Abstract:
A system for tracking an object in space for position, comprises a transponder device connectable to the object. The transponder device has one or several transponder aerial(s) and a transponder circuit connected to the transponder aerial for receiving an RF signal through the transponder aerial. The transponder device adds a known delay to the RF signal thereby producing an RF response for transmitting through the transponder aerial. A transmitter is connected to a first aerial for transmitting the RF signal through a first aerial. A receiver is connected to the first, a second and third aerials for receiving the RF response of the transponder device therethrough. A position calculator is associated to the transmitter and the receiver for calculating a position of the object as a function of the known delay and the time period between the emission of the RF signal and the reception of the RF response from the first, second and third aerials. A method is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This patent application is a divisional of U.S. patent application Ser. No. 11/418,114, filed on May 5, 2006, now U.S. Pat. No. 7,327,306, and claims priority on U.S. Provisional Patent Application No. 60/678,190, filed on May 6, 2005, by the present applicants and which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an RF system for tracking objects in space for position and orientation. The RF tracking system described in this document is used as an example for tracking tools in computer-assisted surgery, but other uses are also contemplated such as mining, storage inventory retrieval, nanorobotics, neurosurgery, cardiology, endodiagnostics, vehicle tracking and any other industrial application. 
   2. Background Art 
   It is often required to track objects for position and orientation in space. For instance, in computer-assisted surgery, tools are tracked for position and orientation in order to provide a surgeon with useful data pertaining to relative position between bone elements and surgical tools. For instance, orthopedic surgery involving bone implants benefits from the use of a tracking system that will provide precise information pertaining to alterations to bone elements. 
   Known tracking systems either offer inadequate precision, or are not completely suited for the types of maneuvers associated with the use of the tracking systems. For instance, in computer-assisted surgery, optical systems are used to track tools. In such systems, a line of sight is required between the tool and movement sensors in order to provide precise position and orientation data. Accordingly, the position of a patient being operated on is influenced by this line of sight that must be kept between the tool and the movement sensors. 
   Other types of systems, such as magnetic emitters and the like, have been used in computer-assisted surgery. However, such systems typically involve bulky components, or wires that interconnect emitter components. Therefore, considering that the working space in a surgical environment must be sterilized, the use of such systems constitutes a costly solution. 
   SUMMARY OF INVENTION 
   It is therefore an aim of the present invention to provide a novel RF system for tracking objects. 
   It is a further aim of the present invention to address issues of the prior art. 
   Therefore, in accordance with the present invention, there is provided a system for tracking an object in space for position, comprising: a transponder device connectable to the object, the transponder device having a transponder aerial and a transponder circuit connected to the transponder aerial for receiving an RF signal through the transponder aerial, the transponder device adding a known delay to the RF signal thereby producing an RF response for transmitting through the transponder aerial; first, second and third aerials; a transmitter connected to the first aerial for transmitting the RF signal through the first aerial; a receiver connected to the first, second and third aerials for receiving the RF response of the transponder device therethrough; and a position calculator associated to the transmitter and the receiver for calculating a position of the object as a function of the known delay and the time period between the emission of the RF signal and the reception of the RF response from the first, second and third aerials. 
   Further in accordance with the present invention, there is provided a method for tracking an object in space for position, comprising the steps of: emitting an RF signal from a fixed position; receiving with a transponder device on the object the RF signal; emitting from the transponder device an RF return signal consisting of the RF signal with a known time delay; receiving the RF signal with at least three aerials associated to the fixed position; and calculating a position of the object from a distance between each of the at least three aerials and the transponder device as a function as a function of the known delay and the time period between the emission of the RF signal and the reception of the RF response from the first, second and third aerials. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof and in which: 
       FIG. 1  is a block diagram illustrating a tracking system in accordance with the preferred embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a tracking system in accordance with a preferred embodiment of the present invention is generally shown at  10 . The tracking system  10  has a transponder device  12  (displaceable, with its independent power source) and a tracking station  14  (fixed). The tracking station  14  is optionally connected to a computer-assisted surgery system  16 , or other system requiring position and orientation data that will be produced by the tracking station  14 . 
   The transponder device  12  is connectable to a tool or other object to be tracked in space for position, and orientation if required. The interrelation between the transponder device  12  and the object to be tracked is known (e.g., through calibration) such that a tracking of the transponder device  12  will enable the tracking station  14  to obtain position and orientation information pertaining to the object (e.g., tip of a tool). The transponder device  12  and aerials of the tracking station  14  are typically separated by a distance ranging between 0.5 m to 10.0 m in computer-assisted surgery, but could be more or less depending on the type of application. 
