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:

Timing is everything – Which light is the right light?

Shortly after posting my last blog, Which light is the right light, I had a customer call with a problem in a machining cell.  They are using a self-contained through-beam sensor, in the form of a fork sensor, with a red light source. They required a small light spot to detect a tool.  As in most machining centers, there is a lot of coolant flying around in the cell and a fine mist in the air.  When water based coolants dry, they separate and leave a white film on surfaces, including photoelectric lenses.  This customer had to shut down their cell and clean off the lens at least once per shift, which was costing them production, time, and money because of false signals.

As we spoke on the phone, I suggested that they use the infrared version because we can burn through the contamination in the environment, in this case the film left behind from the coolant.  The customer wanted some sort of idea of how much residue we could burn through so I did some simple testing and sent him the following pictures.  Picture 1 is a heavy dusting, of all things, coffee creamer.  Picture 2 is a nice dollop of grease from a grease gun and picture 3 is a film of hand cream.

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Back to the Basics – Which light is the right light?

The emitters in photoelectric sensors give off a light that is received by a separate receiver, reflected back to a receiver by a reflector, or reflected back by the object itself. Back in the good ‘ol days, the light source was incandescent, however they ran hot and tended to have a short life. Now solid state devices, LED’s, are used because they use less energy, they can be pulsed very rapidly and you can use different colors for special applications.

Typically we refer to light sources in photoelectrics as red light, infrared, and laser. All have their advantages and disadvantages, and picking the wrong light source, can either make your application successful, or let’s say less desirable than you had hoped.

Red light photos are probably the most favored because they are easy to set-up, and confirmation that the sensor is working properly is easy since you have a bright light that you can focus on your target. However, it is important that you aim the sensor correctly if you have the sensor installed near an operator as the light can be rather annoying if it is in their eyes.

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Sensor Based Error Proofing – As easy as 1, 2, 3

Error proofing your manufacturing processes can be as easy as 1, 2, 3. You should be able to freely deploy error proofing in all appropriate locations in your plants without concerns regarding costs and long-term support or stability. It all starts by first identifying your trouble spots, then implementing a detection method, and finally establishing a process to handle the discrepancy. Let’s discuss the detection methods using sensors, as well as the process, for handling discrepancies.

By utilizing sensors as opposed to vision systems or other passive approaches, the cost of implementation and maintenance is reduced. With the new generation of low-cost lasers, sensors are now more affordable and easier to implement.  Radio Frequency Identification (RFID) brings new opportunities for handling non-conforming products. By tagging the individual part, assembly, or lot, products can be directed to the appropriate rework or scrap area.

These methods will allow you to implement more error proofing in your manufacturing lines to save thousand of dollars in scrap or rework and avoid the potential for costly containment.

Top 5 questions regarding error proofing…

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