Measurement instruments for finger and thumb forces in hand grip applications including guitars

Abstract

A measurement instrument for in situ measurement and indication of finger and thumb forces when gripping hand held objects comprising one or more thin-film force transducer(s) (13, Fig. 7), one or more electrical circuits which will electronically process the transducer output signal or transducers output signals, a meter (7, Fig. 11) or other display and loudspeaker (9, Fig. 12) so as to indicate the instantaneous force or forces to a human observer in visual and audible form. The data may be stored in a computing device and replayed at a later time. The hand held object may be the neck of a guitar or similar stringed instrument or the handle of a racket or tool. The force can be synchronised to a piece of music. Forces from several fingers and thumb can be detected simultaneously. Another aspect is a method of calibration using guitar string tension.

Description

This invention describes an instrument and associated methods for the measurement of thumb and finger forces using a thin resistive force transducer and the indication of the measured values in a manner convenient for the human observer. The forces to be measured include, but are not limited to, those exerted from thumb to an object, or finger to an object, or the pinch force between thumb and finger or fingers, or finger and thumb grip on an object. The application is universal for hand applications and especially advantageous when the physical space available is limited and the invention solves the problem of measuring finger and thumb forces in situ, whereby the uncertainty of the measurement can be reduced and accuracy increased. The invention accomplishes this by placing one or more thin flexible resistive force transducer(s) into place on the handle or gripping surface(s) of an object in a way which conforms to its shape and connects the resistive output terminals of the transducer into one or more electrical circuit(s) to achieve a novel measurement system. The transducers are lightweight, 0.6mm thick and available in sizes ranging from 10mm circular, through 40mm square to 50mm high and 300mm long. They are flexible and may be fixed permanently or in a demountable fashion using adhesive tape or clips to flat and curved surfaces. One such application is guitar playing where it is required to measure the pinching force exerted by a player’s thumb at the rear of the neck and fingers at the front of the neck as when pressing one or more of the strings of the guitar against the surface and frets - the metal bars set into the front of the guitar neck. Another example application is for the measurement of the hand grip force on the handle of a sports device such as golf club and tennis racquet. A further example is the hand grip force on the handle of a workshop tool such as a hammer or chisel. A further example is the grip force on the handle but by a non-human gripper - such as a robotic hand or manipulator . These force measurements would be of interest to a wide range of people including but not limited to product developers and users, product, sport and music trainers who have a care for optimising the design and use for safety and ergonomic reasons, medical staff- physiotherapists, doctors treating hand and arm disorders and biomechanic design engineers. In this description, the example of the guitar will be used. However, it is to be realised that the invention can be generalised to apply to many other hand-held tool and hand grip situations. The description herein uses the left hand as the objective and it is to be realised that the invention can apply to both hands and thumbs and all fingers. In a guitar playing situation the force exerted by a player’s thumb on to the neck of the guitar is of interest to many parties including the player, guitar teacher, medical practitioner, guitar maker. Objective knowledge of this force in standard scientific terms - i.e. Newtons or kilograms and a means to indicate the force to a human observer are useful in a number of ways as explained in the following. A pupil guitar player and their teacher will be aided by knowing this force during instruction and hence by this invention - it will generally be advantageous to minimise the force, commensurate with musical sound quality, as so doing reduces hand fatigue and improves mobility of the hand. A medical practitioner e.g. physiotherapist, hand surgeon or bio-mechanical engineer will be aided by knowing this force and hence by this invention because it will help to diagnose thumb / finger medical conditions, assist in recovery from treatments and assist in design of biomechanical aids. The invention will now be described