This invention relates to speed measuring devices particularly suited for use in determining velocity magnitude or speed of sports objects, and more particularly, to low cost, low energy radar devices for use in measuring the speed of baseballs, baseball bats, paint balls from paint ball guns, and other sports objects, or the speed of movements by martial artists and other players, particularly during training.
Continuous wave (CW) Doppler radar technology is commonly utilized to detect a moving object illuminated by the electromagnetic field of the radar and producing an electrical signal at a Doppler frequency which is a measure of the relative speed of the moving object. This technology has been pioneered and developed by the defense industry in the United States, is well documented in textbooks and reports, and has found numerous applications in consumer products. Security motion sensors, industrial position sensors and police radar units are examples of current uses of Doppler radar systems.
Doppler radar has been used in sports applications to measure the velocities of sports objects or players relative to one another or relative to a reference point. Examples of sports radar in use are found in U.S. Pat. No. 4,276.548 to Lutz and U.S. Pat. No. 5,199,705 to Jenkins et al. Conventional sports radar includes xe2x80x9cspeed gunsxe2x80x9d for measuring baseball or softball speed, such as disclosed in the Lutz patent. Available sports radar units generally occupy approximately 200 cubic inches and cost several hundred dollars. These units are typically operated by a third person somewhat remote from the thrower and receiver.
Implementation of prior art CW Doppler radar systems is relatively complex, generally involving the use of an RF oscillator and signal generator, an antenna system to radiate the oscillator output into free-space and to receive a portion of the transmitted electromagnetic energy that is reflected by the moving object, a transmit/receive switch, diplexer, or circulator device if a single antenna is used for both transmit and receive rather than separate transmit and receive antennas, and various local oscillators, mixers, phase-locked-loops and other xe2x80x9cfront-endxe2x80x9d circuits to heterodyne, demodulate and detect the Doppler signal. This complexity imposes high cost and size requirements on the radar units, which have heretofore discouraged the utilization of CW Doppler technology in consumer applications where extremely small size and low cost are necessary for practical end product realization.
In electronics applications unrelated to those discussed above, Doppler radar systems using simple homodyne circuits have been known. Such applications include defense applications such as ordnance proximity fuzes and target detectors where Doppler modulation provides evidence of a target encounter. Validation of the presence of target signals within a prescribed Doppler frequency passband and the detection of amplitude build-up as the target encounter distance decreases are sufficient for signal processing and decision making in such systems, obviating the need to accurately measure or calculate the specific velocity magnitude or speed. For example, for general proximity sensing applications, mere detection of an increasing distance signal is satisfactory. However, applications requiring a speed measurement necessitate determination of the specific Doppler frequency and a calculation of a corresponding speed value. Such homodyne circuits are but among hundreds or thousands of circuits and modulation schemes that in some way carry speed information but which have not been considered practical for providing speed measurements. Accordingly, circuits of a size or cost that are practical for consumer applications such as sports object speed measurement have not been known or available.
Existing Doppler speed measuring devices suffer from loss of accuracy due to the inability to place the unit in the line of the moving object, resulting in a reduction in the speed measurement to the cosine of the angle between the object""s velocity vector and the line of the Doppler signal between the unit and the moving object. Further, the Doppler units must be positioned where they are not subjected to damage by collision with the object.
Accordingly, a need exists for a low cost, effective, small size, low power device useful for measuring and displaying the speed of objects in consumer applications such as sports and sports training.
A primary objective of the present invention is to provide a small size, low cost, low power device for measuring object speed that is practical for consumer applications such as sports. It is a particular objective of the present invention to provide a sports radar unit for measuring and displaying the velocity magnitude or speed of a sports object such as a baseball. Further objectives of the invention are to provide such a speed measurement apparatus and method for measurement of baseball speed, baseball bat speed, for calibrating paint ball marker speed, for martial arts punch and kick speed measurements and other applications, particularly in training.
According to principles of the present invention, there is provided a CW Doppler radar speed sensor that is small in size, low in cost, low in power consumption and radiated energy, that measures and displays the speed of an object such as a baseball and displays the measured speed to a user. Further according to principles of the present invention, a device is provided that is adapted for mounting at or near the path or point of reception of the moving object, or at the xe2x80x9ctarget pointxe2x80x9d at which the moving object is directed. Such positioning facilitates the use of a low power, short range signal and accurate velocity measurement. The unit preferably transmits and receives RF energy in a microwave frequency range, preferably of a frequency of approximately 2.4 GHz or 5.8 GHz or higher, such as in the 10-25 GHz range.
