WO2004070336A1

WO2004070336A1 - Punch sensor

Info

Publication number: WO2004070336A1
Application number: PCT/AU2004/000138
Authority: WO
Inventors: Daniel James
Original assignee: M.B.T.L. Limited
Priority date: 2003-02-07
Filing date: 2004-02-09
Publication date: 2004-08-19
Also published as: AU2003900516A0

Abstract

A monitor for sensing punch speed and force uses a piezo electric layer supported on an open cell compressible foam. The foam is mounted on a relatively non compressible surface and the impact of the blow is sensed when the piezo layer is struck and when the foam is compressed to give a time interval to enable the time of flight and thus the punch velocity to be determined. The punch magnitude may also be determined. Preferably a second piezo layer is used on the back of the foam.

Description

Punch Sensor

This invention relates to a device for measuring the force and speed of a blow in boxing or martial arts sports.

Background to the invention

The automatic sensing of hits in fencing has been determined by electrical contact between the foil and a sensor panel worn by the participants.

In martial arts USA patents 4762005 and 4824107 disclose a piezo based transducer arrangement to be incorporated into the gloves, headgear or body suit of the participants. Wireless transmission of the piezo signals sends the data to a computer for analysis and display. The devices simply measure the number of impacts.

USA patent 5099702discloses a force pad also for recording impacts.

USA patent 5336959 uses a piezo film to locate the point of impact of a ball.

WO 99/10052 discloses a boxing training system with sensors on gloves or a punching bag to record the number of hits. The force transducer may also be used to detect the magnitude of the force for a given impact.

USA patent 6056674discloses a full protective boxing suit incorporating a fluid layer. The suit incorporates a sensor and transmitter to transmit data on the impacts to the fluid.

Apart from impact magnitude and the frequency of impact the speed of the blow is also important in measuring performance.

It is an object of this invention to provide an inexpensive device that can measure punch frequency, impact force and punch speed as an aid to sports training.

Brief Description of the invention

To this end the present invention provides a monitor for use in boxing or martial arts which incorporates at least one force sensor supported on a compressible material adapted to be mounted on a more rigid material the monitor being adapted to sense an initial impact on the force sensor and a second impact when the support material is compressed by the impact force. The time between the two impacts is used to derive time of flight and the speed of the blow received by the sensor. The force sensor is also used to derive the magnitude of the blow.

The force sensor may comprise force sensitive resistors, strain gauges, load cells or piezoelectric materials the latter being preferred.

Preferably two piezo layers are used the second film being located behind the compressible material. The time between the impacts on the first and second piezo layers is used to determine punch speed and the impact force measured by the rear sensor is used to determine punch magnitude.

The compressible material is preferably an easily compressed open cell foam which provides little resistance to the impact and is resilient so that it immediately resumes its original thickness.

The monitor of this invention is used in a device that can be incorporated in a punching bag, punching glove, a helmet or combat garments.

To this end in another aspect this invention provides a martial arts or boxing performance monitor which consists of a) an impact surface incorporating at least one force sensor supported on a compressible material said impact surface being adapted to be positioned over a more rigid surface b) means to collect the signals from the force sensor when it is impacted and when the supporting material is compressed c) analysis means to derive from said signals the velocity of the blow delivering the impact as well as the magnitude of the force

The device may include a display and may also incorporate data storage to record the measured force and velocities. For boxing, punch frequency in a particular time interval can also be determined and displayed. Preferably the device also incorporates a transmitter to transmit the signals or the derived measurements to a remote receiver for storage and further processing. The force sensor is preferably a piezoelectric material such as PZT or PVDF

Detailed description of the invention A preferred embodiment of the invention will be described. Figure 1 is a schematic illustration of the sensor construction; Figure 2 is a schematic lay out of an alternative sensor construction; Figure 3 illustrates a punching bag incorporating the sensor and display of this invention;

Figure 4 is a system diagram for the sensor of this invention; Figure 5 is a schematic diagram of the sensor preamp; Figure 6 is a schematic diagram of the LED display for the sensor of this invention;

