US20120238407A1

US20120238407A1 - Methods and apparatus for accurately measuring strike impact on a martial arts training bag

Info

Publication number: US20120238407A1
Application number: US13/423,268
Authority: US
Inventors: David S. Dilworth; Kevin M. Hurlahe
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-03-18
Filing date: 2012-03-18
Publication date: 2012-09-20

Abstract

An instrument is attached to a punching bag that accurately and consistently provides a strike impact feedback score to a user. The instrument is adaptable to existing punching bags, is simple for the user to install, simple to operate, and provides an intuitive user interface. The feedback score is a reliable and consistent interpretation of the intensity of the user's strike, providing meaningful feedback to the user and/or the martial arts instructor for use in improving the quality of the student's strikes.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. provisional patent application No. 61/454,341, filed Mar. 18, 2011, the contents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to martial arts training accessories and, more particularly, to an instrument for martial arts training bags that measures strike intensity while rejecting pre- and after-shocks.
Martial arts training and instruction typically includes a variety of methods. These methods include practicing techniques by oneself, such as punching and kicking in the air (shadow boxing, choreographed forms or “Katas”), interactive sparring and/or self-defense escapes practiced with a partner, as well as the use of training equipment to practice striking, such as hand held targets, body shields, and punching bags.
One of the most common pieces of martial arts striking equipment is a free-standing (as opposed to hanging from the ceiling) heavy punching bag, usually consisting of a hard plastic, water filled base and a padded top for striking. These punching bags are widely used in martial arts schools, fitness centers, and home training. Punching bag training is a relatively safe method used to develop forceful strikes. The problem with this activity is that the punching bag currently does not provide accurate and meaningful feedback that measures the intensity of a martial arts practitioner's (hereafter referred to as “user”) physical strikes, such as punches and kicks.
Conventional devices that try to measure strike intensity either use targets that do not produce significant pre- and after-shocks (and, hence do not need to reject pre- and after-shocks) or work with striking targets that produce pre- and after-shocks, but do not separate the user's strike from the pre- and after-shocks.
As can be seen, there is a need for an instrument that can attach to a punching bag to provide accurate and meaningful feedback that measures the intensity of a user's physical strikes.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method for measuring performance metrics of a strike comprises allowing a striking object to settle to a predetermined settled state; collecting accelerometer data describing motion of the striking object before, during and after the strike; determining a start time and an end time of the strike; and rejecting data before the start time and after the end time of the strike.
In another aspect of the present invention, a system for measuring performance metrics of a strike to a free-standing punching bag comprises at least one accelerometer disposed on the punching bag; an instrument adapted to receive accelerometer data from at least one accelerometer; and computer software, written in one or more computer codes, disposed on a computer readable medium, and adapted to determine a start and end time of the strike, reject data before the start time and after the end time, and compute the performance metrics based on data between the start time and end time.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a punching bag having a device to measure strike intensity according to an exemplary embodiment of the present invention;

FIG. 2 is a block diagram showing various elements of the device to measure strike intensity according to an exemplary embodiment of the present invention;

FIG. 3 is a graph showing three-dimensional acceleration data for a palm strike using the device of FIG. 1 on a punching bag; and

