Open- vs. Closed-loop Control

Several previous articles here on SENSORTECH have mentioned closed-loop control (Servo-Hydraulic Showcase, Linear Feedback Sensor Applications: The Three M’s). But exactly what does “closed-loop control” mean? How does it compare to open-loop control? I recently ran across an article in Control Engineering magazine that does an outstanding job of answering those questions.

Click over and have a look at this excellent article.

RFID – Keep it Simple!

traceabilityMost of us drive an automobile and use a PC daily. However, very few of us could accurately describe the intricate details of how each of those work. They help us get to work and help us do our work. There is not a need for us to know and understand the algorithm that allows us to compose and save an excel spread sheet. As well, there is not much use in knowing the coefficient of friction when using snow tires compared to standard tires. While those factors play a major role in the tools we use every day, we do not necessarily need to be an expert or scientist to reap the benefits.

Much like a car or PC, RFID systems enable us to be more efficient and productive. Specifically, RFID systems in manufacturing enable full visibility into the process. RFID technology provides actionable data to an organization. Having access to actionable data allows an organization to make critical business decisions with a great degree of confidence. Essentially, it takes the guess work out of the process.

So, how does it work? Very simply, a reader reads the information that has been written to the memory of a tag. Yes, it is that simple.

Check out this webex sponsored by SME. This is a very basic introduction to RFID and how it is used in manufacturing.

Do’s and Don’ts For Applying Inductive Prox Sensors

Inductive proximity sensor face damage

Example of proximity sensor face damage

In my last post (We Don’t Make Axes Out of Bronze Anymore) we discussed the evolution of technologies which brought up the question, can a prox always replace a limit switch?  Not always.  Note that most proxes cannot directly switch large values of current, for example enough to start a motor, operate a large relay, or power up a high-wattage incandescent light.   Being electronic devices, most standard proxes cannot handle very high temperatures, although specialized hi-temp versions are available.

A prox is designed to be a non-contact device.  That is, it should be installed so that the target does not slam into or rub across the sensing face.  If the application is very rough and the spacing difficult to control, a prox with more sensing range should be selected.  Alternately, the prox could be “bunkered” or flush-mounted inside a heavy, protective bracket.  The target can pound on the bunker continuously, but the sensor remains safely out of harm’s way.

If direct contact with a sensor absolutely cannot be avoided, ruggedized metal-faced sensors are available that are specifically designed to handle impacts on the active surface.

Be sure to consider ambient conditions of the operating environment.  High temperature was mentioned earlier, but other harsh conditions such as disruptive electrical welding fields or high-pressure wash-down can be overcome by selecting proxes specially designed to survive and thrive in these environments.

Operationally, another thing to consider is the target material.  Common mild carbon steel is the ideal target for an inductive prox and will yield the longest sensing ranges with standard proxes.  Other metals such as aluminum, brass, copper, and stainless steel have different material properties that reduce the sensing range of a standard prox.  In these cases be sure to select a Factor 1 type proximity sensor, which can sense all metals at the same range.

Precision Optical Measurement and Detection

In applications that require precise measurement and detection of one or more objects, what type of sensor should one use? If objects that are very small and far apart need to be detected, what type of sensor provides high resolution over its entire sensing range?

The answer: a laser micrometer.

A laser micrometer can identify, compare, or sort objects based on minimal size or height differences. Similar to a standard micrometer caliper, a laser micrometer provides precise measurements.

But how is this done exactly? Let’s find out!

A laser micrometer consists of two opposed sides, a transmitter side and a receiver side. These two sides sit opposite of each other to detect any object that enters in-between them.

On the transmitter side, a laser light source is positioned so that its emitted light enters a lens. The lens then collimates the light from the laser by refraction into a collimated beam of light (see Figure 1). By definition, a collimated light beam is a light beam where each light path in the beam is travelling parallel to one another. This collimated light beam has minimal divergence, even over large distances.


