1. Field of the Invention
The present invention relates to the field of multi-sensor tracking devices, and more particularly to a method of calculating relative phases between channels of a multi-sensor tracking device.
2. Description of the Relevant Art
Multi-sensor tracking devices are well known in the art and are used to orient moving components of a system employing the device with respect to a known position. Multi-sensor tracking devices have application in, for example, virtual reality systems. More particularly, tracking devices may be employed in a headset of a virtual reality system to generate orientation signals relating an instant position of the tracking device, and thus the headset, relative to a predetermined point in space.
FIG. 1 is a block diagram of a typical multi-sensor tracking device 12 used to track angular motion. Tracking device 12 shown in FIG. 1 includes three signal channels 14A-14C, respectively, coupled to a digital signal processor 16. Channels 14A-14C include motion sensors 20A-20C, respectively, analog signal processing circuits 22A-22C, respectively, and analog to digital converters 24A-24C, respectively.
Although not shown in FIG. 1, each sensor 20A-20C is physically mounted within tracking device 12 such that a sensitive axis thereof substantially aligns with one of the orthogonal axes of the tracking device 12. More particularly, sensitive axis of sensor 20A is substantially aligned with the azimuth axis, sensitive axis of sensor 20B is substantially aligned with the elevation axis, and sensitive axis of sensor 20C is substantially aligned with the roll axis.
In theory, each sensor is sensitive to motion with respect to a single axis of the tracking device 12, but insensitive to motion with respect to the remaining axes. Thus, if tracking device 12 shown in FIG. 1 is subjected to simultaneous movement with respect to all three of its orthogonal axes, each motion sensor simultaneously generates an analog signal the magnitude of which is linear to the component of movement with respect to its corresponding axis. These signals may be proportional to the angular position, angular velocity, or angular acceleration about the given axis
Analog signals generated by motion sensors 20A-20C are provided as inputs to analog signal processing circuits 22A-22C, respectively. Analog signal processing circuits 22A-22C provide one or more functions. In particular, analog signal processing circuits 22A-22C may operate to amplify the sensor analog signal inputted thereto. Additionally, analog signal processing circuits 22A-22C may operate to filter random pattern or fixed pattern noise components of the analog sensor signal inputted thereto. The processed analog signals are then provided to analog to digital converters 24A-24C for conversion. Digital signal processor 16 coordinates function of the analog to digital converters 24A-24C. More particularly, digital signal processor 16 generates a sample signal which is simultaneously received by each of the analog to digital circuits 24A-24C. In response thereto, internal sample and hold circuits within the analog to digital converter circuits 24A-24C, sample and hold the analog signals outputted by analog signal processing circuits 22A-22C. The held sampled analog signals are subsequently transformed into digital format by analog to digital converters 24A-24C and forwarded to digital signal processor 16. At this point the relative phases of the three channels are locked, and remain precisely defined throughout the processing within the digital signal processor.
Digital signal processor 16 operates in accordance with well known algorithms to generate an orientation signal as a function of the three digital signals outputted by the channels 14A-14C in general, and the analog to digital converters 24A-24C in particular. For example, digital signal processor may accumulate digital channel signals representing angular velocity in order to generate orientation signals relating current position of the tracking device 12 to a known start point.
It is important to note that the algorithms employed within digital signal processor 16 may operate to combine data from the three separate channels with the presumption that the channel signals are synchronous. However, this is not always the case. Manufacturing tolerances dictate that corresponding elements within the channels 14A-14C are less than physically identical. These physical differences between components may cause relative signal delay between channels. Additionally differences between analog signal processing circuits 22A and 22B may cause a greater signal transmission time through signal processing circuit 22A when compared to signal processing time through circuit 22B. Thus, analog signal processing circuits 22A and 22B may generate analog signal outputs at different times (i.e., out of sync.) not withstanding sensor signal inputs provided at the same instant of time. The overall relative time delay, also referred to as relative phase, between signals outputted by channels 14A-14C, may impair accuracy of the subsequently generated orientation signals.
The present invention relates to a method for calculating relative phases between channels of a multi-sensor tracking device. The calculated phases can be subsequently used by an internal digital signal processor to generate a more accurate orientation signal. The multi-sensor tracking device comprises a plurality of channels each one of which includes at least one sensor for sensing movement with respect to a corresponding axis. To calculate at least one relative phase between channels, the tracking device is subjected to a known movement constrained with respect to one of the orthogonal axes thereof. As the tracking device moves, a plurality of first signals generated by a first channel is recorded. Concurrently, a plurality of second signals generated by a second channel are likewise recorded. Thereafter, a relative phase between the first and second channels is calculated as a function of the recorded plurality of first signals and recorded plurality of second signals. It is this relative phase which may be used to compensate for relative time lags between signals propagating through the first and second channels in subsequent real time application of the tracking device.