FIG. 4 is a flow chart describing the use of the device of FIG. 1 according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.
Broadly, an embodiment of the present invention provides an instrument that can be attached to a punching bag that accurately and consistently provides a strike intensity feedback score to a user. The instrument is adaptable to existing punching bags, is simple for the user to install, simple to operate, and provides an intuitive user interface. The feedback score is a reliable and consistent interpretation of the intensity of the user's strike, providing meaningful feedback to the user and/or the martial arts instructor for use in improving the quality of the student's strikes.
The instrument of the present invention can revolutionize martial arts punching bag training, as it makes the activity more meaningful to the user. The instrument can help the user gauge the effectiveness of various strikes and quantify progress in training. Martial arts instructors could use the instrument to enhance instruction and to receive feedback on the quality of their instruction. Martial arts promoters can use this instrument in tournaments and competitions, as well as demonstrations. Other uses of the instrument of the present invention could be for fitness training, self-defense training, recreation and entertainment, and possibly by law enforcement or the military. The instrument of the present invention solves the above identified problems by accurately capturing and reporting the intensity of the user's strike on a free-standing punching bag and providing meaningful feedback in a simple to use device.
Referring to FIG. 1, an instrument 10 of the present invention can be a small, self-contained device that can attach to a base 12 of a floor-standing martial arts training bag 14 (also referred to as punching bag 14). Once the instrument 10 is installed and turned on, the user can strike a padded portion 16 of the punching bag in an area above wherein the instrument 10 is mounted. Overall, the user's strike causes the punching bag 14 to accelerate away from the user. The punching bag 14 undergoes a series of accelerations. Although most of the early accelerations are the result of the user's strike, a user's foot motion(s) may also induce small (pre-shocks) accelerations to the punching bag 14. The later accelerations are the result of aftershocks as the punching bag settles into a stable position. The punching bag 14 typically takes several seconds to settle and the aftershocks can be more intense than the user's strike and are not accurate measures of the user's strike.
The instrument 10 of the present invention can measure the punching bag's accelerations and process them to find the start and end of the user's strike while rejecting the pre- and after-shocks. Several meaningful values can be computed from the acceleration data measured between the start and end times, such as maximum acceleration, impulsive acceleration, strike duration and the like. The instrument may display the computed values or may electronically send the strike data to a separate computing device.
The instrument of the present invention can use one or more accelerometer axes to sense punching bag accelerations. As shown in FIG. 2, the processing has eight distinct steps as described in the ten points listed in the processing block.
A first step is to initialize the instrument and report the instrument status. This step prepares the instrument for use, scans the status of internal subsystems, and reports this status information as needed. In some embodiments, the instrument may have a manual on/off switch. For these units with a manual on/off switch, this step begins when the power is applied.
A second step is to wait for the punching bag to settle. The punching bag typically rocks and moves after a strike, or when being moved into position. This step ensures that the punching bag has settled prior to acquiring the user's strike.
A third step is to wait for the user to strike the punching bag. The instrument monitors punching bag accelerations, rejecting pre-shocks, and finds the start time when the strike accelerations begin. This step also supports a battery save and sleep/wake functions. For example, if no strike is detected over a predefined period of time, the instrument may automatically move into a sleep mode, saving battery life. In some embodiments, the instrument may be powered by a power adapter. Where powered in this method, the sleep mode turn-on time may be delayed or suspended.
A fourth step is to collect data after the starting strike has been sensed. For some period of time, typically a fraction of a second, the instrument logs accelerometer data starting just prior to the start time.
A fifth step is to find the ending time for the strike and reject the aftershocks. The instrument searches through the logged acceleration data and finds the time when the user's strike has been completed. The data logged after the user's strike has been completed is considered aftershocks, and are not used to compute the performance metrics.
A sixth step is to compute various performance metrics for the strike. Given the acceleration values between the starting and ending times, the instrument can find and compute various strike performance metrics for the user to interpret.
A seventh step can present the strike's performance metrics to the user. The instrument may include one or more displays that present performance metrics in numeric, text, audio or graphical formats. As used herein, the term “display” may refer not only to a visual display, such as on a screen, but may encompass any presentation to the user, such as via an audio presentation, graphical presentation, numeric presentation, textual presentations, or the like. The performance metrics can be presented until the user acknowledges the values, or it can present information for a short period of time and then proceed to the next step. In some embodiments, the instrument may connect to a computing device (for example, via a cable or wirelessly) to deliver the performance metrics to the computer device to be stored.
An eighth step directs the instrument to loop back to the second step, to wait for the punching bag to settle.
These eight steps were defined as shown above for clarity; different implementations may combine, split, or rearrange these steps into similar processing.
The instrument may be implemented in various manners, including a USB accelerometer on a PC with MATLAB®, a single board computer with built-in three axes accelerometer, or as individual components assembled to produce self-contained units.
Time was chosen in the processing description because the strike occurs over some period of time. Internally the processing may use an index or some other term to distinguish between different acceleration values. In some cases, time refers to the corresponding accelerometer values collected at that time.
Reject means to attenuate effects from pre- and/or after-shocks and is easy to conceptualize in the time domain, but filtering in some other domain (e.g., frequency) may be used.
The above steps are described in greater detail below.
Step 1 can measure the state of the battery and verify the accelerometer is operational. The display can show the instrument version number, the battery state, and accelerometer or other error conditions that may be detected.
Step 2 can compute values that are used in Steps 3, 4, 5, and 6. Step 2 can compute means and standard deviations from the accelerometer(s). In some embodiments, the punching bag is considered “settled” when the standard deviations fall below a “settled threshold value”. This small threshold value could be set by the accelerometer noise level, computed in real-time, be a predetermined noise level of the training environment, or be a fixed preset factory value. When the “settled threshold value” is met, the instrument moves to the next steps where mean(s) are used as offset(s) and subtracted from accelerometer values. Other techniques could be used to determine mean and standard deviation like values, such as IIR or median filter, max/min values, or the like.
Step 3 can determine when the user strikes the punching bag. The strike begins when the signed and offset accelerometer values exceed a “signed start of strike threshold” that indicates the punching bag is moving away from the user. The “signed start of strike threshold” is a small trigger value that may have a time duration constraint. The time duration can prevent a short noise burst from triggering the start time. The “signed start of strike threshold” could be preset or calculated similar to the threshold value in Step 2. The signed acceleration value (not the unsigned acceleration magnitude) is needed to determine which direction the punching bag is moving, and in this case moving away from the user. If an unsigned (e.g., magnitude) value were used, then the instrument may incorrectly include extraneous motions, for example when the user steps toward the punching bag, the floor supporting the punching bag may deflect, and the punching bag will actually move toward the user just prior to the strike. Although an unsigned threshold value could be used, it may produce less accurate results.
Once the starting time for the strike is detected, the instrument can log the starting time, include a small number (e.g. 10) of samples prior to the strike for diagnostic purposes, and move to Step 4. If after some period of time (e.g., a few minutes) a strike is not detected, the instrument can enter a battery save mode that reduces the power consumed by the LED display. Further inactivity can cause the instrument to enter a sleep/wake mode where power consumption is minimized. In either case, the instrument can be activated by a (gentle) strike or by cycling the optional on/off switch.
Step 4 can start collecting accelerometer data after the start of the strike has been detected in Step 3. The data collection duration is typically a fraction of a second. The duration could be preset (e.g., 0.1 seconds) or could be dependent on the observed acceleration values. After the accelerometer values for the strike and the likely aftershock data have been collected, the instrument moves to Step 5.
Step 5 can determine when the strike has ended. The current technique finds the time where the signed and offset accelerometer values cross a “signed end of strike threshold” that may have a time duration constraint. The time duration and threshold can be determined as discussed in Step 3. Crossing this threshold indicates the punching bag direction (for the one or more axes that are being monitored) has reversed. This time is the ending time of the strike, but may be corrupted by aftershocks. The ending time is used to set an upper bound on processing in Step 6.
Step 6 can use the accelerometer values (signed and offset), between the starting and ending times, to compute various strike performance metrics. These performance metrics include:

