DE4435174C2 - Device for recording the applied forces and power on a crank - Google Patents

Description

The invention relates to a device for measuring the force and power on a Pedal crank, especially that of a bicycle, the sizes being force and Angular velocity converted into electrical signals and an evaluation device be fed.

A device that detects pedal forces on the bottom bracket by means of strain gauges and processed together with the angular speed of the crank in a microcomputer, in order to calculate the power from this is already described in DE 37 22 728.

Due to constructive measures, only the tangential part of the force, thus the torque on the crank is detected. A component of force in another Direction, approximately in the radial direction, is by means of the strain gauges load-bearing deformation element is not detected and can not be measured become.

An extension of the function for measuring other directions of force with regard to the crank was aimed for for the following reason:
As is known, the tip of the foot describes a simple circular movement in cycling. This is fixed by the pedal cranks. In contrast, the force curve for the rotational movement, which is applied by the muscles, is more complex. The driver lacks an opportunity to check this qualitatively. He only has his feeling here as an aid. So far, only the result of pedaling has been measured, the output and the output, the pulse.

But the display of the forces and provides precisely at the interface between the driver and the bike Forces additional information. Work is only done on the force component performed parallel to the direction of movement, but forces are perpendicular to the direction of movement suitable to tire the muscle without performing propulsion.

Test arrangements for have already been published in specialist magazines for doctors, athletes and trainers Pedal force measurements on stationary bicycle ergometers are described. From the Descriptions show that a pedal with a piezoelectric crystal sensor for Force measurement used in three directions between the subject's shoe and pedal has been. The disadvantage of this solution is that the rocking angle of the foot when pedaling between the pedal and crank must be taken into account with an extra angle sensor. (McLean Brian, Improving pedaling technique with "real time" biomechanical feedback, Excel, Belconnen, 5 (1988), Vol. 1, S 15-18 / / Ericson M O, Efficiency of pedal forces during ergometer cycling, international Journal of Sports Medicine, Stuttgart, 9 (1988), 2, pp. 118-122).

It is an object of the invention to design a pedal force measuring arrangement such that in addition to the power, the angle at which the force acts on the crank can also be measured. For this purpose, the respective crank angle should also be determined.

Another favorable embodiment is seen in the fact that the measuring arrangement can be retrofitted into any bike without using the crank or Pedal crank bearing to make changes.  

solution

According to the invention, this object is achieved by that the force by measuring the shear deformation on the crank pin with on it arranged strain gauges is determined, and in addition at least two pairs of strain gauges associated with each other different direction of force absorption at an angle directly on the Crank pins are arranged.

The measurement of shear deformation on a crank pin with it arranged Strain gauges are known per se. For example, in the Laid-open specification P 29 48 774.9 a force measuring device for a machine with Cantilever described, in which also by means of strain gauges on one The shear deformation is measured. Strain gauges are also here arranged on the pin so that the force of only one of particular interest Direction is measured.

On the one hand, the pin does not have the function of a crank pin on a pedal crank and on the other hand the one angular velocity is not detected, for example to calculate a performance.

Any force directions in the plane perpendicular to the tenon cannot be determined because the measurement is one-dimensional.

On a bicycle, the pedal axle has the function of the crank pin. The pedal axis (...) turns with the crank. A measured angle with respect to the pedal axis is consequently also an angle with respect to the crank.

The pedal body and the foot, however, lead primarily a circular but translational movement.

Achieved advantages

Because of the symmetry of the pedal axle, the deformability of the axle is under one applied force is the same in all directions. Therefore, the force angle and the force can be here can be determined equally well in all directions perpendicular to the axis.

Another advantage of placing the sensors on the pedal axis is seen in that the pedals are relatively easy to replace compared to the crank and crank axis can be and therefore a retrofitting of a bike so special is easily possible.

An essential feature of the invention is that it does not bend the axis but the shear deformation is measured.

Shear deformations on axes are generally less than deformations Bend. They require a more precise transducer, but have the advantage of being independent to be from the distance of the causal force.

