Augmented reality technology is developed on the basis of virtual reality technology, so it has the same inheritance and consistency as the hardware of virtual reality system in hardware structure. Like most VR systems, graphics processors are also essential for AR systems. In addition, the AR system also includes human-computer interaction devices such as data gloves, 6D mouse, eye tracker, force feedback device, voice recognition and synthesis system, etc. Each device has a wide variety and performance.
The manner in which the hardware tracking device acquires the tracked target position and direction information is also often applied to the augmented reality system. These hardware tracking devices include electromechanical trackers, electromagnetic trackers, ultrasonic trackers, photoelectric trackers, and inertial trackers, which are implemented in different ways, each with advantages and disadvantages, and have applications in existing augmented reality systems. Example.
The electromechanical tracker is an absolute position sensor. It is usually composed of a small mechanical arm, one end is fixed on one reference frame, and the other end is fixed on the object to be tested. A potentiometer or an optical encoder is used as a joint sensor to measure the rotation angle at the joint, and then a dynamic calculation is performed according to the measured relative rotation angle and the arm length connecting the two sensors to obtain a six-degree-of-freedom azimuth output. This tracker is more reliable, with fewer potential sources of interference and shorter latency. However, the disadvantage is that the tracking accuracy of the tracker is affected by changes in the ambient temperature, the resolution of the joint sensor is low, and the operating range of the tracker is limited. In some specific applications (such as surgical training), this tracker has an advantage when the user's range of activity is not an important indicator.
2. Electromagnetic trackerElectromagnetic tracker is a widely used type of azimuth tracker. It uses a three-axis coil to emit low-frequency magnetic field, and uses a three-axis magnetic receiver fixed on the object to be measured as the change information of the sensor induced magnetic field, using the transmitting magnetic field and induction. The fused relationship between the signals determines the spatial orientation of the object being tracked. According to the form of the three-axis excitation source, the electromagnetic tracker is divided into an AC electromagnetic tracker and a DC electromagnetic tracker.
The excitation source of the AC electromagnetic tracker is composed of a bipolar magnetic source generated by alternating currents of three magnetic field directions, and the magnetic receiver is composed of three sets of coils respectively testing three excitation sources. The magnetic receiver senses the magnetic field information of the excitation source and calculates the spatial orientation of the magnetic receiver relative to the excitation source based on the electromagnetic energy transfer relationship from the excitation source to the magnetic receiver. The operating frequency of the excitation source is usually 30-120 Hz due to factors such as calculation performance, reaction time and noise. In order to ensure the signal-to-noise ratio under different environmental conditions, the excitation wave is usually modulated by using a carrier of 7-14 kHz. The transmitter of the DC electromagnetic tracker (corresponding to the excitation source) consists of three sets of coils wound orthogonally around the cube core, and sequentially inputs DC current to the transmitter coil, so that each group of transmitter coils respectively generates a pulse-modulated DC. Electromagnetic field. The receiver is also a periodic variation of the direction of the DC magnetic field formed by three sets of independent coils wound orthogonally around the cube core. An alternating current is generated in the three-way receiver coil, the current intensity being proportional to the resolvable component of the local DC magnetic field. Nine data can be obtained in each measurement cycle, which represent the magnitude of the magnetic field induced by the three sets of receiver coils. The electronic unit performs a certain algorithm to determine the position and direction of the receiver relative to the transmitter.
The receiver of the AC electromagnetic tracking system is usually small and suitable for mounting on a head-mounted display, but the most deadly drawback of this type of tracker is its vulnerability to environmental electromagnetic interference. The alternating magnetic field generated by the emitter is very sensitive to nearby electron conductors, especially ferromagnetic materials. The alternating rotating magnetic field generates eddy currents in the ferromagnetic material, thereby generating a secondary alternating magnetic field, which causes the magnetic field pattern generated by the alternating current excitation source to be distorted. Distortion can cause serious measurement errors.
The biggest advantage of DC electromagnetic trackers is that eddy currents are generated only at the beginning of the measurement cycle, and once the magnetic field reaches a steady state, no eddy currents are generated. As long as the eddy current attenuation is awaited before the measurement, the eddy current effect can be avoided, so that the measurement error generated by the distortion eddy current field can be reduced.