- a) Finding the maximum acceleration magnitude. The processing can find the largest strike magnitude between the start and end times. The magnitude could be computed from one or more accelerometer axes, for example:
  - Sum of the absolute value of accelerometer values, or
  - Sum of squared accelerometer values, possibly with square root applied to that sum.
- b) Strike duration can also be computed in different ways, for example:
  - Subtracting the strike start time from the end time: This approach may be corrupted by the effects of aftershocks when finding the ending time,
  - Subtract the start time from the maximum acceleration time, and then double this difference: This approach uses the parts of the acceleration waveform prior to aftershocks potentially corrupting the accelerometer signal and assumes the peak occurs at the midpoint of the strike, or
  - Compute both duration values described above and compare for consistency, or combine (e.g. average) to improve the estimated strike duration.
- c) Computing the impulsive acceleration, a value that parallels the well-known impulsive force, and integrating (with respect to time) the acceleration magnitude between the start and end times. As was discussed in (b) above, the time integration may be limited to the time between the start and peak time and the reported value scaled accordingly (e.g., use the midpoint and double the computed impulsive acceleration).
- d) The accelerometer values could be fit to a curve (e.g., a polynomial) using a least squares or other curve fitting technique. The peak magnitude, strike duration, impulsive acceleration, and the like, could be computed from the curve fit solution. As discussed in (b) and (c), the values between the start, midpoint, and end may be adapted to fit the curve.

These found and computed values are available for display to the user in Step 7. Step 7 can present the full, or a subset, of the values acquired, found, or computed in the steps above. Numeric and text values can be presented on multiple displays, or displayed sequentially. Plots can be presented on graphical displays. This information can be presented immediately after each strike, compared with recent values, or logged for presentation or analyses at a later time, possibly using a computer to process individual or related strikes. After the performance values have been displayed, the instrument moves to Step 8.
Step 8 completes the processing of the strike, logging any needed values, and automatically loops back to Step 2 where the sequence is repeated.
The instrument, in its simplest form, is an easy to use upgrade to an existing free-standing punching bag. The user removes the instrument from the shipping box, removes the hook and loop (such as Velcro®) tape covers, and attaches the instrument to one of the ledges at the base of the punching bag, where it can be easily seen. When the unit is turned on, the user first observes the status of the unit, followed by a message that the unit is calibrating (e.g., message says “Cal-” with the hyphen toggling on and off). When the punching bag has settled, the average acceleration value for each accelerometer axis is saved and the “Go” message is displayed. The user can then strike the padded portion of the punching bag in the area above where the instrument is mounted. The instrument will immediately compute performance values and present them to the user. Each presented value is displayed briefly, and when all values are presented, the instrument cycles back to the “CAL-” step while the punching bag settles.
The sample rate for data collection should be high enough to support accurate computation of the performance metrics. For example, the instrument may sample 1000 3D acceleration samples per second. The accelerometer values acquired during the settling time can be used as offsets (subtracted from accelerometer data) for the actual processing. Gravity imposes a 3D 1 g acceleration that is vectorally sensed by the accelerometer. The actual reported values are dependent on the 3D orientation of the single or multi-axes accelerometers. Gravity provides a readily available calibration source, but is always vectorally affecting the acquired accelerometer values. The offset(s) acquired during the settling time include the gravity effects, and these offset(s) can be vectorally subtracted from the acquired accelerometer data. Hence, with offsets applied, the computed acceleration for a settled punching bag is zero for all axes. After a strike, non-zero computed acceleration values indicate the punching bag is moving and the sign of the acceleration indicates which direction the punching bag is moving, including the rotated effects of gravity.
The signed acceleration values are used for determining the start and end times for the strike, and these start/end times are used to help reject the pre- and after-shocks that floor standing punching bags exhibit. It is common for the aftershocks to be stronger than the user's strike that set the punching bag in motion, and hence the most accurate values are obtained when the aftershocks are not processed as part of the user's performance metrics. Similarly, failure to reject pre-shocks may reduce the accuracy of the performance metrics.
The ending time approximates the time the strike ended, but depending on the type of strike that was executed, it is possible that aftershocks and gravity may affect sensing the strike end time. Although the strike end time is always measured, it is possible to use it only to bound the search for the maximum acceleration, and then, for example, replace the data between the maximum and end strike with a mirror image of the data from the start to the maximum acceleration. In any case, the search for the maximum acceleration is bounded by the start and end times, and after the maximum acceleration value and time are found, those values can augment the computation of other performance metrics.
A minimal system, with limited capabilities, could be implemented using a subset of the elements shown in FIG. 2. For example, motion can be sensed with a one-axis accelerometer (1D). The accelerometer data rate can be reduced from the current 1000 3D updates per second to a slower rate, possibly 100 1D updates per second. One or more elements may be removed from the processing block, such as check/report status, wait for punching bag to settle, wait for start of strike, battery save or sleep/wake, find end of strike, find peak acceleration, compute performance metrics and present results.
A somewhat more automated approach would be to use “hard-coded” or “dynamically allocated” bounds for the start and end times with respect to sensed motion (either magnitude or signed). The signed and offset method described in Steps 1-8 was developed to provide more robust detection of start and end times.
The instrument could be modified by developing a model for the dynamic response of the punching bag for various types of strikes. A strike's forces, height, and trajectory affect the punching bag acceleration. The punching bag acceleration has two major components: tipping and skidding. A model that could decompose strike accelerations into tipping and skidding effects may produce more accurate results.
A related modification is that the punching bag accelerations are the convolution of the punching bag impulse response function and the vector force applied by the user as function of time. If the impulse response function were known, the applied user force could be computed by de-convoluting the punching bag impulse response function from the measured accelerometer values. The computed user force result would expand the performance metrics that could be analyzed.