solely by way of example application to guitar playing and with reference to the accompanying drawings in which: Figure 1 shows the bones of the human hand which are of interest. Figure 2 shows a guitar and the left hand on the neck of the guitar gripping the guitar neck Figure 3 shows in plan-view a guitar playing left hand the left hand and the direction of the forces required of the thumb and fingers Figure 4 shows a drawing of a square thin resistive transducer 50 x50 x 0.6mm Figure 5 shows the resistance vs force characteristic or the 50 x 50 mm transducer Figure 6 a drawing of a larger thin resistive transducer 50 x200mm Figure 7 shows a guitar with the placement of the smaller square thin resistive transducer on the neck of the guitar Figure 8 shows a guitar with the placement of the longer thin resistive transducer on the neck Figure 9 shows an electrical block diagram with visual meter indicator of force with an optional continuous recording and playback functional element Figure 10 shows a circuit block diagram with both visual meter and acoustic indicators of force Figure 11 shows a simple circuit schematic with visual meter only Figure 12 shows a circuit schematic with visual meter and acoustic output Figure 13 shows a block diagram arrangement of the continuous measurement system with a data recorder and playback Figure 14 shows a force diagram with a relatively low string deflection Figure 15 shows a force diagram with a relatively high string deflection Figure 16 shows a realisation for calibration of force using a cam in a low position Figure 17 shows a realisation for calibration of force using a cam in a high position Figure 18 shows an application scenario of a guitar with a force indicator box Figure 19 shows an application scenario of a guitar with a continuous recorder I playback Figure 20 shows the invention for a multi sensor and finger tip force sensors Figure 21 shows the functional block diagram for a multi sensor and fingertip force sensor system. Referring to Figure 1, one significant medical condition is arthritic degradation of the thumb joints, and there is a well-known and prevalent problem at the three main joints - 1) carpometacarpal (CMC), 2) metacarpophalangeal (MCP) and 3) interphalangeal (IP). Such degradation is accelerated by over use and over exertion of the thumb when gripping objects -handles etc., including the neck of a guitar, especially when executing the so - called “barre” chord shapes at the first and second fretboard positions e.g. Fmajor, Bmajor etc. Similarly the muscles, tendons and joints of the fingers may all suffer damage and hence be the objects for studying forces acting on them. Referring to Figure 2 this shows the front elevation view of a guitar and player’s left hand and Figure 3 the corresponding plan view showing the left-hand position when playing such barre chords. These positions require the greatest thumb force pressing on the rear of the guitar neck to facilitate the opposing force of the finger or fingers to simultaneously press down the strings on to the fret board at the front of the neck, securely enough so to avoid any string buzz (when a string is not quite clamped against the metal fret). Figure 3 indicates the direction of the forces by the arrows. This type of barre chord shapes and other left hand manipulations are well known to guitar players as hard to achieve, take much practice and cause much fatigue. Frequently players will over - exert the thumb to achieve the musical effect - and when this is repeated during the career of the player, leads to premature failure of one or more thumb joint causing pain and loss of musical ability. Figure 4 shows a drawing of a thin, relatively low-cost resistive transducer, purchased as a standard manufactured item, such as the type SF45-65 from the seller Fungee33. It is 50 mm square and 0.6mm thick with two output terminals for connecting to ensuing circuitry. It has an active area of 45 x 45mm, meaning this is the space which responds to force, by varying the resistance measured at the terminals. The transducer has the characteristic of resistance versus force applied to its active area given by Figure 5. Figure 6 shows a drawing of a longer transducer of similar thickness and width. Figure 7 shows how the invention conveniently measures force and indicates the force to a human observer, by placing the thin resistive force measurement transducer 1 at the rear surface of the guitar neck 2 in a demountable and unobtrusive way using thin PVC or other adhesive tape. Figure 8 shows how a longer transducer 3, running over a great length of the neck 2 can be demountably and unobtrusively affixed to the rear of the guitar neck, for measurement of thumb and finger