Figure 7 is a schematic diagram of the display mode selection buttons; Figure 8 is a schematic diagram of the power supply; Figure 9 illustrates process flowchart for the firmware used in the sensor; Figure 10 is a graph of force vs time obtained from the rear sensor response for a fast punch;

Figure 11 is a graph of force vs time obtained from the rear sensor response for a slow punch;

The punch sensor consist of two thin-film PVDF sensors separated by open- cell foam as shown in figure 1. The PVDF sensors20 and 25 as illustrated are attached to the open-cell foam 23 with double-sided adhesive tape. The open- cell foam block 23 is 130x77x45mm and is made from toluene diisocyanate (TDI) with polyols. Protecting the rear PVDF sensor 25 is a 130x77x15mm layer of closed-cell foam 21 made from polyolefins. Protecting the front PVDF sensor 20 is a 130x77x3mm layer of closed-cell foam 26. When small PVDF sensors are used in the punch sensor to reduce the cost of the device it makes the sensors significantly more difficult to trigger due to the relative disparity in surface area between the PVDF sensors and a boxers fist. This problem is addressed by the addition of the rectangular protrusion 27(T- bar) to the front closed-cell foam layer 21 as shown in Figure 2. The figure 2 embodiment has the same rear foam 26 and sensor 25 but the front sensor 20 is mounted over the 15x50x12mm rectangular protrusion 27 in the centre which also made from polyolefins. The rectangular protrusion is embedded 12mm into the open-cell foam 23 where the front PVDF sensor is also attached in order to present a flat surface for impact. Without the protrusion 27, the PVDF sensor 20 undergoes minimal stress on initial impact due to being surrounded by foam. With the protrusion, the PVDF sensor is bent over the protrusion on initial impact causing significantly more stress in the length direction. The result is a much more discernible voltage peak produced by the front PVDF sensor 20 on initial impact.

The punch sensor is mounted on a boxing bag and configured to sense an impact as shown in figure 3. The sensor of figure 1 Or 2 is located at 30 and the control and display panel 33 is mounted at head height. The display 33 may be fastened by strap 39 to the bag. The display and control panel 33 includes a power indicator LED 34 a power switch 35 and for scrolling the display a forward button 36 and a vback button 37. The front PVDF sensor is configured to produce a discernible voltage peak on initial impact and another voltage peak resulting from a second impact when the impact force compresses the open-cell foam. Punch speed can then be calculated using the separation distance between the front PVDF sensor and boxing bag and the measured time of flight between the two impacts. Punch force is calculated by directly integrating the signal produced by the rear PVDF sensor during the impact.

As can been seen in Figure 4, the punch sensor is interfaced to the processor board with a sensor pre-amp. The sensor pre-amp conditions the signal and provides necessary impedance matching appropriate for the processor board. The sensor pre-amp connects to the processor board via the on-chip analog- to-digital converter (ADC). A 3-digit, seven segment LED display is used for displaying punch measurements made by the Micropunch. The specific measurement to display (e.g. speed, force) is selected using two display mode select buttons. Power is supplied to the processor board, sensor pre- amp, and LED display from a 5V regulated, battery, power supply unit (PSU). Due to the high output voltage of PVDF sensors and the AC nature of their output signal, an interface circuit is required to attenuate the signal and add a DC bias. The interface circuit match's the impedance of the PVDF sensor and microcontroller. An inverting voltage amplifier configuration is the most suitable since it is the simplest circuit that provides all of these characteristics. A schematic of the signal conditioning circuit is shown in Figure 5. The sensor pre-amp consists primarily of two inverting voltage amplifiers with adjustable gain. The front sensor channel is configured for attenuating the input signal, since the front PVDF sensor typically produces a voltage much greater than the limits of the processor board. The rear sensor channel is configured for a small amount of gain since a typical output signal from the rear PVDF sensor is less than the operating voltage of the processor board. The dual packed L 358 was chosen since it has the required number of op- amps and can operate from a single polarity supply rail. Furthermore, it is cheap and readily available. Although there are op-amps on the market with superior performance, the LM358 has sufficient performance for this application.

The AC signal from the PVDF sensors is added to a 2V DC bias provided by the voltage reference. The LM317 voltage regulator was used for the voltage reference since it can be configured to provide 2V output and is a widely available and inexpensive IC.