With the pedal, the distance between the foot and the strain gauges depends on the placement on the pedal differently, and thus also the lever arm with regard to the applied force. Even when the shoe is fixed on the pedal, the lever arm can be different, depending after whether the force is more with the inside or outside of the toe is initiated.

The embodiment of the invention according to claim 2, that the shear deformation on the crank pin is absorbed in two directions perpendicular to each other and that the strain gauges are oriented at 45 ° with respect to the pin axis has the following advantages:
Because of the symmetry of the crank pin, deformations in a certain direction are a measure of the component of the force in this direction. By measuring in two known directions, two force components are obtained, of which the direction is known. The total force and the angle can be determined from this by vector addition. If you measure in two directions perpendicular to each other, the calculation is simplified:
Total force F _ges = √f + F and the angle a = arctan (F ₁ / F ₂₎ relative to the extensometer.

The strain gauges must be applied at an angle of 45 ° to the pedal axis, i.e. at an angle advantageous because the expansion maxima of the shear deformation are below 45 ° to the axial direction are located and the greatest sensitivity is achieved.

It has a particularly advantageous effect that strains that occur due to arbitrary moments the axis, do not affect the result of the shear deformation determination. If the strain gauges are exactly mirror images of the axis, then compensate for all bending strains within the measuring bridge.

To the particularly interesting power components: tangential and normal to the crank determine a microcontroller is used according to claim 3. From the tangential Force component can be multiplied by the angular velocity and the Crank radius calculate the power. The normal force component does not contribute to performance at.

If the pedal axis is provided with sensors and then into any one Screwed crank, the angle of rotation of the sensor to the crank is from Orientation of the thread dependent and not known beforehand. To not one here To make constructive change necessary, according to claim 4, the angle of rotation Sensor related to the crank after assembly, e.g. B. with a protractor, measured and, e.g. B. entered on the evaluation unit.

An embodiment according to claim 5 enables the angle of the pedal axis in screwed-in state with respect to the crank via a calibration measurement with known Determine force angle. For this, z. B. the pedal at the bottom dead center with a downward weight load. The difference between the angle at the calibration measurement is determined by the microcontroller, and the perpendicular would then be the Defined angle of rotation of the pedal axis to the crank, the calibration angle.

Another advantage of measuring the force on the pedal axis is that the angle of the Pedals or the foot can be disregarded with respect to the crank. It is only one Determination of the current crank position necessary.

An advantageous solution to this problem is characterized according to claim 6, that the crank angle at the time of measurement, from a position encoder, with a time measurement is determined. With continuous pedaling, the speed changes are within one period low. Therefore, especially when the period is constant constant angular velocity are assumed. Out of time and Angular velocity, the current angle is determined. Pedal breaks are on the changed period is recognized.

For determining the position, z. B. a permanent magnet as a signal generator on Frame and a reed switch as a signal sensor on the crank, or vice versa.

It is advantageous to display the measurement results in a field of view of the vehicle user arranged display field for feedback of the size, type and efficiency of its Force deployment.  

In the embodiment according to claim 7, the device has the possibility of a measured values record one or more pedaling periods and e.g. B. in a RAM or EEPROM save and the difference between two force profiles, especially between one currently measured and a previous one. If a force curve has been saved, where you only went very loosely, you get the muscle strength of the current kick. All constant forces, e.g. B. Weight of the leg, fall at the Difference formation away.

Due to the inertia of the legs, pedaling speed also occurs dependent forces on the pedals. These are compensated if there are two Pedal force curves with the same period are subtracted from each other. This is in Claim 8 proposed. , , ,  

Especially the not inconsiderable weight of the leg (and clothing / shoe) was the representation of the forces acting tangentially and normal to the crank circle and the actual information content cover up.

The pedal force display should not only serve to show the absolute force and power, but also to the cyclist also provide additional information about the type of kick: is only kicked down or also after pulled back or up to add an extra moment.

In the event of damage to the leg apparatus, the maximum values could be monitored; if under a Rehabilitation should not exceed the strength values specified by the doctor. The independent measurement Differences in leg strength and pedaling technique on both legs can be seen on both crank sides. It is also possible to compare your own pedal force curve with that of a better driver.  