Acquisition of data collected from additional accelerometers or gyroscopes on the punching bag would provide a basis for resolving the actual motions the punching bag was undergoing. This type of processing would expand the instrument's processing capabilities, including resolving punching bag motion with respect to the earth's gravity vector.
If the accelerometer is mounted along the center vertical axis (up-down) of the punching bag, then the user could strike the padded part of the punching bag from any side and receive comparable performance measures. Currently, with the accelerometer displaced from the center axis, the acceleration values are affected by the strike location.
Future versions of the present invention could have the instrument (with internal or external sensor, for example) integrated directly into the punching bag, possibly as part of the punching bag manufacturing process.
For proof of principle, the first instrument was implemented using a USB accelerometer, PC notebook computer, and MATLAB. The USB accelerometer plugged into the notebook and attached to the base of the punching bag. The accelerometer included MATLAB compatible drivers. A specialized program was written in MATLAB to implement the instrument's functions shown in FIG. 2.
The second generation unit was based on a module that had the accelerometer, processor, and display on a single computer board. The processing was written in (the computer programming language) C, and implemented the instrument's functions shown in FIG. 2.
In some embodiments, the instrument has the accelerometer housed in the box with the processing unit and display. This approach provides the most compact and simplest configuration, but limits the accelerometer values to the accelerations imparted upon the box at its position and orientation. In other embodiments, the instrument has an external accelerometer wired to the box. This external sensor can then be mounted at the bottom of the base of the floor standing punching bag, at the top of the punching bag, or the like. The mounting point affects the acceleration sensed by the accelerometer. In should be noted that the box (instrument), processing, and display could also be separated.
A swinging mass (e.g., dumb bell) could be attached by a swing to just touch the padded part of the punching bag when at rest. The mass could then be swung back some distance and released, striking the punching bag, and performance metrics could be computed. This swinging mass technique could be used to validate and calibrate the instrument of the present invention.
Using a swinging mass setup similar to the above, the punching bag can be replaced by a device that holds a wooden board (e.g., 12″×12″×1″ board), with the resting swinging mass just touching the board. The device that holds the board could replace the padded portion of the punching bag. By experimenting with multiple boards and swing cycles, the starting position of the swing to break a board could be determined (similar results may also be obtained by dropping a mass from height (h) onto the board). Next, the board can be replaced with the punching bag, and the mass can be swung from the same height position. The resulting performance metrics provide an estimate of the score needed to break a board. This process could be duplicated with thinner, thicker, or multiple boards, other objects, such as cement slabs, or objects that even simulate bones.
At the top of the swing position, the mass has a potential energy that is the product of the mass, gravity, and height (PE=m g h). When the mass swings down and is at the bottom of its swing arc, it has a kinetic energy that is the product of ½, mass, and velocity²(KE=½ m v²), to a first approximation, PE=KE and hence the swing's mass and height can be adjusted to deliver desired amounts of energy and velocity at the bottom of the swing arc.
FIG. 3 shows partially processed plots of 3D instrument data for a “palm strike”. The larger plot has 0.1-second duration and is sampled at 1000 3D points per second. Each increment on the horizontal axis is 0.001 seconds. The larger plot is the full dataset, and the smaller inset plot has data for x=1 through x=15.
The Gx, Gy, and Gz plots are the accelerations along the x, y, and z axes, respectively. The G2′ plot is the squared sum of the three accelerations (G2′=Gx²+Gy²+Gz²). The G2′ plot dominates the positive values. The Gz plot is the most negative going plot and crosses positive near x=31. In this configuration, the Gz term is used to find the start and end times of the strike. The Gx and Gy terms only contribute to the G2′ value.
The sign of Gz is important, as it shows the punching bag is moving away from the user when Gz<0 and towards the user when Gz>0. From the large plot it can be seen where the Gz values go negative, thus the strike actually begins at x=10 and continues until x=31.
The inset on the right side shows a subplot from x=1 to x=15. It should be noted that Gz has both positive and negative values caused by the user planting a foot near the punching bag just prior to the strike. The punching bag can be seen tipping slightly toward the user (positive value) in the vicinity of x=4, then tipping away (negative) near x=6, and after additional pre-shocks, finally tipping away (negative) at the actual start time near x=10.
Aftershocks are shown that peak near x=35, x=45, and X=50. Two of these (x=35 and x=50) have higher values than the strike.
For this data set, the instrument processing would reject pre-shock values from x=1-9, process strike values from x=10-31, and reject aftershock values from x=31-99.
FIG. 4 is a simplified flow chart for an instrument of the present invention. Although shown as one flow process, in some embodiments, the instrument implements this flow as three threads running in one CPU. This figure corresponds to the descriptions given above.
It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

