Utilizing Holographic Scanning Technology for Noncontact Dimensional Measurements

Introduction

As global competition continues to intensify, the need to increase productivity and reduce waste is critical to the survival of today's manufacturer. Being able to better measure and control the dimensional characteristics of your product is one way to provide the edge necessary to achieve that goal.

 

This paper describes a unique highly accurate, noncontact technology, developed by TSI Incorporated, to measure product dimensions in many product applications. Details of the technology and its advantages for measuring diameter, lumps, neck downs, shape, profile and more will be discussed.

 

Dimensional Measurements

The intent of dimensional measurements is to assure with the highest degree of confidence the stated form, fit and function of the product is indeed correct. Accurate dimensional measurement is important for many different reasons including:

 

· To assure the product will dimensionally conform to all other parts or processes where the product is used.

· To provide means of reducing the amount material needed for the product and yet have the product still function properly.

· To provide a means of monitoring process parameters for process capability and process control.

 

History of Optical Dimensional Measurement Gauges

Optical Dimensional Measurement gauges have been used widely in industrial applications for well over 30 years These systems have been accepted as industry standards.

 

Analog Diameter Measurement Systems:

Early optical gauges used for dimensional measurement of extruded products such as wire and cable was based on an analog design. This system consisted of a light source, a collimating lens, a collecting lens and a single point photo-detector.

Analog Based System

The collimating lens and light source are used to generate a bright sheet of light across plane in space. The collection lens focuses that sheet of light on to a detector. The intensity of light is measured by the detector. If there is nothing in the plane of light, the intensity is quite high. However, when a product enters the light sheet, the intensity of light seen by the detector decreases proportional to the size of the product. The larger the product, the more light that is blocked and the lower the light intensity on the detector. Therefore, one can correlate the dimension of the product to the amount of light the detector sees.

 

Pros and Cons:

The advantage of this technology is the speed at which a change in dimension can be measured. The limiting factor is the detector speed, which is usually in the 10Khz range or higher. Therefore, if a sudden change in dimension occurs, say a lump or neck down, this type of detector will see it. There are two disadvantages to this system design. First is the sensitivity of the system to measure small changes in product dimension. Small changes will appear as electrical noise and will be ignored by the detector. Second is the system constantly need calibration. With time, the light source will decrease in intensity and the system needs to be adjusted accordingly.

 

Linear Diode Array Systems

The next generation gauging systems were designed around using a linear diode array as the detector rather than a single point detector.

Diode Array based System

As with the analog system, this design uses a light source and collimating lens to generate a bright sheet of light in space. What is different is instead of focusing the light sheet on to a diode, the light sheet is imaged on to an array of diodes. The diode array actually captures an electronic image of the light sheet. As the product enters the sheet, the shadow created by the product is captured by the array. The larger the shadow the larger the product.

 

Pros and Cons:

The advantage of this technology is it is now a digital device. Each individual diode in the array is either electrically off or on, depending on the image of the product. Therefore, the system is less susceptible to electronic noise. It can detect smaller changes in product dimension. There are several disadvantages. First, although the measurement resolution is much better then analog devices, it is still limited by the number of diode in the array. Second, the speed is limited by how fast the array can be read. This is typically less then 1000 times a second. This limits the ability to measure quick changes in dimension.

 

Scanning Laser Systems

About 20 years ago a new technology was developed that revolutionized optical dimensional measurement techniques. This system uses a scanning laser beam to "image" the product rather then a continuous light sheet.

Mirror Based Laser Scanning System

 

The laser scanning system scans a laser beam through a collimating lens past the product being measured, through a receiving lens on to a photo detector. When the beam enters the measurement region, the detector will see a sudden increase in light. As the beam clips the first edge of the product, the intensity dramatically decreases. As the beam exits the second edge of the product, the intensity increases again. Finally, as the beam exits the measurement region, the intensity drops again.

 

Pros and Cons:

The advantage of this technology is accuracy. The signal from the detector can now be broken into many small segments separated by time. Time segments can be very small, therefore, the edge of the product can be measured very accurately.

 

There are several disadvantages to this technique. First, the measurement rate is very low. There are typically 6 to 8 sides to the rotating mirror and even with a 6000 rpm motor, that limits us to 600 to 800 scans per second. Second, the multi faceted mirror is a machined part and each face is different. Therefore, each scan has a slightly different scan rate. This effects the timing of each scan and so it effects the accuracy of every scan. Typically, most systems will average many scans together in order to get a good average measurement. This results in a lower measurement rate. It also eliminates the ability to look for real peak dimensional such as fast moving flaws and tapered slopes. Finally, as the motor starts to wear out, the mirror will begin to wobble. This wobble will change the timing of the scan and therefore change the measurement. Scanning systems require periodic calibration to compensate for motor wear.

 

Using Holographic Technology to Scan the Laser Beam

Target Systems, now TSI Incorporated, introduced a system that utilizes a holographic technique to measure product dimension very fast and accurately. This system is called the Holix. The Holix offers all of the advantages of a scanning laser system while also providing many features available only with faster analog gauges.

 

How the Holix Works

Holix Gauge Model XY5007HP

 

The heart of the Holix is a patented (Patent 4,329,326) holographic disc. The holographic disc consists of many holographic optical elements (HOE). Each of the HOE elements acts as a refraction lens. The laser beam is projected through the disc and the HOE. The first order refracted beam exits the HOE at an angle of roughly 22° off axis. As the disk rotates, the angle of the gratings in the scan HOE rotate, causing the beam to scan from side to side.