The device according to one preferred embodiment of the invention, includes a radar transmitter and receiver that employs a single simple CW Doppler homodyne circuit preferably having an oscillator-detector that is based on a single transistor, which utilizes resonant circuit elements of the oscillator as an antenna to radiate energy into free-space. A portion of the radiated energy strikes the nearby moving object and is reflected back to the oscillator-antenna circuit where it is mixed with the oscillator signal. The coherent relationship of the transmitted and received signals in a simple homodyne circuit produces a Doppler frequency modulation as the distance to the moving object changes.
The preferred embodiment of the present invention makes use of the phenomena whereby, at a given separation distance between the radar and the moving object, the received object-reflected signal is exactly in-phase with, and reinforces, the oscillator signal, but as the separation distance changes by each one-quarter wavelength of the transmitted signal, the total two-way travel distance to the object and back changes by one-half wavelength, resulting in an out-of-phase or canceling relationship between the received and transmitted signals. Each distance change of one-half wavelength results in a two-way radar round trip change of one wavelength, thus producing one complete cycle of modulation. As the distance to the moving object changes by successive one-half wavelength increments, multiple cycles of modulation are produced. The frequency of this modulating signal is the Doppler frequency, which is equal to the velocity of the moving object expressed in terms of one-half wavelengths of the transmitted signal as follows:       f    D    =            v                        λ          t                /        2              =                  2        ⁢                  vf          t                    c      
where: fD is the frequency of Doppler modulation,
v is the relative velocity of the moving object,
xcext is the wavelength of the transmitted signal,
ft is the frequency of the transmitted signal,
c is the magnitude of the velocity of electromagnetic energy propagating in surrounding medium (free-space in this case) and is equal to the product of frequency and wavelength.
In the preferred embodiment of the invention, this resulting Doppler signal which modulates the oscillator signal is detected by filtering it out of the incoming signal, amplifying it, filtering it again and converting it to a digital signal, preferably using a zero-crossing detector (ZCD). The output of the ZCD is ideally a square wave having a frequency that is the Doppler frequency. The detected digitized Doppler frequency signal is applied to the input port of a microprocessor, which measures the time between negative-going zero-crossings using an internal timer. The measurement of zero-crossing intervals are compared to certain criteria to verify that a valid signal is being processed. Then a Doppler frequency value is calculated from the measured zero-crossing information by taking the time between zero-crossings in the same direction as is equal to the period of the Doppler frequency. Using the above formula, the velocity of the moving object toward the sensor, for example, the speed of a thrown ball approaching the sensor, is then calculated. The calculated velocity magnitude is displayed on a small liquid crystal display (LCD).
The radar unit of the preferred embodiment of the invention is preferably located in approximately a direct line with, or at only a slight angle to, the flight of the ball or other object whose speed is being measured. It is also preferably located such that the object passes within one or a few feet of the device somewhere in the path of the object, such as at the endpoint or point of catch. This arrangement preferably places the object within a few inches of the radar unit and moving directly toward the unit so that the speed of the object is measured within close proximity to the unit. In certain embodiments of the invention, the antenna of the unit is positioned in or very close to the path of the moving object with a signal processing portion of the unit positioned remote from the antenna and connected to the antenna by a transmission line. The antenna is preferably of a fixed length and, when remote from the other circuitry of the unit, is connected to the circuitry with a coaxial, parallel conductor or other transmission line that is impedance matched and designed into the RF detector circuitry.
In the case of a baseball, a preferred location for the unit, or at least the antenna portion of the unit, is on the catching forearm of the person catching the ball, preferably on the hand or wrist of the catching player. In one preferred embodiment, the unit is supported on or in a baseball glove worn on the hand of the user, preferably at the web portion thereof or behind the fingers of the user in line with the path of the ball. By so locating the unit, or providing the unit with a short range of effectiveness of less than ten feet, and preferably of from one to three feet, velocity errors due to off-line location are minimized, since the Doppler frequency represents the velocity of the object in a direction toward or away from the radar unit. Glove location of the radar also allows detection of the approaching ball within very close proximity to the unit and just before the baseball enters the glove pocket. Alternatively, the unit can be mounted on the catcher""s wrist, hand or elsewhere on the user""s forearm. This positioning fixes the relationship of the unit to the path of the ball and minimizes transmitter output power requirements, and corresponding battery power supply needs. With one preferred embodiment of the invention, transmitter output power can thus be in the order of microwatts, which is much less than the radiated power levels of most wireless consumer products such as cellular and portable telephones. Short range detection also avoids false readings of speed due to the motions or movement of the thrower.
