New laser sensing technologies economically solve previously impossible applications

Michael Dean, Technical Marketing Manager, Banner Engineering Corp.

 

The recent evolution and application of Class 2 lasers for industrial sensing, measurement and inspection is now having significant impact in factories. These new technologies and self-contained sensors are now able to solve applications previously not possible without large, costly systems, many requiring a separate large controller module. These new sensors include both analog and discrete outputs in the same sensor that can be programmed independently and operated simultaneously. Some models now include a patented scalable analog output that distributes the analog signal evenly over the width of the programmed sensing window to optimize sensing performance.

 

Push-button programming has greatly simplified the application of these technologies. With a single button, users can program a unique sensing window size and position, or a unique set-point threshold centered within a sensing window. This article discusses the principles of operation of two of the latest laser technologies.

 

1. Optical Triangulation

This popular technology uses an emitter to transmit visible laser light through a lens, towards a target. The laser light beam from the emitter bounces off the target, scattering some of its light through another lens to the sensor's PSD (Position Sensitive Device) receiver element. The target's distance from the receiver determines the angle at which the light travels to the receiver element. This angle, in turn, determines where the received light will fall along the PSD receiver element.

 

The position of the light on the PSD receiver element is processed through analog and/or digital electronics to calculate the appropriate output value (or distance to the target). The analog output varies in proportion to the target's position within the user-programmed analog window limits. The discrete (switched) output energizes whenever the target is located between the user-programmed discrete window limits. Analog and discrete window limits may be the same, or programmed independently.

 

Optical triangulation is a relatively short range technology but can offer ranges up to approximately one-half meter (20 inches), however, longer ranges require much larger and more expensive sensors costing several thousand dollars. Resolution is extremely high, oftentimes achieving maximums up to 3 microns (0.0001") for flat white targets. This makes it an excellent choice for precision applications including calibrating robot arms, wafer profiling, measuring diameter or thickness, and assembly dimension inspection. Because these sensors are non-contacting, they can be used with moving processes, hot parts and sticky parts. They also include measuring features that are unavailable on contacting probes, giving them a distinct advantage over probes, such as LVDTs. The technology has a few limitations that are discussed below.

   

Color effects

The color of the object being measured can affect the resolution and accuracy of the readings. White, red, yellow, and orange targets will reflect more light than green, blue, or black targets. The resolution for dark targets may be up to four times worse than for white targets.

 

Target requirements

Triangulation sensors depend on the diffuse reflections of light from the target. A diffuse reflection is one in which the light tends to scatter equally in all directions from the target. If the target surface is mirror-like, then light will tend to reflect in only one direction and the sensors will not work properly. Triangulation sensors also require a non-porous, opaque surface for accurate operation. Measurement errors will result from semi-transparent targets such as clear plastic, or from porous materials such as foam.

 

Metal surfaces

Bare metal surfaces, even though they may be somewhat diffuse, typically do not exhibit consistent reflectivity across their surfaces; consequently, the repeatability from one point on a metal surface to another, even at the same distance from the sensor will degrade. This effect varies from metal to metal and is dependent upon surface finish. The user should bench-test a sample of the metal in order to estimate the expected repeatability for the application.

 

Total expected measurement error

Keep in mind that the overall expected accuracy of an analog sensor is the combination of several performance parameters, not simply the sensor's resolution. For example, consider a laser triangulation sensor measuring the position of a dark colored plastic part, at medium response speed, in an environment that varies +/- 3˚C. The individual errors would be:

 

Resolution      48 µm  (4 x 12 µm, the resolution of a white target)

Linearity         60 µm 

Temp effect    21 µm  ( 7µm/˚C x 3˚C )

 

Since these errors are independent, they may be combined using the Root-Sum-of-Squares (RSS) method as follows:

 

Total expected error = √482 + 602 + 212 = 80 µm

 

  2. Time-of-Flight

Time-of-flight laser sensing technology is an extremely promising new technology that allows very long range yet accurate gauging of distances. A short electrical pulse drives a semiconductor laser diode to emit a pulse of light. The emitted light is collimated through a lens, which produces a very narrow laser beam. The laser beam bounces off the target, scattering some of its light through the sensor's receiving lens to a photodiode, which creates an electrical pulse. The time interval between the two electrical pulses (transmitting and receiving the beam) is used to calculate the distance to the target, using the speed of light as a constant.

 

Multiple pulses are evaluated by the sensor's microprocessor, which calculates the appropriate output value. The analog output provides a variable signal (4 to 20 mA or 0 to 10V dc, depending on model) that is proportional to the target's position within the user-programmed analog window limits. The discrete (switched) output energizes whenever the target is located between the user-programmed discrete window limits. Like laser triangulation, window limits for the analog and discrete outputs may be the same, or they may be programmed independently.

 

Ranges for time-of-flight sensors are typically 0.3 to 3 m (1'-9.8') for gray targets; 0.3 to 5 m (1'-16.4') for white targets. Utilizing a retroreflector, ranges can be extremely long, up to 50 m (164') or more depending on the target. These sensors can pulse up to one million times per second, with the sensor's microprocessor averaging 1,000 pulses every 0.001 second to provide extremely precise gauging data. They provide extraordinary measurement resolution relative to range of 1 mm (.04") depending on response speed programmed—the faster the response, the lower the resolution.

 

Time-of-flight laser sensing technology offers value and flexibility for a variety of applications, including determining position of automated storage/retrieval cars, positioning fixtures, measuring levels in bins, measuring distance to objects and heights of objects on conveyors, measuring diameters of logs, protecting overhead cranes from collision, measuring roll diameters, controlling loop tension, and long-distance sensing of parts in fixtures.  Especially significant, is the ability of these sensors to reliably detect severely angled targets, versus ultrasonic solutions that require the target to be approximately perpendicular to the target.

 

Size, Cost and Performance

Most laser sensors are still large two-piece systems requiring a separate controller and costing several thousand dollars. The latest laser sensing technologies are now in self-contained housings about the size of a pack of cigarettes and available at prices typically below $900. There are also non-laser LED-based triangulation solutions that offer an analog or discrete output for under $300. Users must carefully evaluate their needs, however, they now have numerous excellent new solutions for their more challenging sensing applications.

 

This article is written and provided by Michael Dean, Technical Marketing Manager, Banner Engineering Corporation. Banner Engineering offers a

complete and integrated line of sensing and machine safety products. For more information about Banner or their products please visit their website

at: http://www.bannerengineering.com.