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Measuring Surface Roughness with an Optical Sensor

A new optical detector designed to measure the surface roughness of objects on the production line operates by comparing the amount of light reflected in the specular direction with that reflected at an angle normal
to the surface of interest.

Walter Bloechle, Hohner Corp.

The surface roughness of an object can be measured either mechanically or optically. Mechanical devices based on the profilometer principle are expensive, can be unreliable in certain applications, and require

Figure 1. A simple, inexpensive optical sensor has been developed to measure the roughness of a surface under production line conditions.

physical contact with the surface of interest. The surface damage that may result can corrupt the measurement data. Noncontact optical techniques eliminate the problems of surface damage and inaccurate data, but they require very precise optical elements that must be realigned continually.

A new optical device (see Figure 1) performs noncontact surface measurements inexpensively and without requiring high-precision mechanical or optical components. Fast data acquisition and the use of a computer allow the sensor to be used in continuous production operations.

Principle of Operation

To understand how the detector works, let us briefly review some basic optics (see Figure 2). Light from a point source is directed toward the surface of interest.

Figure 2.An absolutely smooth surface reflects light from a point source at an angle qr equal to the angle of incidence qi.

If the surface is absolutely smooth, the light (minus some absorbed fraction) will be reflected at an angle equal to the angle of incidence (ur = ui). This is specular reflectance. Reflectance in any other direction is zero.

If the surface is "absolutely rough," the light will scatter, with equal amounts reflected at multiple angles (see Figure 3). The light reflected at angle ur will therefore be equal to that reflected at an angle normal to the surface, 90. (It is important to note that there are no naturally occurring absolutely rough surfaces. Bulk reflecting materials approach the theoretical standard, and milk glass is often used as a model.) The degree of roughness of a real-world surface is therefore a continuum; its value at any given point must be determined by comparing the amount of light reflected at ur to that reflected at the perpendicular or some other angle. The difference between the amount of light reflected in any two directions is inversely proportional to the degree of surface roughness.

The components of the sensor (see Figure 4)

Figure 3. Light from a point source is scattered by a rough surface into multiple reflections of equal intensity. The amount of light reflected at angle qr is equal to that reflected at an angle normal to the surface.

are an LED light source, two photodetectors, a computer, and application software. Light from the point source is reflected from the surface. One photodetector is aimed in the specular direction of reflectance (ur) and the other in a position normal to the surface. The computer compares the signals from the two photodetectors and calculates the degree of roughness.

To obtain an accurate value, the user must calibrate the sensor against samples with known roughness. The function, F, used as the calibration parameter is expressed as:

F= SSN
 (1)
SS + SN

where:

SS = signal from photodetector placed in specular direction

SN = signal from photodetector placed in normal direction

Figure 4. The primary components of the surface roughness sensor are an LED light source (red, in Figure 1), two photodetectors, and a computer. One photodetector senses the amount of light reflected at qr and compares it to that reflected at an angle 90 to the surface of interest. The difference is inversely proportional to the degree of roughness.

Software

The sensor software consists of the USB (universal serial bus) driver for Microsoft Windows. It allows calibration and measurement. The calibration program creates a user data base that the measurement program uses to calculate the degree of roughness.

The software kit also includes a DLL (dynamic link library) unit for the Microsoft Windows operating system that allows users to build their own applications using program development environments such as MS Visual Basic, Delphi, and C++.


 SURFACE SENSOR SPECS
Operating temperature
–20C to 60C (–4F to 140F)
Current consumption
80 mA (max)
Power supply
24 V
Humidity: up to 98% permissible
Capacity: 2 X 10 bits
Test compliance to
 IEC 801 standard

IEC 801-2 electrostatic discharge
IEC 801-3 radiated field req.
IEC 801-4 electrical fast transient
IEC 801-5 surge immunity
IEC 801-6 conducted RF
Mechanical specifications
Weight: 8 oz. (0.2 kg)
Protection: IP 54, NEMA 3
Housing: aluminum
Shock: 10 g (6 ms)
Vibration: 5 g (500 Hz)

Walter Bloechle is President, Hohner Corp., 5536 Regional Rd. #81, Beamsville, Ontario, Canada LOR 1B3; 905-563-4924, fax 905-563-7209, hohner@hohner.com or www.hohner.com

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