With the baseball speed measuring radar unit mounted on the receiver""s forearm, such as on a baseball glove, the display is preferably positioned on the unit itself facing rearwardly so that the receiver can read the output upon catching the ball. Mounted on the receiver""s wrist, the antenna portion of the radar unit is preferably worn on the front of the wrist facing the thrower while the display is mounted on the back of the wrist so it is visible to the catcher, with both the antenna and display portions being secured by the same wrist band, with the band containing a flat cable interconnecting the two portions. The LCD, battery, power supply, and the two switches are located in a module on the back of the wrist. In this embodiment, the unit can include a real time such as that of a conventional digital wristwatch, which can share the battery and power circuit with the speed measuring device and utilize the display of the device to display time of day or elapsed time.
The radar velocity sensor can be operated from a 2.5 VDC battery power supply, requiring an average current of less than one milliampere. Thus, a single 3 volt nominal lithium cell capable of 160 milliampere-hours can power the sensor for a relatively long duration. Small, inexpensive cylindrical and button configuration lithium cells with this energy capability are readily available and are widely used in consumer products. Power xe2x80x9cON/OFFxe2x80x9d and xe2x80x9cResetxe2x80x9d switches are provided which are easily operated by the nongloved hand of the receiver before each succeeding throw is delivered.
The velocity measurement device of the present invention is capable of being miniaturized and produced inexpensively so that it can be used in consumer applications, which, up to now, have not heretofore been addressed by the prior art, It can be built into, or attached to, a baseball or softball glove, to measure the speed of the ball being caught. The radar can be worn on throwing arms of persons xe2x80x9ctossingxe2x80x9d a ball or by others batting, throwing, catching or otherwise dealing with moving objects in sports or other recreational uses. Gloves can be designed to conveniently incorporate the radar in a pouch, within a glove thumb, finger or heal pad, or held by straps, bands, hook and loop fasteners or effective means. A radar unit can be built directly into the glove. Gloves may be used in various sports applications, and may be considered to include hand and other forearm garments or body fastening structures or devices. In the preferred embodiments, the unit or the antenna of the unit is situated behind the glove or other target with the radiated and reflected signals passing through the glove or target.
In certain embodiments of the invention, a bat speed measuring device and method are provided in which an RF antenna is positioned in or near the path of a bat, such as on a post upstanding from a home plate or other base. The post may be a ball support post of a batting tee. The antenna is a fixed length antenna having a defined radiating length and connected to the end of a transmission line. Preferably, the antenna is at the top of the tee or other post and the transmission line extends from the antenna through the post to a signal processor in or near the base or beyond the base at a location that is out of the path of the swinging bat and immune from being struck and damaged by the swinging bat. The signal processor includes an RF detector matched and tuned to the transmission line and the antenna and a digital processor that converts the RF Doppler signal to numerical speed measurement data. The processor may include or be linked to a display or computer, either by wire or other solid link or through a wireless circuit so the measurement data can be read by a coach or the person swinging the bat.
In another embodiment of the invention, speed measurement is provided in martial arts training to measure the speed of punches and kicks. Preferably, a speed measurement device or antenna thereof is provided behind a target pad that is held by a coach or trainer or is fixed to a support, so that a trainee punching or kicking the pad can have the hand or foot speed measured.
Also according to the present invention, speed measurement of other sports objects is also provided in applications where small portable, devices may be used. For example, paint ball guns used in survival games and training, use air pressure to propel paint balls or markers at other players. To avoid injury to the players being shot with the markers, the velocity of the markers at the barrels of the guns is limited to, for example, 300 feet per second. To optimize the trajectory of the markers, it is desirable to calibrate the guns so that the marker is as close to the upper velocity limit without going over the limit. One embodiment of the invention contemplates the fixing of a speed measuring unit or the antenna thereof on the barrel of the marker gun closely adjacent the barrel with the antenna aimed parallel to the barrel and the path of the marker. The device is adjusted to process Doppler readings for a speed range of, for example, 150 to 400 feet per second.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the of the preferred embodiments of the invention, in which: