Detection based on “Contact Measurement”
Principle and types of contact positioning sensors
Principle and major types
Contact Positioning Sensors are generally divided into the following two groups depending on the detection method:
- ・“Differential transformer” method ⇒ Method using a magnetic coil
- ・“Scale” method ⇒ Method using an internal scale (ruler).
In addition, there is another method originally developed by KEYENCE:
- ・“Scale Shot System”
⇒ World’s first method in which a CMOS sensor records a unique pattern on an absolute value glass scale.
Differential transformer method
The sensor using the differential transformer method has an internal coil that generates a magnetic field when a current flows. When a core is inserted into it, the impedance of the coil changes according to the distance the core is inserted, resulting in the change in the signal level. The sensor detects this change in signal level and converts it into travel range.
Advantage
- ・The “absolute position” can be obtained based on the signal level that changes in accordance with the spindle position.
(No zero point adjustment necessary, no tracking errors)
Disadvantage
- ・The accuracy decreases at the end of the spindle. Since the system is coil-based, the magnetic field is applied evenly around the centre but is not consistent near the end.
- ・Consideration should be given to the linearity and temperature characteristic, as they can alter results.
Scale method
Advantage
- ・High accuracy (The accuracy basically depends on the resolution of the scale markings.)
- ・No need to consider the linearity because the scale markings are consistent along the entire scale.
- ・The temperature characteristics are good because the scale markings do not change with varying temperature.
Disadvantage
- ・When the spindle moves abruptly due to vibration or other reason, the response of the photoelectric sensor may delay, resulting in tracking errors.
Scale Shot System (KEYENCE original principle)
Like typical scale method sensors, KEYENCE’s GT2 Series includes a transmitter, a receiver, and a scale within the unit. However, its slits are not simple ones used by typical scale method sensors. The scale of the GT2 Series uses slits with complex and unique patterns. The sensor can read this pattern to identify the spindle position.
- (1) When the spindle moves, the absolute value scale moves accordingly.
- (2) The CMOS sensor reads the complex pattern in the scale at high speed.
- (3) The spindle position information is sent to the amplifier.
Advantage
- ・Absolute position detection
- ・Since the sensor detects position information, it does not require zero point adjustment and not produce tracking errors.
- ・The scale method ensures high accuracy in the entire measuring range.
- ・Good temperature characteristics
Disadvantage
- ・Nothing in particular
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Detection based on “Contact Measurement”
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