A LM358 op-amp has an output voltage swing of 0V to V⁺ - 1.5V. Thus, with the sensor pre-amp and processor board powered from the same voltage, 1.5V of the voltage range will be unusable. To improve on this, the sensor pre-amp power supply is taken directly from the slightly higher voltage battery terminals allowing greater utilisation of the 10-bit resolution of the processor board's ADC. A 2V reference is chosen since 2V is approximately half the output voltage swing of the LM358 op-amp.

The main technologies that may be used for the display are LED and LCD. LED displays have the advantage of higher visibility, and superior ruggedness, while LCD has the advantage of lower power consumption and smaller size. An LED display is used in this embodiment since high visibility and ruggedness is more important than low power consumption and smaller size for this application.

For the LED display decoder/driver IC is the 74LS47. The 74LS47 has open- collector outputs capable of sinking 24mA, and is capable of driving the LED's with only the addition of current-limiting resistors. The display will require three, single digit, LED display devices. Since the chosen display decoder/driver was a 74LS47 with open-collector outputs, the display devices must be common anode, so that the 74LS47 outputs can pull the displays' LED cathodes to ground, turning the LED's on. The common-anode A-521H single digit display was chosen since it is inexpensive. As shown in Figure 6, the LED display has 12 inputs composed of 3 BCD numbers. The decimal point of the centre display device is made permanently on, by tying it to ground via a current-limiting resistor. All unused inputs are tied to VCC since they are all active low.

The display mode buttons are two push-button switches that allow the user to browse forward and backward through the available display modes. The push-button switches used are generic, normally open, widely available, and inexpensive. Furthermore, they have a tall profile allowing them to protrude from the protective packaging of the device and are relatively easy to operate with the thumb of a gloved hand.

As shown Figure 7, the output end of the push-button switch is tied to VCC via a pull-up resistor while the other end is tied to ground. Pushing the button will short the output of the push-button switch to ground. The power supply shown in Figure 8, consists of a National Semiconductor LM2940CT-5.0 1A low dropout regulator , 4xAA batteries, on/off switch, power LED, and filtering capacitors. The circuit requires a constant 5V, but the 4xAA batteries placed in series produces 6V. A low dropout regulator was chosen since it only requires a minimum input voltage of 5.5V while other regulators require at least 6.7V. The on/off switch disconnects the batteries from the circuit, and the power LED will light up when the batteries are connected to the circuit. The filtering capacitors suppress any noise on the power supply line.

The firmware for the sensor includes a program to analyse the signal from the punch sensor, and display measurements of punch speed and force. A simplified flowchart representation of the main algorithm used in the firmware is shown in Figure 9. This algorithm continuously samples the output signal of the punch sensor at a fixed rate of 2000 samples/sec. A punch is detected by comparing the sample taken from the punch sensor to a fixed threshold. When the sample taken exceeds this threshold, the program enters a "punch occurring" mode. The affect of this mode is to cause the program to store 400 bytes (i.e. 100 samples x 2 bytes/sample x 2 PVDF sensors) of sampled data at the fixed rate. The sample buffer size was experimentally determined during sensor testing so as to allow the capture of the entire collision of fist and sensor/boxing-bag at the specified sample rate. When the sample buffer is full, the program returns to a "punch not occurring" mode. When in this mode, the program returns to the punch detection algorithm.

Punch speed and force are calculated after the signal from the punch sensor has been sampled for the required sampling duration.

Punch speed is calculated by first finding the voltage peaks corresponding to the initial and second impacts imparted on the punch sensor. Knowing the sample rate, the time delay between the two peaks can be calculated. Using this time delay and the fixed distance between the front PVDF sensor and the boxing bag, the average speed of the punch can be calculated as follows, where s = average speed, d = distance, and t = time: d s = — t

Punch force is calculated by integrating the output signal of the rear PVDF sensor with respect to time. This produces a force vs. time curve, which is integrated again and divided by the change in time to get an average force measurement for the punch.

