In this column, I will discuss an overview of dimensional and displacement measurement.
What comes to mind when you hear the phrase "measuring dimensions or displacement"? Considering everyday objects, for example, when building shelves or tables as a DIY project, you need to accurately measure the dimensions of the boards you'll be using. Also, when setting up a table, you need to measure beforehand whether there's sufficient distance between the chair (displacement) and the wall. In this way, measuring whether the dimensions (length, thickness, depth, angle, etc.) of various parts of machinery, equipment, and manufactured objects fall within a certain standard range, and whether the distance moved, position, and rotation angle perform as intended, is crucial for ensuring the performance and specifications of machinery and equipment. This is dimension and displacement measurement.
While "dimensional measurement" and "displacement measurement" sound similar, they each have different meanings.
"Dimensional measurement" refers to measuring the absolute values of an object, such as its length and thickness.
"Displacement measurement" refers to measuring relative values such as the amount of movement from a reference position. We will introduce when and what kind of measuring instruments are used, using Ono Sokki 's measuring instruments as an example.
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Types of sensors used for measurement
[Dimensional Measurement]
Ono Sokki There are three types of sensors for dimensional measurement available from
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Contact-type sensors: Linear gauge sensors (BS/GS series)
Measurement examples: Measuring the thickness and height of metal processed products, the depth of holes, etc.
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Contact-type sensors: Rotary encoders (RP/SP series)
Measurement examples: Angle measurement of rotating bodies, and measurement of feed amounts of steel plates or paper using roller encoders.
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Non-contact sensors: Capacitive non-contact thickness gauges (VE/CL series)
Measurement examples: Measurement of the thickness of steel plates, silicon wafers, resin films, etc.
[Displacement Measurement Law]
There are also three types of sensors for displacement measurement.
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Contact-type sensors: Linear gauge sensors (BS/GS series)
Measurement examples: Measurement of deformation of materials due to load, axial runout of rotating bodies, etc.
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Non-contact sensors: Capacitive displacement gauges (VE/VT series)
Measurement examples: Measurement of axial runout of a rotating body, amount of runout during disk rotation, etc.
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Non-contact sensor: Laser length measuring instrument (LV-9000 series)
Measurement examples: Measurements such as performance evaluation and positioning accuracy of machine tools, where resolution and precision of micrometers or less are required.
For information on measuring instruments related to dimensions and displacement, please refer to this page.
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Sensor operating principle and precautions for use
This section explains the operating principles and precautions for using linear gauge sensors and rotary encoders, which are relatively common among users.
Linear gauge sensor
[Operating Principle]
Figure 1 shows the basic structure of a linear gauge sensor. Inside the sensor, a movable slit that moves in conjunction with the spindle and a fixed slit fixed in a fixed position are arranged opposite each other. Brightness and darkness scales are printed on the slits at regular intervals. The fixed slit has two slits, A and B, offset by a 1/4 pitch, which allows for the determination of the spindle's direction of movement (+ and - directions). A light source (LED) and a photodetector are positioned opposite each other, with the slits in between.
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Figure 1 Basic structure of a linear gauge sensor
As the moving slit moves relative to the fixed slit, the light passing through the fixed slit's window alternates between light and dark. At this time, two square wave signals with the same period and a 90° phase difference are output.
The direction is determined from whether the phase is leading or lagging, and the displacement is measured by adding or subtracting using a counter.
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Figure 2. Displacement detection principle of a linear gauge sensor.
[Precautions for Use]
Avoid applying excessive force to the spindle in the lateral direction.
(Impacts during spindle raising and lowering can cause errors or damage.)
The linear gauge sensor unit should be installed in a location that avoids areas with high vibration.
Please position the spindle so that its axis is perpendicular to the surface to be measured (reference surface).
Rotary encoder
[Operating Principle]
Figure 3 shows the basic structure of a rotary encoder. A slit disc (moving slit) with equally spaced grid markings is attached to the rotary shaft of the rotary encoder, and a fixed slit with the same spacing markings is fixed to the main body opposite it (Figure 4). A light-emitting element (light-emitting diode) and two light-receiving elements (phototransistors) are installed on either side of these two slits. As the rotary shaft rotates, the light path is blocked by one slit pitch at a time, causing the light to blink on and off a number of times proportional to the amount of rotation.
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Figure 3 Basic structure of a rotary encoder -
Figure 4 Positional relationship between the slit disk and the fixed slit
The light and dark areas are captured as electrical signals by two light-receiving elements, and the resulting waveform is shaped into a square wave, which becomes the output signal of the encoder. The two output signals are phase-shifted so that they are 90° apart from each other, and the direction of rotation of the axis is determined using the same principle as the linear gauge sensor explained earlier.
[Precautions for Use]
Avoid applying strong impacts or loads exceeding the permissible load to the rotating shaft.
Please install the unit so that the rotating shaft and the coupling are aligned.
It may be affected by noise from the power line. Use filters, use a power line free from noise from other systems, and isolate it from power lines carrying high power during wiring.
When performing dimensional and displacement measurements, the sensor may malfunction depending on the installation conditions.
Please pay particular attention to the sensor's mounting condition.
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Precautions when taking items out of the country
Please note that non-contact thickness gauges (VE/CL series) *1 and laser length measuring instruments (LV-9000 series) are subject to regulations, and an export license issued by the Ministry of Economy, Trade and Industry is required when exporting or taking these products out of the country.
*1 A combination of sensor and display unit that achieves a resolution of 0.2 μm or less.
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The dimensional and displacement measuring instruments we handle are as follows:
Please take a look if you are interested.
GS/BS Series Linear Gauge Sensors
VE/CL Series Non-Contact Thickness Gauge
RP Series Rotary Encoder / General Industrial Use
RP/SP Series Rotary Encoder / Compact, Low Torque Type
VE/VT Series Capacitive Displacement Gauges
LV-9000 Series Laser Length Meter
(Excerpt from the email newsletter issued on December 15, 2021)