3. Ultrasonic trackerThe time difference or the sound pressure difference of the sound of different sound sources to a specific location can be used for positioning and tracking. Generally, there are two methods: pulse wave time-of-flight (TOF) measurement method and continuous wave phase coherence measurement method. Ways. TOF measurement is a method of determining the propagation distance by measuring the propagation time of sound waves from the transmitter to the receiver under specific temperature conditions. Most ultrasonic trackers use this measurement method. The data refresh rate of this method is limited by several factors. The transmission speed of sound waves is about 340m/s, and effective measurement data can be obtained only when the wavefront of the transmitted wave reaches the sensor. Moreover, the transmitter must be allowed to emit a few milliseconds of acoustic pulse after the pulsation is generated, and wait for the emission pulse to disappear before the new measurement begins. Since each transmitter-sensor group requires a separate pulse flight sequence, the time required for the measurement is equal to the single flight time multiplied by the number of combinations. The accuracy of such a time-of-flight measurement system depends on the ability to detect the exact moment at which the transmitted sound wave reaches the receiver. Sounds such as key squeaks in the environment can affect measurement accuracy, and air flow and sensor lock-up can also cause measurement errors.
The continuous wave phase coherence measurement determines the distance between the source and the receiver by comparing the phase between the reference signal and the received transmitted signal. The method has high measurement accuracy and high data refresh frequency, and can overcome the influence of environmental interference through multiple filtering without affecting the accuracy and time response characteristics of the system.
Compared with electromagnetic trackers, the biggest advantage of ultrasonic trackers is that they are not affected by external magnetic fields and ferromagnetic substances, and the measurement range is large. The tracker based on the sonic time-of-flight method is susceptible to interference from pseudo-sound pulses and has good accuracy and time response characteristics in a small working range. However, as the distance of action increases, the data refresh frequency and accuracy of such trackers decrease. The tracker based on continuous wave coherence measurement has a higher data refresh frequency, which is beneficial to improve the accuracy, responsiveness, measurement range and robustness of the system, and is not susceptible to interference by pseudo-pulses. However, both types of trackers can cause errors due to air flow or sensor blocking. However, if appropriate modulation measures are taken, the environmental characteristics of the continuous wave phase measurement method can be improved, and an acoustic tracker with high precision, high data refresh rate and low delay is expected.
In 1966, Roberts of the MIT Lincoln Laboratory in the United States developed an ultrasonic displacement tracker, LincolnWand. The eye tracker is based on sonic time-of-flight measurement method, using four transmitters and one receiver. The tracking accuracy and resolution are only 5mmoLogitech. Another TOF-based ultrasound tracking system, also known as RedBaron, has been developed with eye tracking accuracy and resolution of only a few millimeters.
4. Photoelectric trackerA photoelectric eye tracker (also known as a visual eye tracker) calculates the orientation of a tracked object by using ambient light or controlling the light emitted by the light source at different times or at different positions on the image projection plane. In the case of a controlled light source, infrared light is typically used to avoid interference from the tracker to the user.
From a structural point of view, the photoelectric tracker is divided into two types: "outside-in" (OI) and "inside-out" (inside-out, 10). For the "outer-inner" approach, the sensor is fixed and the transmitter is mounted on the object being tracked, which means that the sensor "sees inward" the target of distant motion, which requires extremely expensive high resolution sensors. For the "inside-out" approach, the transmitter is fixed and the sensor is mounted on the moving object, which means that the sensor "looks outward" from the moving target. Using multiple emitters within the working range improves accuracy and extends the working range.
The internal-one external photoelectric tracker has good time response characteristics, and has potential advantages such as high data refresh frequency, wide application range, and small phase lag, which is more suitable for real-time applications. However, the optical system has potential error factors such as false light, surface blur or light occlusion. In order to obtain a sufficient working range, a short-focus lens is used, and the system measurement accuracy is lowered. Multi-transmitter architecture is a solution at the expense of complexity and cost. Therefore, the photoelectric tracker must make a compromise between factors such as accuracy, measurement range, and price, and must ensure that the optical path is not blocked.
5. Inertial trackerThe inertial tracker uses the gyro's direction tracking capability to measure the angular change of the three rotational degrees of freedom; the accelerometer is used to measure the displacement of the three translational degrees of freedom. Previously, such azimuth tracking methods have been used in navigation devices for aircraft such as aircraft and missiles, which are relatively cumbersome. With the miniaturization of gyros and accelerometers, this tracking method is increasingly favored in the civilian market. The need for the emission source is the biggest advantage of the inertial tracker. However, the traditional gyro technology is difficult to meet the measurement accuracy requirements. The measurement error is easy to produce angular drift with time, and the temperature-dependent drift is also more obvious. The new piezoelectric solid-state gyroscope has greatly improved in the above performance.
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