1. Field of the Invention
The present invention relates to a distance measuring apparatus and, more particularly, to a distance measuring apparatus for compensating for an error of the distance measured by a triangulation measuring method in accordance with a TOF method.
2. Discussion of the Related Art
A sensor for measuring the distance includes an infrared sensor using infrared rays, an ultrasonic sensor using ultrasonic waves, and a TOF sensor. The infrared sensor may receive focusing light of infrared rays, which is radiated by a light source and then reflected by a surface of the object to be measured, in accordance with a triangulation measuring principle and may measure the distance using a position sensitive detector (PSD) capable of calculating a light-receiving point as an output current. The ultrasonic sensor may measure the distance from the object to be measured by measuring the time that is taken for an ultrasonic wave pulse generated by a sensor to be reflected by a surface of the object to be measured and to then return back to the sensor.
A TOF sensor includes a light source, such as an LED for emitting an infrared pulse of a very short width, and a sensor for detecting light reflected by the object. The distance from the object may be calculated using an equation d=c*tTOF/2 (d is the distance from the object, c is speed of light, and tTOF is the time taken for light radiated by the light source to be reflected by the object and to then return back to the sensor) by measuring the time that is taken for light radiated by the light source to be reflected by the object and to then return back to the sensor. Since speed of light is too fast and thus it is difficult to measure the time tTOF, the distance is indirectly calculated by modulating and radiating light through the light source and using two or more phases.
FIG. 1 shows the principle in which the distance is measured in accordance with a TOF method.
When a light source radiates light in a pulse form of a specific width T0, light reflected by the object reaches a sensor after a lapse of a specific time Td. The sensor detects the reflected light in synchronization with the pulse emitted by the light source (phase 1) and also detects the reflected light with a phase difference of 180 degrees from the pulse emitted by the light source (phase 2). The sensor may calculate the distance from the object based on the amount of light Q1 detected in synchronization with the output light and the amount of light Q2 having the phase difference of 180 degrees from the output light.
A cell forming the sensor may include two switches V1 and V2, two capacitors C1 and C2, and a diode D for generating charges in response to reflected light. The switches S1 and S2 operate in response to the phase 1 and the phase 2, respectively, and alternately connect the diode D for generating charges in response to reflected light to the capacitors C1 and C2. Charges generated from the diode D are stored in the capacitors C1 and C2 as the amounts of charges Q1 and Q2. Accordingly, the voltages V1 and V2 of the capacitors C1 and C2 are proportional to the amounts of charges Q1 and Q2 accumulated in the capacitors C1 and C2. In this case, the distance from the object may be calculated as a value proportional to(1/2)*c*T0*V2/(V1+V2)
A distance measuring unit for measuring the distance using the TOF method may be problematic in that a cell forming a sensor is saturated when measuring a short distance and may be problematic in that the amount of light is insufficient when measuring a long distance.
There is an attempt to solve the cell saturation problem in a short distance in measuring the distance using the TOF method by adopting a triangulation measuring method for calculating the distance from the object using a sensor including a plurality of cells in which TOF distance measuring units are aligned in one direction.
In such a triangulation measuring method, however, after a calibration operation is initially performed on the relationship between the distance and the cell, a distance characteristic of the sensor may be changed due to the lapse of time, an external impact, and a change of an environment. Although there is an error in a measured distance, it is difficult to check the error, and there is no method for compensating for the error. In order to solve an error problem, a distance meter must be repaired by an expert.