forces over the greater length of the neck which is advantageous for analysis of hand forces in the so-called higher positions on the neck. It is to be appreciated how such a thin flexible transducer can be fixed in place so as to be minimally noticeable to the player and minimally affect the application of the players hand forces, which serve greatly to improve measurement capabilities returning representative measurements with minimal uncertainties and maximal repeatability. Figure 9 shows a simple application block diagram whereby the thin transducer is connected into an electrical circuit to a source of DC power, such as a 9 volt battery, a processing circuit which translates the resistance of the transducer to current (for example) and indicates the instantaneous force visually on a current meter, which is calibrated to indicate force in kilograms (kg), for example in the range 0 to 6 kg. Or in Newtons over the range 0 to 60N. To make the measurement indication more easily comprehended by any user - for example with the player or the guitar teacher in a real teaching scenario - it is an objective of the invention to provide an audible indication of the force. This is achieved by extending the circuit to include a resistance-to-audio musical sound function and a loudspeaker, for example whereby the force range 0 to 10kg is scaled to an audio musical chord sequence or pitch range, with the following mapping of force to audio tone pitch: Force in kg 0 1 2 3 4 5 6 7 8 9 10 Audio Frequency, Hz 82.4 92.5 103.8 123.5 164.8 207.6 247.0 277.2 329.7 415.4 440.0 Musical Note E2 F#2 G#2 B3 E3 G#3 B4 C#4 E4 G#4 A4 This block diagram is seen in Figure 10. The simplest electronic circuit schematic is seen in fig 11. The voltage regulator integrated circuit 4 provides a stable voltage which supplies the circuit including the thin resistive force transducer 5 and the adjustable element 6 is provided for calibration purposes. The meter 7 provides a visual indication of the force. Figure 12 shows a schematic for the electronic circuit which provide both acoustic and visual indication of force such circuitry can be built and housed in a single case. A voltage to frequency (pitch) conversion is based here on the well-known “555” timer chip 8, and the acoustic output is on the speaker 9. It is to be appreciated that these are singular examples for how the force indication can be provided - other realisations and embodiments are possible. These examples are based on a wired metallic connection between the thin film transducer on the guitar neck however a wireless connection can also be realised using radio or optical signals and suitable additional circuitry. Such wireless connections would be advantageous to remove the physical constraint between the guitar, or other type of object on which force is to be measured, and the measurement apparatus. Yet a further application example for this invention is where the measured thumb force is not only indicated real time - visually and audibly as explained before - but also recorded over a period of time in a real chart recorder or on a laptop or pad or mobile phone application. As shown conceptually in the block diagram of Figure 13. This is useful in that it also provides a valuable insight into the force on the thumb and fingers during the epoch of a particular piece or pieces of music, which can be time correlated to the score of the music. This would help the player develop their playing technique. One useful technique employed by guitarists and taught by teachers to minimise the force on the thumb is to grip the guitar body and neck between the right arm and the player’s body, thereby to define the neck position so that the fingers can press on the strings with less or no thumb force required. The technique is assisted by the following development of the invention. The thin film transducer and system will be calibrated using a number of weights, for example at least 3 in the range 1 to 6 kg; each weight is set on to the transducer and the electrical reading noted on the measuring device and used to define the force to be measured. The circuit may use adjustable components to provide calibration function as described earlier and seen in Figures 11 and 12. It is also an objective of this invention, for applications to the guitar or similar instrument, to provide a means to quickly check the force transducer and its calibration, without the need for heavy and inconvenient weights. Such a calibration would aid users should there be any doubt about the values being shown. The method uses the tensioned strings on the guitar as the force. Guitar strings are carefully made and known to be tensioned in a calibrated way to establish the correct pitches, and