Spartan-3 Goes Golfing
His son graduates from Junior High next week – blank it out… His wife’s car needs the oil changed – blank it out… His mother sounded tired when she called last week – blank it out… He tries to empty his conscious of all the noise, the clutter, and the dreary detail of his daily life. He blanks his mind and visualizes himself as if he were watching his own actions from a short distance away. He fast forwards several seconds into the future. He sees himself address the ball and begin the back swing. He feels the pendulum action of the graphite-and-titanium club just as it reaches the perfect azimuth and then starts its arc downward, striking the sphere at the sweet spot from precisely the right angle. His mind’s eye watches the track of the ball as it sails perfectly toward his target.
A second later, his conscious goes completely empty as his body starts through the carefully choreographed motions his mind has just visualized. Years of practice and tens of thousands of strokes have trained his muscles to execute the maneuver perfectly – as long as his thoughts don’t get in the way. The sound of the club face striking the ball finds his ears, and immediately his finely-tuned intuition knows he has succeeded.
This time, another eye watches his every move. Instead of the golfer’s mental visualization, a briefcase-sized unit mounted near the tee takes over the observation task. At 10GHz, electromagnetic waves strike the ball and echo back at slightly different frequencies to a phased array of sensitive antennae. The tiny Doppler shift is almost imperceptible in a sea of seemingly random noise, but when the resulting audio-frequency-difference signal is gathered, queued, and synchronized by a high-performance FPGA and then sent on its way through a USB connection to a waiting signal-processing algorithm running on a PC, the picture becomes clear. Every aspect of the club’s movement and the ball’s flight is accurately recorded. Even the spin of the ball creates differential signals that tip the tool as to the reason behind the slightly curved flight path. Before the ball hits the ground, its precise trajectory is being plotted on a nearby monitor.
When Interactive Sports Games came to Axcon for help with their TrackMan golf radar system, they were looking for custom A/D conversion hardware with a very high signal to noise ratio (SNR). Current state-of-the-art ball-launch monitors were video-based, taking 2-4 pictures of the ball immediately after launch and extrapolating shot distance from those images. TrackMan was instead based on phased-array radar and had the potential for much higher accuracy – if a good enough S/N ratio could be achieved. “Picture a golf ball flying past at a distance of 200 yards, and you want to know how much it is spinning,” says Axcon’s Anders Enggaard. “We needed 110-120db SNR to get the accuracy we needed.”
Typically, when we think of radar applications, we think of super-high performance digital signal processing (DSP) hardware (think FPGAs with big arrays of hardware DSP blocks). In this case, however, only the difference signal from the Doppler radar was being captured, so the crucial data was reduced to several channels of audio-frequency information. That means that the Spartan-3 board was assigned a task akin to a high-end sound card – gather data through an 8-channel, 24-bit, 192kSa analog-to-digital converter (ADC), line it up and buffer it, and squirt it out through a USB2.0 connection to a PC, where sophisticated signal processing algorithms are waiting to analyze and display the exact 3D flight path and spin of the ball as well as the precise movement of the club.
In addition to SNR, the challenges of the application included keeping the overall cost down, creating a single-board solution for the ADC-to-USB2.0, and making sure that the data was buffered sufficiently to account for the very non-real-time behavior of Windows on the PC without dropping any samples. The board also contains a GPS receiver used to get precise location data so that multiple radar units could have a basis for a meaningful discussion of the location of the ball.
The ADC pipes the data into the Spartan-3 FPGA at about 5Mbytes/sec. Altough Axcon works with a wide variety of FPGAs, the Spartan-3 was chosen because of its relatively large memory capacity. The USB2.0 interface to the PC could deliver as much as 480Mbps, so the unpredictable element requiring the FPGA buffering was the Windows application, which was slurping data from the other end of the connection in unpredictable gulps. The system was more demanding than a simple buffer, however. The synchronization of data from multiple ADCs was critical, requiring both deterministic and low skew between samples.
There was also a tight schedule constraint on the design project, and it was important that the board be completed successfully on the first spin. By basing the system design on an FPGA, the board could be worked before the final details of the FPGA were complete, and any last-minute problems could be corrected by modifying the FPGA design. In this case, the strategy paid off, as the ADC hadn’t passed characterization at the time the prototype was implemented. Several major changes, including changes to the clocking strategy, were required as a result of the ADC characterization.
The FPGA was connected to the 8-channel ADC, the GPS receiver, the USB microcontroller/PHY, some external memory, and a custom control interface (see figure 1). Key to the success of the design was careful crafting of the analog and digital portions of the board so that digital noise didn’t compromise the SNR. That’s one place where Axcon’s considerable design experience paid off. “We came in at about four months from project specification to working prototype,” continues Enggaard. “We always try to control costs by getting the design right the first time. That works in the best interests of both Axcon and our clients.”
(Image courtesy of Axcon.)
TrackMan has already seen a great deal of interest since its introduction. The briefcase-sized unit is perfect for golf club manufacturers that want to analyze their equipment during the design phase. It is also highly desirable to advanced golf players for careful analysis of their swing and ball flight. Training facilities are installing the systems and applying them in advanced training programs. Additionally, TV broadcasters are using them in order to enhance their broadcasts of professional tournaments with accurate, real-time tracking of the ball’s speed and flight path in their on-screen graphics.
Denmark-based Axcon has been in business for almost three years, specializing in FPGA-based board and system design. They use a variety of FPGA vendors and technologies, depending on what fits the bill for each specific project. They also provide training to customers in FPGA design and in signal integrity.