The new thing about this device is that the pedaling force is determined not only by size but also by angle done and separated for both legs. This is made possible by measuring the pedal axis.

The display should be on any bike during normal use. This is the bike with the pedals belonging to the measuring device, the associated measuring transmitters on the cranks and the Equip display unit.

Further favorable configurations of the device are specified in the following exemplary embodiment.

The invention is explained in more detail below with reference to the drawings; in this shows:

Fig. 1 is a flow chart for the evaluation of the electrical signals obtained from the strain gauges and the magnetic switches (arrangement of Fig. 6).

Fig. 2 shows a force (F) applied to the pedal ( 12 ) or the pedal axis ( 11 ), which is divided into the components (Ft) in the tangential direction and (Fn) in the normal direction with respect to the crank ( 10 ); the crank angle (a) with respect to the position encoder (magnet) ( 15 ).

Fig. 3 shows the shear deformation of two square elements of the pedal axis ( 11 ), which deforms under the force (F). The shear deformation is exaggerated. The strain gauges ( 1 , 2 ), which are applied at an angle of 45 ° to the axis, are stretched ( 1 ) and compressed ( 2 ).

Fig. 4 shows in perspective how the strain gauges ( 1-8 ) are arranged on the pedal axis ( 11 ). The strain gauges are attached to the crank ( 10 ) and pedal bearing in the area between the pedal axle mounting. The force (F) symbolizes the force introduced into the pedal axis ( 11 ) via the pedal bearing.

Fig. 5 shows the deflection of the pedal axis ( 11 ) by the force (F) seen from the front. The component in direction 1 is detected by the strain gauges ( 1-4 ) and the component in direction 2 by the strain gauges ( 5-8 ). The total deflection and the deflection angle can be determined by vector addition of the two deflections. The deflection is approximately proportional to the force.

Fig. 6 shows a perspective view of the pedal crank with the pedals (12), the housing for the, sense amplifier and transmitter (14) to the crank (10), the housing for the data receiver (16) on the frame and the display unit (17 ) on the handlebars in the driver's field of vision.

Fig. 7 shows how the weight of the leg and the muscle strength at different crank positions add to the measured total force under these crank positions.

An exemplary embodiment of the invention is described in detail below with reference to the drawings:
Fig. 1: The two measuring devices for the left and right crank side are constructed identically. Therefore, only one (right) side is dealt with. The strain gauges ( 1-8 ) on the pedal axis ( 11 ) are combined to form two Wheatstone bridges. (The position of the strain gauge on the pedal axis can be seen in FIGS. 4 and 5.) The microcontroller switches on the supply voltage for the measurement via an electronic switch. The measurement signal amplifier and the sensors are only supplied with current for the measurement carried out periodically or at certain crank angles, in each case only 50 us. No electricity is consumed during the much longer breaks in between. With 20 measurements per second, the power consumption is reduced by a factor of 1000.

The bridge diagonal voltage is amplified separately for each bridge and by one channel of each A / D converter read. The time of measurement is saved together with the bridge measured values and evaluated in the later presentation.  

At the end of each pedaling period, when the magnet ( 15 ) triggers a signal in the measuring transmitter ( 14 ), a new time measurement is started in the microcontroller and the last one is checked for validity: if the period is in the measuring range and corresponds to the previous one, then no pedal interruption before.

The ratio of the measurement time to the period corresponds to the ratio of the crank angle to Full circle. The measurement times are converted as a percentage of the period.

The strain gauge measurement values are standardized using the angle of rotation that was determined during the calibration measurement. (A description of this follows at the end)

The data measured and buffered during the last pedaling period are transferred bit by bit to the Sent to recipient. To do this, the controller outputs pulses via the transmitter coil on the crank. These will picked up by the receiver coil on the frame, via an amplifier circuit and frequency division again sent to a microcontroller for further processing; means two pulses with a short interval a 0, two pulses with double spacing mean a 1. So the measurement data can be without Loss of accuracy can be transmitted.