Figure 1

On the other side, the receiver side, a CCD (charge-coupled device) is positioned to collect the light emitted from the transmitter side. CCDs are made up tiny light-sensitive cells. These cells convert the amount of light intensity received into a corresponding electric charge, which can then be measured (see Figure 2).


Figure 2

The combination of these two components, a collimated light beam and a CCD, make up the foundation of a standard laser micrometer. The collimated light beam, which consists of a homogeneous light band, is directed at the CCD, which consists of hundreds of tiny light-sensitive cells. With this configuration, even a slight change in an object (e.g., its diameter, height, position, etc.) causes a change in the object’s corresponding shadow that is projected onto the CCD. This slight change can then be measured.

A few examples of the measurement capabilities for a laser micrometer are listed below, along with a video.


Position Monitoring


Diameter Detection


Gap/Height Measurement


Edge Guide — even with semi-transparent materials

The following video showcases the capabilities of the Balluff Light Array sensor: For more information on this sensor, please click here.

Servo-Hydraulic Showcase

48959254_woodbannerIn a previous installment here on SENSORTECH, we explored the three M’s of linear position feedback application (Linear Feedback Sensors – The Three M’s).  One of those three M’s stands for Motion Control.  When we talk about motion control applications for industrial linear position sensors, we’re often referring to closed-loop servo-hydraulics.  In these applications, the linear position sensor, which is usually installed into a hydraulic cylinder, plays a key role in the ability to accurately and reliably control the motion of very large, heavy loads.

Nowhere is closed-loop servo hydraulics more prominently utilized than in primary wood processing – where raw logs are transformed into all manner of finished board lumber.  Applications such as saws, edgers, planers, along with many more, rely heavily on closed-loop servo-hydraulics.  In many cases, hydraulic actuators get the job done when other types -electric, pneumatic – simply can’t.

If you’d like to get a look at some of these application, or to learn more about how linear positions sensors are used in the applications, a good place to start would be at an event where many of the machinery builders and suppliers gather in one place for a few days.  Does such an event exist? (I hear you asking).

Well of course it does!  It just so happens this very thing will be taking place in Portland, OR in the middle of October 2014.  If you would like to learn more about these interesting applications in general, and how linear position sensors are used in particular, you might want visit Balluff at the Timber Processing and Energy Expo.  Click the link below for more information.

Timber Processing and Energy Expo, October 15th through October 17th

Ultrasonic Sensors with Analog Output

Many times in an application we need more than a simple discrete on/off output. For a more accurate detection mode we can utilize analog outputs to monitor position, height, fill-levels and part presence typically found in object detection assemblies. When utilizing Ultrasonic sensors with an analog output we can simply measure the distance value that is proportional to the distance of our target within the operating range of the sensor. Typically 0…10V or 4…20mA outputs are available with the option of rising or falling characteristics. Rising and falling is a way to invert the view of the output, so 0…10V would simply be inverted to 10…0V or 4…20mA would be 20…4mA.

Ultrasonic sensor offerings are a great alternative as they can deal with difficult targets that are typically a challenge for other sensor technologies. They also offer very good resolution with the options of long and short range detection. Below is an example of a 4…20mA linear output. As you can see the closer our target gets to the sensor face it indicates an output closer to 4mA and the further away from the sensor it will provide and output closer to 20mA. For more information on Ultrasonic sensors, click here.


We Don’t Make Axes Out of Bronze Anymore

Every technology commonly in use today exists for a reason.   Technologies have life cycles: they are invented out of necessity and are often widely used as the best available solution to a given technical problem.  For example, at one time bronze was the best available metallurgy for making long-lasting tools and weapons, and it quickly replaced copper as the material of choice.  But later on, bronze was itself replaced by iron, steel, and ultimately stainless steel.

