What Is a Length Measuring Machine?
A length-measuring machine is, as the name implies, a device for measuring length.
Currently, length is defined by the distance light travels in unit time, based on the speed of light. There are two methods of measuring length: direct and indirect.
- Direct Method
This is a method of measuring length by comparing it to a standard length or scale using a commonly used ruler, tape measure, caliper, micrometer, etc. - Indirect Method
This is a method of measuring length by using other physical quantities related to length or by using electrical or optical methods.
In most cases, length can be measured by the direct method, but in the case of long structures or objects on the order of microns, the indirect method is used because it is difficult to prepare a standard length (scale). The indirect method is also used when the object is complicated in shape, inaccessible, or not allowed to be touched.
Uses of Length Measuring Machines
Length-measuring machines are used in a variety of fields, and the best one should be selected for each application. The types are as follows:
- A few millimeters to several tens of millimeters and small enough to fit in the palm of your hand or on a tabletop: e.g., rulers and calipers
- Several hundred millimeters to several meters long and rather large: e.g., tape measures, etc.
- Micrometers for microscopic observation of the finished product with an accuracy on the order of microns
- Out in the field, several meters to several tens of meters: e.g., optical methods (triangulation, laser length meter) measuring fine irregularities in precision industrial products such as lenses or semiconductor wafers (laser interferometry)
- In technologies such as X-ray CT: applied for measurements inside objects that are inaccessible by light or stylus
- In the nanotechnology industry: which requires measurements at the nanometer level, scanning electron microscopy is applied in this way
As a handy application, length measurement methods based on image analysis are also being developed, such as the recent development of an application for measuring length from a smartphone camera.
Principle of Length Measuring Machines
The meter is defined as the length that light travels in a vacuum in 1/299,792,458 of a second. The metric prototype based on this is the standard for length. In principle, the direct method is a comparison to this metric standard.
A measurement principle based on the definition of length is to measure the time of flight (ToF) of light. Since light is very fast, sophisticated electronic technology is required. Many laser-type instruments now commonly use a measurement method based on the phase difference between the intensity-modulated incident light and the reflected light.
By definition, this is the behavior of light in a vacuum, so in practice, it must be corrected for the refractive index of air. Laser interferometry uses the phenomenon of interference between laser beams.
By analyzing the interference fringes produced when the reflected light from the reference surface interferes with the reflected light from the measurement surface for the same laser irradiation, the distance of the measurement surface from the reference surface can be measured on the nm order. We have exemplified several length measuring instruments, but there are numerous other methods.
Other Information on Length Measuring Machines
1. How to Use Length Measuring Machines
The horizontal model employed by many length measuring machines consists of a bed, a reciprocating table with a built-in standard scale that moves on the bed, a measuring microscope that observes the standard scale, a measuring surface on which the specimen is placed under a constant measuring force, and a measuring table that supports the specimen to be measured. There are two types of horizontal length measuring instruments: one satisfies Abbe’s principle and the other satisfies Eppenstein’s principle.
In horizontal length measuring machines with a structure that satisfies Abbe’s principle, measurement is performed by placing the measurement axis of the specimen and the scale face of the standard scale on the same straight line so that measurement error due to angular deviation from the measurement axis line of the reciprocating table based on non-straightness of the bed is negligible.
On the other hand, in horizontal length measuring machines having a structure that satisfies the Eppenstein principle, to eliminate measurement errors due to the non-straightness of the bed, measurement is performed by configuring the machine so that the focal length of the objective lens for the standard scale is equal to the distance between the measurement axis of the specimen and the standard scale when they are separated, and by optically placing the focal plane of the lens on the standard scale. Measurement is performed by optically placing the focal plane of the lens on the standard scale.
2. Laser Length Measuring Machines
Laser-length measuring machines irradiate a laser beam onto an object and use the reflected light to measure the distance of the object. Laser length measuring machines are called “displacement sensors” or “distance sensors” depending on the distance to be measured.
- Displacement Sensors
Displacement sensors are length-measuring machines that measure short distances (tens to hundreds of millimeters) in microns. - Distance Sensors
These are length-measuring machines that measure long distances (several millimeters to several meters) in millimeters.
The two known measurement methods for length-measuring machines are the triangulation method and the time of flight (ToF) method.
Triangulation Method
This is a measurement method that uses the principle of triangulation based on reflected light and consists of a light emitter and a light receiver. A semiconductor laser is used as the light emitter. In the measurement method, a laser beam focused from the semiconductor laser through a projection lens is irradiated onto the specimen. A portion of the diffuse reflection of the laser light irradiated on the specimen forms a spot image on the photosensor through the light-receiving lens. By detecting and calculating the position of the imaged spot, the amount of displacement from the specimen can be measured.
A system that uses a CMOS (Complementary Metal Oxide Semiconductor) light-receiving element is called a CMOS system, while a system that uses a CCD (Charge Coupled Device) light-receiving element is called a CCD system.
Time of Flight (ToF)
This method measures the distance by measuring the time it takes for the irradiated light to reflect off the specimen and be received by the light-receiving part. There are two methods: the phase difference distance method, which uses the phase difference between the wavelength of the emitted light and the wavelength of the received light, and the pulse propagation method, which emits a laser beam with a fixed pulse width.