   The transponder device  12  has an aerial  20  connected to a transponder circuit  21 . The aerial  20  is provided to receive incoming RF signals, and to emit response signals as a function of the incoming RF signals, as directed by the transponder circuit  21 . 
   The transponder circuit  21  receives the incoming RF signals and controls the emission of a response through the aerial  20 . More specifically, between the receipt of a signal and the transmission of a response signal from the transponder device  12 , a delay of time occurs, which delay of time is known. The delay of time is, for instance, caused by one or two SAW filters, a delay line or other delay method on a delay circuit. The transponder circuit  21  may also amplify the incoming RF signal. 
   The tracking station  14  has a controller  40 . The controller  40  is a processing unit (e.g., micro-controller, computer or the like) that controls the operation of the tracking station  14 . The controller  40  is connected to a user interface  41 , by which an operator may command the tracking system  10 . The controller  40  transmits position and orientation associated data to the user interface  41  as output from the tracking system  10 . 
   The controller  40  is also connected to a transmitter/receiver  42 . The transmitter/receiver  42  is provided for emitting modulated RF signals through aerials  43 ,  44  and  45 , and for receiving a return RF signal from the transponder device  12  using the aerials  43  to  45 . 
   Accordingly, as shown in  FIG. 1 , the aerials  43  to  45  are all connected separately to the transmitter/receiver  42 . It is preferred to minimize the distance between the transmitter/receiver  42  and the aerials  43  to  45  to minimize any dephasing. However, value tables may be used for the compensation of any delay in transmission due to a non-negligible distance between the transmitter/receiver  42  and the aerials  43  to  45 . Operation of the transmitter/receiver  42  is commanded by the controller  40 . 
   A position/orientation calculator  46  is connected to the controller  40 . The position/orientation calculator  46  is typically a software or a drive associated with the controller  40 , by which position and, if required, orientation pertaining to the transponder device  12  is calculated. Operation of the position/orientation calculator  46  will be described hereinafter. 
   An obstruction detector  47  is also connected to the controller  40 . The controller  40  commands the obstruction detector  47 , which will detect any obstruction between the transponder device  12  and the tracking station  14 . More specifically, it is possible that obstruction-inducing objects cause interference between the aerials  43  to  45  of the tracking station  14  and the aerial  20  of the transponder device  12 . Accordingly, the obstruction detector  47  is provided so as to take into account any obstruction, and any obstruction will be considered in position calculations by compensation software in the position/orientation calculator  46 . For instance, noise and the level of the RF signal received by the aerials  43  to  45  is monitored to determine the level of interference, which information is used thereafter by the compensation software. The obstruction detector  47  may signal that a non-negligible level of interference is present (sound signal, visual signal), so as to advise the operator person to remove any interfering object from the field of operation. 
   The computer-assisted surgery system  16  is optionally connected to the controller  40  (e.g., wireless connection) so as to receive position and orientation data, which will be used by the computer-assisted surgery system  16  in order to provide such information in various forms to the operator of the computer-assisted surgery system  16 . 
   Now that the various components of the tracking system  10  have been described, a general operation of the tracking system  10  follows. 
   In order to obtain position and, if required, orientation information pertaining to an object, the controller  40  will initiate a transmission to the transponder device  12 . The controller  40  will send a signal to the position/orientation calculator  46 . 
   For instance, an actuation pulse is sent to the position/orientation calculator  46 . The position/orientation calculator  46  has a cycle counter (i.e., internal clock) and the counter values at the time of transmission (Tx) and at the time of reception (Rx) will be used in the position calculations. Phase measurement is also considered by a phase comparator in the position/orientation calculator  46 , as will be described hereinafter. 
   Simultaneously, a transmitter actuation pulse is sent from the controller  40  to the transmitter/receiver  42 . Accordingly, the transmitter/receiver  42  will send an actuation signal to one of the aerials  43  to  45 . For instance, the aerial  43  will emit a modulated RF signal (e.g., RF pulse) from this actuation of the controller  40 . 
   The modulated RF pulse from the aerial  43  will be received by the aerial  20  of the transponder device  12 . The modulated RF pulse received by the aerial  20  will be forwarded to the transponder circuit  21 , which will return the signal in the form of a delayed return pulse emitted by the aerial  20 . As mentioned previously, the delay between the receipt of the signal by the aerial  20  and the emission of a return signal by the aerial  20  is known. The modulated RF pulse is a wave train of short length, as a function of the size of the transponder circuit  21 . 