When it comes to detecting the presence of an object, such as a moving component on a piece of machinery, the dominant technology used to be electro-mechanical limit switches.  Mechanical & electrical wear and tear under heavy industrial use led to unsatisfactory long-term reliability.  What was needed was a way to switch electrical control signal current without mechanical contact with the target – and without arcing & burning across electrical contacts.

Example of an inductive proximity sensor

Example of an inductive proximity sensor

Enter the invention of the all-electronic inductive proximity sensor.  With no moving parts and solid-state transistorized switching capability, the inductive proximity sensor solved the two major drawbacks of industrial limit switches (mechanical & electrical wear) in a single, rugged device.  The inductive proximity sensor – or “prox” for short – detects the presence of metallic targets by interpreting changes in the high-frequency electro-magnetic field emanating from its face or “active surface”.  The metal of the target disrupts the field; the sensor responds by electronically switching its output ON (target present) or OFF (target not present). The level of switched current is typically in the 200mA DC range, which is enough to trigger a PLC input or operate a small relay.

In my next post, I will explain the do’s and don’ts for applying inductive prox sensors.

Aw, yiss…BiSS!

BiSSThere’s a cool new serial data interface coming on the scene.

It’s called BiSS (Bi-Directional Synchronous Serial), and is an open-source, free-to-license digital interface for sensors and actuators. BiSS is hardware compatible to the industrial standard SSI (Serial Synchronous Interface) but offers additional features and options. Here are a few highlights:

  • Serial Synchronous Data Communication
  • 2 unidirectional lines Clock and Data
    o Cyclic at high-speed (up to 10 MHz with RS422 and 100 MHz with LVDS)
    o Line delay compensation for high-speed data transfer
    o Request processing times for data generation at slaves
    o Safety capable: CRC, Errors, Warnings
    o Bus capability for multiple slaves and devices in a chain
  • Unidirectional
    o BiSS C (unidirectional) protocol: Unidirectional use of BiSS C
  • Bidirectional
    o BiSS C protocol: Continuous mode
  • Actuators
    o Operate actuators via two additional lines

bmlFor more information and FAQs, visit

Click here to learn about absolute, magnetically coded position measurement systems for large measurement sections with BiSS interface.


Optical Window Sensors

Optical window sensors are utilized where reliable part counting is needed. This type of sensor technology is based on an array of LEDs on one side, opposite an array of phototransistors on the other side. This array covers the whole area of the window’s opening with an evenly as possible distribution of light. The more evenly distributed the light is throughout the window, the higher the resolution.

Optical window sensors are usually assigned a particular term to reveal their specific functionality type. The two typical functionality types for an optical window sensor are either static or dynamic. The differences between the two functionality types are briefly outlined here.

Static functionality looks for unchanging events. In the case for an optical window sensor, static means it detects the percentage of signal blocked by an object present in or passing through the window. Dynamic functionality looks for changing events. In the case for an optical window sensor, dynamic means it detects moving objects in the window and ignores non-moving objects. Still, in either case whether static or dynamic, the sensor detects objects as they pass through the window.

A common follow-up question is: what are the pros and cons for using either functionality over the other? This is a good question, because there are definite benefits and disadvantages to both approaches. A few of these benefits and disadvantages are briefly outlined below.

Read more of this post

Asset Tracking – Top 10

The goal of plant-based asset tracking is to reduce non-productive time and asset losses, while increasing overall productivity and utilization by accurately tracking assets. Bar code and RFID technologies track changes to an asset’s location, condition, conformity status, and availability.

Balluff has been in this business for over 25 years. Based on that experience, we have compiled the top 10 list of commonly tracked plant-based assets:

1. Dunnage containers
2. Machine tools
3. Plant-floor Equipment
4. Stamping dies
5. Torque Wrenches
6. Plastic Molds
7. Storage tanks and vessels
8. IT equipment
9. Automated Guided Vehicle (AGV)
10. Modular automation sub-systems

If you are looking to gain tighter control of your assets, visit




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