the total tension in all 6 classical guitar strings of “normal” tension will be a known value around 45kg. High and low tension strings are available, and in any case, this precise value will be known by the user. Figure 14 is a force diagram and shows how the normal tension strings, assumed here as 45kg total in a guitar with a standard 650mm scale length, i.e. distance between the saddle and the nut, if raised at the 7th fret by a vertical distance of 6.35mm, there will be a force acting vertically of 1.99kg. Figure 15 shows that if the strings are raised vertically by a greater height of 12.85 mm, then the force acting vertically will be 4.03kg. Figures 16 and 17 show a practicable embodiment for this force calibrator. It shows the force transducer placed with its central planar axis over the 7th fret on a thin metal plate of steel or aluminium or brass. A rotatable cam with an elliptical profile having a major axis 12.85mm and a minor axis 6.35 can be inserted under the strings and over the force transducer, connected to its processor and indicator circuit. When placed with the lower height of 6.35mm, the force indicated can then be read off a meter or the output device and verified that it reads 1,99kg; or if not, the calibration is adjusted accordingly. Then the cam is rotated 90 degrees to raise the strings to the new height of 12.85mm, when the force shown should be 4.03kg. Thereby with this two - level method the accuracy, range and linearity of the forces indicated can be checked for increased confidence. Figures 18 and 19 show two overall application scenarios possible according to the descriptions given earlier; Figure 18 shows the simple visual and acoustic box connected to the transducer for instantaneous values and Figure 19 the arrangement for continuous recording and playback. It is to be realised that this latter system in a guitar (or other musical instrument) could be used to show forces synchronised against the music score being played. This could be beneficial to optimise a player’s performance. It is to be realised that other embodiments exist including where the thin force transducer is permanently built into a measurement instrument and where it is alternately temporarily fixed. In a further development of the concepts previously described, it is an objective of this invention to extend the measurement of force to the finger tips in addition to the thumb - such that an indication of overall thumb / finger force measurement is obtained. This has the use of showing the nett force acting in the pinch or grip action between thumb and fingers. The benefits and purpose are explained next. In a guitar playing application it is beneficial to reduce thumb force to reduce wear and tear on the thumb joints, provide more agility when playing and reduce fatigue. Guitarists can be trained to achieve this instead of using pinch force between thumb and fingers, by holding the guitar body more rigidly by clamping it between their body and right arm, so that the fingers press on an immobilised neck. Figure 20 shows how this is to be achieved by placing small thin force -resistance transducers, 10mm diameter on the index, middle, ring and optionally little finger tips on the left hand using flexible finger-tip sleeves made from possible materials including thin rubber of a type used in domestic rubber gloves. Figure 21 shows how the transducer outputs can be combined. The resistance of each of these three or four transducers is individually measured, converted to voltages, scaled to represent a kilogram force, in the processing circuits 1 to 3 or 4. These individual voltages Fl, F2, F3 and optionally F4 are summed to represent a total finger force Ff acting from the front to the back of the guitar fretboard, pressing down on the strings.. Ff=E(Fl,F2, F3) The thumb force Ft is measured and as it is acting from the back toward the front of the fretboard, it is processed as a negative quantity. Such that if only the thumb and fingers are providing the activation of forces Ff-Ft = 0 i.e. the force forwards balances the force rearwards However, by using the players body to hold the guitar between the player’s body and right arm, Ff can be maintained yet Ft reduced so that Ft<Ff This invention allows Ft to be measured and signalled to the player in -situ to help develop this playing technique and to improve the body grip and further to signal the force balance between Ft and Ff, by calculating and indicating Ff-Ft Guitar teachers will or should encourage players to use the lowest thumb force, however until this invention there has not been a method to objectively indicate to a teacher, nor the pupil, the force actually used and how the player can optimise their playing position.