Since both measuring transmitters are synchronized by their own magnets ( 15 ) on the pedal circumference and the cranks are offset by 180 °, and the transmission time is only a fraction of the period, there is a time separation of the transmission of both sides.

An identifier for the left or right crank side enables both data streams to be transmitted to the display unit via a common data line and processed further there separately. The further data processing is carried out by a microcontroller in the evaluation unit:
Storage and display of the measured values, calculation of the performance etc. , ,

Conversion of the strain gauge measured values into a standardized coordinate system.

The measured values are in a coordinate system with tangential and normal alignment to the crank converted. For this purpose, a calibration measurement is carried out after mounting the pedals in order to determine the Determine the angle of rotation of the strain gauge bridges with respect to the crank. The calibration program in the microcontroller is called up automatically after changing the battery. Below the defined crank angle, bottom dead center (UT) a vertical force (e.g. weight of the leg) is applied.

Designated for any force angle and crank positions
F ₁ : force component measured by strain gauge bridge ( 1-4 ), in direction 1 (according to FIG. 5)
F ₂ : Force component measured from strain gauge bridge ( 5-8 ), in direction 2
α: The angle of rotation of the strain gauge bridges with respect to the crank was determined in the calibration measurement

Force components determined during the calibration measurement with vertical force in the crank position UT (absolute value of the force is arbitrary, but could be used for calibration of the display)
F ₀₁ Force component measured by the strain gauge bridge ( 1-4 ), in the UT with vertical force
F ₀₂ Force component measured by the strain gauge bridge ( 5-8 ), in the UT with vertical force

Conversion formula into the tn coordinate system rotated relative to the strain gauge

F _n = F ₁ .cosα + F ₂ .sinα;

F _t = -F ₁ .sinα + F ₂ .cosα;

With

As an alternative to the inductive transmission of the measurement data via coils on the bottom bracket, this could also be done with a transmit high-frequency carrier frequency directly to the receiver on the handlebars or on the support vehicle become.

The present invention is in no way applicable to the application according to the exemplary embodiments limited, but can be used with any type of crank drive.

Claims (8)

1. Device for measuring the force and power on a pedal crank ( 10 ), in particular that of a bicycle, wherein

• a) the quantities force and angular velocity are converted into electrical signals and fed to an evaluation device,
• b) the angular velocity associated with the force and / or the crank angle is determined from a time measurement of a signal from a position sensor ( 15 ) which acts periodically with the crank rotation,
• c) the force is determined by measuring the shear deformation on the crank pin with strain gauges arranged thereon and in addition
• d) at least two pairs of mutually associated strain gauges ( 1 , 2 , 5 , 6 ) with different force absorption directions are offset at an angle directly on the crank pin ( 11 ).

2. Device according to claim 1, characterized in that that the shear deformation on the crank pin in two perpendicular to each other standing directions is recorded and that the strain gauges are aligned at 45 ° with respect to the pin axis. 3. Device according to claim 1 or 2, characterized in
that from two recorded shear deformation components, the absolute force and the angle of the force with respect to the crank pin are calculated by a microcontroller and / or
that the force component in the circumferential direction and in the normal direction with respect to the crank pin is calculated by a microcontroller from the two shear deformation components recorded. 4. Device according to one of claims 1 to 3, characterized in
that the angle of the pedal axis in the screwed state with respect to the crank is measured via an angle measurement after assembly and
that the angle is entered using an input option. 5. Device according to one of claims 1 to 3, characterized in that the determination of the angle of the pedal axis when screwed in with regard to the crank via a calibration measurement with a known force angle he follows. 6. Device according to one of claims 1 to 5, characterized in that the crank angle at the time of measurement from a position encoder with a Time measurement is determined. 7. Device according to one of claims 1 to 6, characterized in that a force curve of a period is stored and that the difference between a currently measured force curve and the stored one Force curve can be displayed. 8. Device according to one of claims 1 to 7, characterized in that that force profiles of different pedal frequencies are stored and that the difference to a force curve with the current force curve same cadence can be formed and displayed.