1. A method for measuring performance metrics of a strike, the method comprising:

allowing a striking object to settle to a predetermined settled state;

collecting accelerometer data describing motion of the striking object before, during and after the strike;

determining a start time and an end time of the strike; and

rejecting data before the start time and after the end time of the strike.

2. The method of claim 1, further comprising computing the performance metrics of the strike to a user.

3. The method of claim 2, wherein the performance metrics include at least one of a maximum acceleration magnitude, a strike duration and impulsive acceleration.

4. The method of claim 2, further comprising presenting the calculated performance metrics to a user.

5. The method of claim 1, wherein the striking object is a free-standing punching bag.

6. The method of claim 1, wherein the accelerometer measures data in three dimensions.

7. The method of claim 1, further comprising calibrating an instrument attached to the striking object prior to collecting accelerometer data.

8. The method of claim 7, wherein the step of calibrating the instrument includes swinging a mass that hangs adjacent to the striking object while at rest, into the striking object.

9. The method of claim 8, further comprising delivering kinetic energy to the striking object with the mass in an amount known to achieve a particular result, including strike velocity.

10. A system for measuring performance metrics of a strike to a free-standing punching bag, the system comprising:

at least one accelerometer disposed on the punching bag;

an instrument adapted to receive accelerometer data from the at least one accelerometer; and

computer software, written in one or more computer codes, disposed on a computer readable medium, and adapted to determine a start and end time of the strike, reject data before the start time and end time, and compute the performance metrics.

11. The system of claim 10, further comprising a display to show the performance metrics to a user.

12. The system of claim 10, wherein the computer software is adapted to further determine a settled state of the punching bag and provide a go signal to a user to begin their strike.