Claims

1. A measurement instrument for indication of finger and thumb forces when gripping hand held objects which comprises: one or more thin-film force transducer(s) whose electrical resistance is dependent on the compressive force exerted thereupon, a battery or batteries, an electrical processing circuit(s) which convert the resistance of the thin-film force transducer(s) to an analogue voltage or current, an electrical connection interface between the thin film force transducer circuitry and the succeeding circuitry, a meter or other visual display, a voltage to audio frequency converter circuit connected to a loudspeaker, wherein the battery or batteries supply power to the circuitry; wherein the thin-film force transducer(s) and its electrical processing circuit is placed in one enclosure; wherein the meter or other visual display, battery, voltage to audio frequency converter and loudspeaker are placed in a second enclosure to indicate the force measurement to a human observer; wherein the electrical connection interface between the first and second enclosures uses metallic wires or alternately wherein the electrical connection interface between the first enclosure and second enclosure uses wireless or optical means by the addition of electrical circuitry to provide the wireless or optical connection2. A measurement instrument according to claim 1 in which the electrical processing circuit comprises the thin-film force transducer element or elements, the battery, signal processing and scaling components and a meter which is scaled, calibrated and marked to show to a human observer the instantaneous thumb and finger force or forces acting on the thin-film force transducer or transducers in a visual manner shown in units of kilograms or Newtons25 10 243. A measurement instrument according to claims 1 and 2 which shows to the human observer the force acting on the thin-film force transducer or transducers using electrical circuitry to convert the thin-film force transducer output signal to an acoustic tone or tones whose pitch and harmonic relationships are proportional to the instantaneous finger and thumb force, said relationships being scaled and calibrated according to the force acting on the thin-film force transducer shown in kilograms or Newtons4. A measurement instrument according to any of the preceding claims whereby in addition to the measurement and indication of instantaneous finger and thumb force by visual and acoustic means, the force over time is recorded continuously in a chart recorder, or digitally in a computing device such as laptop, pad or phone or other data logging device as an analogue or digital data representation which will provide for replay of the force over-time at a future date and time5. A measurement instrument according to any of the preceding claims 1 to 4 whereby the thin-film force transducer(s), battery, electrical processing circuitry, the meter or other visual display and loudspeaker components are integrated into a single physical embodiment enclosed in a single casing6. A measurement instrument according to any of the preceding claims which may include a means to calibrate it within the electrical processing circuitry or within the meter or other visual display by inclusion of an adjustable element which sets the value shown when loaded with a known force to provide measurement indications to specifiable accuracy limits to known standards in kilogram or Newtons.

7. A measurement instrument according to any of the preceding claims whereby two or more thin-film force transducers are connected to two or more corresponding25 10 24electrical processing circuits so that multiple fingertip or thumb tip forces are simultaneously measured and displayed to a human observer8. A measurement instrument according to any of the preceding claims wherein the handheld object is a guitar or other stringed necked instrument or a piano and other keyboard instruments wherein the physical size and force measurement range is made appropriate to the musical instrument to measure the finger and thumb forces9. A measurement instrument according to any of the preceding claims 1 to 8 whereby the thin-film force transducer or transducers, electrical processing circuit, battery, voltage to audio frequency converter circuit, loudspeaker, meter or other visual displays are fixed in a removeable manner on to the object’s handle or the rear of the neck of a guitar or similar necked musical instrument at positions along its length determined by the required hand position or positions, thereby to measure the instantaneous force applied by the user’s thumb on the handle or guitar player’s thumb to the neck.

10. A measurement instrument according to the preceding claim 8 whereby the thin-film force transducer or transducers, resistance to a voltage or current transforming circuit, battery, voltage to audio frequency converter circuit, loudspeaker, meter or other visual displays are fixed in a permanent manner on to the object handle or rear of the neck of a guitar or similar necked musical instrument11. A measurement instrument according to the preceding claims 8 to 10 whereby the force applied by the user’s thumb on a handheld object’s handle or player’s thumb to the guitar neck is recorded continuously over a period of time for replay at a later time and whereby the continuous force is synchronised to a piece of music.

12. A measurement instrument according to the preceding claims 8 to 11 whereby in addition to the thin film force transducer affixed to the rear of the guitar neck, small25 10 24diameter thin film force transducers are removably fixed to the player’s finger tips and each of their outputs processed in electrical circuits and combined with the thumb output so that finger and thumb forces can be simultaneously instantaneous measured and indicated or simultaneously continuously measured for replay of finger and thumb forces over a period of time13. According to claims 8 to 11 a means to calibrate the measurement instrument for greater accuracy using the tension in the strings of the guitar or similar stringed instrument by employing additional components including wedges, elliptical cams and mounting plates to translate the known axial tension of the strings to a known force or forces acting perpendicular to the thin-film force transducer, whereby the known force in kg or Newton can be used to set an electronic circuit, meter or other visual displays or computer readable output, to a numerical value equivalent to the known force or forces.

Images