good. If shiny objects are an issue, consider…
1.3. POLARIZED RETRO-REFLECTIVE. Uses a reflector to create a beam. Very similar to above, but the light is polarized, and the sensor will not be fooled by shiny objects.
Those are three options for Photo Optic Sensors. A term you may have heard is “dark operating”. The three devices listed above are triggered when light is blocked, and so are called “dark operating” devices.
Photo Optic Sensors – Three Other Options
OK. What if you just don’t have space for a reflector or receiver in your application? Let’s look at some other options, each with unique considerations.
1.4. DIFFUSE SENSOR. A diffuse sensor is also a send/receive device but it uses the object to reflect light back to itself. It triggers when it sees light, so it’s called a “light operating” device. Diffuse are sometimes considered the least effective of photo sensors. Considerations: You’ll have to adjust the gain of the light source to detect the part at its farther possible location. A black part will reflect less light than a white part, and so could fool the sensor. A shiny background could cause issues for this sensor. If the background, or color of object is a concern, you may want to try another version of diffuse sensor…
1.5. BACKGROUND SUPPRESSION SENSOR. This type of sensor is adjustable to only see a certain distance and NO farther. Regardless of the background this sensor will ignore anything past a set distance. A background suppression sensor may be appropriate if you need to see all colors at the same distance or if you have a shiny background. Considerations: What if you have a clear object? For example, a bottle. This item will allow light to pass through it, rending a regular photo sensor useless. What to do?
1.6. CLEAR OBJECT DETECTOR. This is a super sensitive photo sensor that looks for any change in light between it and a reflector.
Photo Optic Sensors – Types of Light
We just reviewed quite a few types of sensors above. For almost all photo sensors, there are three ways you can deliver the light:
- LED. This is the typical photo sensor used for basic applications. The LED can be a visible color, or infrared.
- Laser. Lasers allow a very thin beam to get into tight spaces, or see small objects
- Fiber Optic. Allow access into tight spaces. Also, useful in very hot spaces where other materials may risk melting.
If we are going to answer the question of “What smart sensor should I use?” we must always consider the application and the challenges. Photo optic sensors sometimes face serious challenges in dusty or misty environments. Really, any environment that might impede light travel presents a challenge for photo optic sensors. Also, we started discussing applications where a clear object would allow light to pass right through it. All applications where photos might not be ideal, but where an ultrasonic sensor works well.
2.1. ULTRASONIC. An ultrasonic sensor bounces high frequency sound off a target, similar to a diffuse photo. In the same way that you can talk to a friend through dust and fog, the sound waves of an ultrasonic will be unimpeded through these media. Ultrasonics are often used to sense the level of something from above. For a liquid this works well, and it can also be used to tell the height of a fine solid. For those types of uses, consider an ultrasonic sensor.
- Considerations: Ultrasonics have their own set of limitations. In level applications with a powdery solid material some of the sound waves maybe absorbed, reducing the range of the sensor. A real-world example: I have found that sawdust will absorb about 50% of the sound.
- When locating your sensor, be careful to not place it close to the exhaust of a pneumatic valve. The high pitch of the exhaust air could mimic the sensors sound frequency. Or place a muffler in the exhaust port.
Capacitive sensors can be a nice option for your tough applications. Using an electric field, they will detect the capacitance of a material. When might we consider a Capacitive sensor?
3.1. CAPACITIVE. They are good for seeing through a material that has low capacitance, like plastic or cardboard, to detect something on the other side with higher capacitance.
- Ex: Measuring water level behind a plastic port in a tank.
- Ex: Checking to see if product was deposited inside a box.
- Ex: Detecting the presence of water in a plastic bottle.
- Considerations: A challenge for this type of sensor can be distance. A capacitive sensor needs to be placed close.
Inductive Proximity Sensors
Colloquially “A Prox”. These are very common devices. Using a magnetic field, the presence of any metal can be detected. These sensors are often used not only to detect parts, but also components of a machine.
4.1. INDUCTIVE PROXIMITY. A non-contact device. For the general prox sensor, all metals will not be detected at the same range. Non-ferrous metals will require the prox to be mounted closer to the target. Since most applications are detecting steel, this is not often an issue. However, if detecting different metals at the same distance is important in your application, there are options.
- Considerations: Machine parts wear out over time. This will cause misalignment and a prox can be destroyed by collision.
How can you avoid device destruction? The larger diameter prox you use, the farther from the face it will sense. This allows more clearance for the device. Simple rule of thumb: the larger diameter, the further away you can mount.
How are you mounting this sensor? When it comes to proxes, you can flush mount versus versus non-flush mount. If your device is mounted flush to the metal, the prox will detect that metal before it is able to detect anything else. This is not in line with intended use. If you need a flush mounted prox, make sure you specify one that is designed for it. Adding shielding to a prox will directs the magnetic field outward to enable a flush mount.
Limit Switches are electro-mechanical devices that consist of an actuator mechanically linked to a set of contacts. When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection.
5.1. LIMIT SWITCHES. Limit switches work in a variety of applications and environments because of their ruggedness, simple visible operation, easy installation and reliable operation.
- Considerations: Limit switches are sometimes used in hazardous locations.
- Considerations: While limit switches are very commonly used, it’s one more point of wear and possible failure. In most cases you should use a non-contact option that will last much longer.
Smart Sensors: Taking Your Discrete Sensors to the Next Level
Using a new communication protocol, traditionally discrete (on/off) devices can give you more diagnostics. Think about a sensor that is performing a critical role. If it fails there may be a big crash and an expensive fix.
–What if you could detect a photo eye was getting dirty and clean it before it failed.
–What if you could tell a normally open prox was destroyed the moment it happened, rather than waiting until the machine wasn’t working.
–What if when replacing a sensor, the PLC would immediately detect a new device and down load the settings of the old one? No need for a skilled technician to set it up.
–What if you run various products on a machine on different days? Imagine if you could download new settings to all the sensors instantly from the PLC. Big savings in set up time.
The above is all possible with Rockwell sensors using a new protocol called IO-Link . And it’s important to note you don’t need to buy special sensors or connectivity. The technology is embedded in the sensors already and you decide whether or not to activate the functionality for your critical applications. Reach out to the Horizon Solutions team for more details.