   The modulated RF pulse may be amplified into the delayed return signal. More specifically, in order to reduce the effect of reflections, it is considered to provide gain to the return signal. Any gain at the transponder device  12  is as a function of reception sensitivity of the transmitter/receiver  42 . It is also considered to provide a gain as a function of any magnitude loss in the incoming modulated RF pulse. 
   The emitted return RF signal from the transponder device  12  will be received by all three aerials  43 ,  44  and  45 . Accordingly, by triangulation, the position of the transponder device  12  can be calculated. 
   Each of the three aerials  43  to  45  will send notification of the delayed return signal to the transmitter/receiver  42 , which will forward this receiver end signal to the controller  40 . 
   The controller  40 , having received the signal, will actuate the position/orientation calculator  46 , by way of an end pulse, so as to obtain a time value for the reception of a signal with cycle counter. The signal will be recognized by the position/orientation calculator  46 , whereby the position of the transponder device  12  can be calculated using triangulation with the distance between the aerials  43  to  45  and the transponder device  12 . The time delay at the transponder device  12  is taken into account when calculating a distance between the aerials  43  to  45  and the transponder device  12 . 
   It is pointed out that if orientation information is required, the object should be equipped with three of the transponder device  12 , in a non-linear arrangement or orthogonal arrangement. Alternatively, a transponder device  12  having three aerials  20  for the transponder circuit  21 , via appropriate RF switches can be used. The three transponder aerials would be orthogonally oriented. A single one of the transponder device  12  or the transponder device  12  with a single aerial will provide position information only. 
   In the event that the position/orientation calculator  46  uses a cycle counter, the amount of time between the emission of the modulated RF pulse and the receipt of the return RF signal by the transmitter/receiver  42  is calculated as a function of the number of cycles measured by the cycle counter. The phase comparator is then used to transform an incomplete remaining cycle into a time value, which will be used to calculate with the number of cycles the total time between emission and reception of a signal by the transmitter/receiver  42 . 
   As mentioned previously, the distance between the aerials and the transponder device  12  is calculated as a function of this time value, and considering the time delay at the transponder device  12  and the speed of light. 
   Although the tracking station  14  has been described as having three aerials, namely aerials  43  to  45 , it is contemplated to provide the transponder device  12  and/or the tracking station  14  with additional aerials to ensure the precision of the position and orientation measurement. Moreover, the type of aerials used can be selected as a function of the level of precision required. In one embodiment, the tracking station  14  typically has a printed circuit board of rectangular shape having aerials at its corners (with circuitry for each aerial), as well as the required circuitry of the transmitted/receiver  42  and other components of the tracking station  14 . However, other configurations are contemplated, such as independent printed circuits for each aerial. Any three of the aerials are arranged to form a plane. The signal frequency is typically of 915 MHz. The various actuation signals are of suitable frequency. As an example, it is contemplated to use YAGI aerials for the tracking station  14 . 
   The obstruction detector  47  is connected to the controller  40  so as to feed obstruction data to the controller  40 . More specifically, it is contemplated to use a visual sensor (or audio, ultrasound, laser sensors or the like) that will detect the presence of objects between the aerials  43 ,  44  and/or  45  and the transponder device  12 . As a result of any obstruction, the position/orientation calculator  46  will take into account such data in the calculation of the position and orientation of the transponder device  12 . If the tracking station  14  is provided with more than three aerials, it is possible to remove signals from one of the aerials in the calculation of the position and orientation by the position/orientation calculator  46 , if it is considered that there is obstruction between that given aerial and the transponder device  12 . It is contemplated to provide the position/orientation calculator  46  with a database of tabulated information pertaining to the effect of various types of obstruction. This information could be used to correct the position and orientation calculation as a function of the type of obstruction. 
   The above-described operation of the system involves the emission of a modulated RF pulse by one of the aerials  43  to  45 . However, in order to provide constantly updated position and orientation information about the transponder device  12 , it is pointed out that the tracking station  14  is constantly cycling modulated RF pulses by sequentially changing the emission from the aerials  43  to  45 , or any other suitable sequence. 
   Other contemplated uses for the tracking system  10  include mining, storage inventory retrieval, nanorobotics, neurosurgery, cardiology, endodiagnostics, vehicle tracking and any other industrial application. It is contemplated to attach the transponder device  12  to a probe. Such a probe could be an injectable probe (e.g., injectable in living beings such as humans and animals).