The front mounted PVDF sensor generates a voltage peak when a fist collides with the foam block and a second voltage peak when the fist collides with the boxing bag. The time delay between the two peaks is calculated from the sensor data. Using the distance between the front sensor and the boxing bag and dividing by the time delay the average speed of the fist can be calculated.

A plot of force vs. time for the punch can be obtained by integrating the voltage waveform obtained directly from the sensor. Integrating the force vs. time plot will give the impulse force of the punch. Dividing the impulse force by the contact time of the punch will give the average force of the punch.

Once the punch speed and force has been calculated, the appropriate reading is displayed on the LED display. The type of measurement displayed depends on the current display mode. The forward/back display mode buttons described in the hardware design allows the user to switch between display modes. The state of the buttons is checked periodically whenever a punch is not occurring. The buttons are debounced in the firmware.

Before calculating punch speed and force, the sampled signal from the punch sensor is analysed to detect false punches. Flicking the punch sensor is a common example of a false punch. The PVDF punch sensors are highly sensitive to rapid changes in force (i.e. flicks), which produce large signals. False punches can be distinguished by their relatively short duration compared to a true punch. When a false punch is detected, it is ignored. Figures 10 and 11 show the results, for both a fast and slow punch, of calculated instantaneous force. When the plots are compared, the difference between the fast and slow punch can clearly be seen. The fast punch produces an impact force over a smaller time period than the slow punch as expected.

The sensor data provides a linear reading of punch force above a minimum force which sufficient to fully compress the open cell foam. As Athletes in boxing and martial arts are more interested in the upper ranges of punch force the non linearity at low forces is not a problem. The features of the embodiment of the invention described include:- β Powered by 2 AA batteries

• 3 digit display to show punch velocity and punch frequency

• the display is flashed momentarily after a punch or sequence of punches is complete

• optionally cradle based charging and communications via magnetic and infrared coupling

• the unit can be sealed in a package suitable for location on the punching bag or in close proximity to the punch sensing unit

Those skilled in the art will realize that the invention may be implemented in a number of embodiments different to that described without departing from the core teachings of the invention. The sensor may be varied in size and used in a boxing glove, head gear or vest as well as a boxing bag.

Claims

1. A martial arts or boxing performance monitor which consists of a) an impact surface incorporating at least one force sensor supported on a compressible material said impact surface being adapted to be positioned over a more rigid surface b) means to collect the signals from the force sensor when it is impacted and when the supporting material is compressed c) analysis means to derive from said signals the velocity of the blow delivering the impact as well as the magnitude of the force.

2. A monitor as claimed in claim 1 wherein the force sensor is a layer of piezoelectric material.

3. A monitor for use in boxing or martial arts which incorporates at least one force sensor supported on a compressible material adapted to be mounted on a more rigid material the sensor being adapted to sense an initial impact on the sensor and a second impact when the support material is compressed by the impact force.

4. A monitor as claimed in claim 3 in which a first sensor is mounted on the outer face of the compressible material and used to measure the timing of the first and second impacts and a second sensor mounted on the rear face of the compressible material is used calculate punch force.

5. A monitor as claimed in claim 3 wherein the force sensor is a layer of piezoelectric material.

6. A boxing glove incorporating at least one force sensor supported on a compressible material adapted to lie between the outer and inner lining of the glove the sensor being adapted to sense an initial impact on the sensor and a second impact when the support material is compressed by the impact force.

7. A boxing glove as claimed in claim 6 in which a first sensor is mounted on the outer face of the compressible material and used to measure the timing of the first and second impacts and a second sensor mounted on the rear face of the compressible material is used calculate punch force.

8. A boxing glove as claimed in claim 7 wherein the force sensors comprise a layer of piezoelectric material.

9. A boxing vest incorporating at least one force sensor supported on a compressible material adapted to lie between the outer and inner lining of the vest the sensor being adapted to sense an initial impact on the sensor and a second impact when the support material is compressed by the impact force.

10. A boxing vest as claimed in claim 9 in which a first sensor is mounted on the outer face of the compressible material and used to measure the timing of the first and second impacts and a second sensor mounted on the rear face of the compressible material is used calculate punch force.

11. A boxing vest as claimed in claim 10 wherein the force sensors comprise a layer of piezoelectric material.