Scanning Characteristics of the Holix Patented Disc

 

The basic concept behind the Holix is similar to the scanning laser technology. Using a laser diode, a bright laser beam is projected through the holographic disc causing the beam to scan.

 

The scanned beam is transmitted through a collimating lens, which generates a plane in which the beam passes. A collection lens picks up the laser beam plane and focuses the beam on to a detector.

Diagram of Holix Dimensional Gauge

 

The light picked up by the photo-detector is converted to an electrical signal and digitized. The digitized signal is analyzed using a complex edge detection algorithm and the exact position of the edge of the product is found for each and every scan.

 

Advantages over Mirror Based Scanning Systems

There are several important advantages to the Holix Technology. They are listed below.

 

An Accurate Measurement with Every Scan

There are a couple reasons why the Holix can provide an accurate measurement with every scan. First, the holographic disc used in the Holix is reproduced photographically using hologram technology. This means every HOE on every disc is nearly identical. At least as identical as is photographically possible. Therefore, unlike machine produced mirrors, every scan from the Holix is repeatable. Since the scans are repeatable, there is no need to average several scans to get an accurate measurement. Therefore, every scan generates an accurate measurement.

 

Second, the signal for each axis of the Holix is digitized and analyzed by individual high speed Digital Signal Processors. The high speed DSP allows each HOE on the disc to have its own calibration, eliminating any small variations that even the holographic technique could not over come.

 

A very high scan and measurement rate:

The Holix disc can be manufactured with small holographic elements allowing for up to 23 elements on a disk. This far exceeds the six to eight faces on a rotating mirror. Therefore, almost three times as many scans are generated for every revolution of the disc. In real terms, if we have a 7,500 RPM motor speed and we have a 23-element disc, we can measure the product almost 3,000 times a second for each axis. Most scanning laser system measure at a rate of roughly 200 times a second.

 

The Holix never requires calibration:

The primary reason that the accuracy of a scanning laser system would change is if the scan rate of the laser were to change. The main cause for the scan rate to change is due to wobble from the motor or motor speed changing. The Holix technology eliminates both of these problems.

 

First, the high speed DSP monitors the rotational speed of the disc each and every revolution. This is done using a small space between two of the elements as a marker pulse. The speed of the motor is then used as part of the edge detection algorithm. If the motor slows down, the DSP can compensate for it.

 

Second, the holographic element is insensitive to tilt. The beam passing through the element will only be effected by the rotation of the element, not the angle at which it enters the element. Therefore, any small amount of tilt caused by wear on the motor will be ignored. The amount of beam deflection with a mirrored system is one to one based on the angle change of the mirror face.

 

Profiling Tests

Because of the limited scan rate, it is difficult for conventional scanning systems to provide an accurate profile of a product. The following test demonstrates how the Holix has the ability to measure the diameter or profile of products at a higher scan rate, therefore providing more information about the product. To perform this test, we decided to profile a center-less ground wire used in Medical procedures as a cathider guide wire. Medical guide wire has a unique shape with very precise characteristics such as accurate tapers and very small lumps in the wire.

In order to do this test, we asked for the assistance of a company called Advance Control Electromechanical (ACE). ACE manufactures a special traversing system, which can pull the guide wire through the opening of the scanning gauge and reads the diameter from each and every scan. The diameter information is then plotted in a graph of diameter vs. length.

 

A special design guide wire was used for this test. This guide wire had a diameter taper from 0.034" (0.86 mm) to 0.016" (0.41 mm). It also has two small bumps located on the taper and a special shape to the end of the wire. The bumps on the taper are useful because they represent a sudden change in the diameter of a product, such as a flaw in an extrusion process.

 

The guide wire was pulled through the gauge at a constant speed and the scan rate of the gauge was changed for each test. In the first test, we used a scan rate of 100 scans per second. The results are shown in the next plot.

100 Scans per Second

 

Notice how the taper of the wire is at best an approximation the real shape of the wire. Also, notice how the bumps in the wire were partially missed by the scans and how the shape of the end of the wire is indistinguishable.

 

The next test used a scan rate of 666 per second, the speed of the fastest laser-scanning gauge available on the market today. Again, the profile was plotted.

666 Scans per Second

Enlargement of the Bump Profile at
666 Scans per Second

 

This scan rate was better, but still the actual size and shape of the bumps could not be determined. In addition, the shape of the end of the wire was not very clear.

 

Finally, we measured the profile of the wire using a Holix gauge with 2833 scans and measurements per second.

28 33 Scans per Second

Enlargement of the Bump Profile at
2833 Scans per Second

 

At this scan rate, the profile of the product is perfect. The taper of the wire is smooth and precise. The size and shape of the bumps is clear. The shape of the end of the wire can be determined.

 

Conclusion

The Holix has shown the ability to measure the dimensional characteristic features of a product at a much higher rate then conventional diameter gauges. The higher the measurement rate, the more information about the dimension of the product and more information, the better measurement. The Holix has the ability to not only provide accurate diameter measurements, but in any continuously changing process, it has the ability to see more information about the product dimensions. In many cases, the Holix can also be used to provide flaw detection as well as diameter information.