The present invention relates to an incremental encoder and, more particularly, to an incremental encoder having a rotor provided with a plurality of indices and capable of detecting a reference or zero point without turning the rotor one full turn.
Encoders have been widely used for electrically measuring distances and angles. Optical encoders and magnetic encoders are quite analogous in basic configuration.
Rotary encoders have been prevalently used for the electrical measurement of angles. In particular, optical encoders incorporate advanced optical techniques, they are capable of measurement with high accuracy and high resolution, they have high resolving power and are immune to disturbances such as external magnetism, and they have a long lifetime owing to their noncontact measuring operation.
Optical encoders having such exemplary features are used in surveying instruments for measuring angles, for example.
Optical encoders employed in current surveying instruments are classified into those of absolute type, i.e., absolute optical encoders, and those of incremental type, i.e., incremental optical encoders.
In an absolute optical encoder, angle and position on a circumference are in one-to-one correspondence, and positions on a circumference are registered as absolute addresses. Therefore, position information about any position can be obtained. However, an absolute optical encoder has a complex construction, and it is very difficult to construct an absolute optical encoder in a compact, lightweight manner for a surveying instrument.
As shown in FIG. 6, an incremental optical encoder comprises a rotor 9100 provided with a main scale 9110, a stator 9200 provided with a subscale 9210, and a detecting means having components 9310, 9320 and 9330. The main scale 9110 formed on the rotor 9100 has graduation lines arranged at equal angular pitch, e.g. of 80 sec, at the periphery of the rotor 9100 in an occulting pattern. The subscale 9210 formed on the stator 9200 has graduation lines arranged at an angular pitch equal to that of the main scale 9110 in an occulting pattern.
The detecting means comprises an LED (light emitting diode) 9310, a collimator lens 9320 and a photosensor 9330. The LED 9310 and the photosensor 9330 are disposed with the rotor 9100 and the stator 9200 therebetween. When the rotor 9100 rotates, the light emitted by the LED 9310 is interrupted once every time the rotor 9100 turns through an angle corresponding to one graduation of the main scale 9110, and the photosensor 9330 provides an electric signal indicating the interruption of the light. The electric signals provided by the photosensor 9330 are counted to detect the angle through which the rotor 9100 has turned.
The incremental optical encoder is able to start counting the output signals of the photosensor 9330 from any position of the rotor 9100 and to measure the angle of turn of the rotor 9100 from the position where counting the output signals of the photosensor 9330 is started.
A surveying instrument employing an incremental optical encoder will be described with reference to FIGS. 7(a) and 7(b). A surveying instrument 10000 comprises a base unit 8100, a standard unit 8200 rotatably mounted on the base unit 8100 in a horizontal plane, a sighting telescope 8300 supported on the standard unit 8200 for turning in a vertical plane, a first detector 8400 for detecting the horizontal angle of the standard unit 8200, and a second detector 8500 for detecting the elevation angle of the sight line of the telescope 8300.
The base unit 8100 is connected by leveling screws 8160 to a leveling plate 8150 to be fixedly mounted on a tripod or the like. The level of the surveying instrument 10000 can be adjusted by turning the leveling screws 8160. The base unit 8100 is provided with a lower adjusting knob 8120 and a lower clamp knob 8130 to adjust and fix the base unit 8100. The standard unit 8200 is provided with an upper adjusting knob 8220 and an upper clamp knob 8230 to adjust and fix the standard unit 8200. The sighting telescope 8300 is provided with an elevation adjusting knob 8320 and an elevation clamp knob 8330 to adjust the elevation angle in the sighting direction of the sighting telescope 8300 and to fix the sighting telescope 8300 in an adjusted sighting direction.
An optical encoder included in the second detector 8500 detects the elevation angle of the sighting direction of the sighting telescope 8300 with respect to zenith, for example. The optical encoder of the second detector 8500 is provided with an index for zero detection to determine an angle from a reference. The signals are counted using the index as a reference or zero point to measure the angle. An optical encoder included in the first detector 8400 for measuring the horizontal angle of the standard unit 8200 has no particular reference direction and hence does not need any reference point. Therefore, the optical encoder is an ordinary one not provided with any index.
The incremental optical encoder provided with an index of the second detector 8500 will be described with reference to FIG. 8. The incremental optical encoder of the second detector 8500 comprises a rotor 8510, a stator 8520, and an optical detector comprising components 8531-8533 and 8535-8537 disposed with the rotor 8510 and the stator 8520 there between as shown. The rotor 8510 is provided at its periphery with a main scale 8511 having graduation lines formed at equal angular pitch, and a zero detection index 8512. The stator 8520 is provided with a first subscale 8521 for use in combination with the main scale 8511, and a second subscale 8522 for use in combination with the zero detecting index 8512. The optical detector 8530 comprises an index detecting unit and a main scale detecting unit. The index detecting unit comprises a first light emitting device 8531, a first collimator lens 8532 and a first photosensor 8533 and is capable of detecting the zero detecting index 8512 of the rotor 8510. The main scale detecting unit comprises a second light emitting device 8536, a second collimator lens 8535 and a second photosensor 8537. The main scale detecting unit detects light pulses produced by the occulting pattern of the main scale 8511 of the rotor 8510, and the second photosensor 8537 converts the light pulses into corresponding electric signals. The electric signals are counted to determine an angle from a zero detection point.
A method of using the conventional surveying instrument 10000 thus constructed will be described. The leveling plate 8150 of the surveying instrument 10000 is mounted on a tripod, and the leveling plate 8150 is leveled by turning the leveling screws 8160. Then, a main switch is closed and the sighting telescope 8300 is turned one full turn to complete preparations. When the sighting telescope 8300 is turned, the rotor 8510 of the second detector 8500 is turned, and the zero detecting index 8512 sets a zero point to measure an angle from the zero point.
This conventional surveying instrument 10000 needs to turn by one full turn the sighting telescope 8300 provided with the second detector 8500 including the incremental optical encoder provided with the index; that is, the zero detecting index 8512 of the rotor 8510 must be detected by the index detecting unit of the optical detector 8530 by turning the sighting telescope 8300 one full turn. The sighting telescope 8300 needs to be turned by one full turn because the index 8512, in general, is invisible from outside. If the sighting telescope 8300 is turned quickly, the index detecting unit of the optical detector 8530 is unable to detect the zero detecting index 8512 of the rotor 8510 and hence measurement cannot be made. Accordingly, the sighting telescope 8300 must be turned at an appropriate turning speed below a certain turning speed, with the appropriate turning speed unavoidably being dependent on user intuition, which is very burdensome.