DE9415162U1 - Electronic device for determining pedaling force - Google Patents

Description

W. Petzke, Dipl.-Ing. 1

Description

Electronic device for determining pedal force

The invention relates to an electronic measuring device for recording the magnitude and direction of the forces applied by the cyclist to the pedals at any crank angle during the periodic pedaling process and displaying them on a display panel arranged in the field of vision of the vehicle user, with the purpose of providing the driver with information about the magnitude, type and efficiency of his use of force.

An electronic device for measuring the forces applied to a continuously rotating bicycle crank or a bicycle pedal, particularly during the biomechanical process of pedaling, and which change in direction and magnitude, consists, as taken from the literature: force sensors in the area of the pedal, the crank or the crank bearing, a signal amplifier, a device for determining the crank angle and for transmitting the measurement signal from the rotating measuring point to the stationary display device.

These devices are used in research facilities and make it possible to measure pedal forces on mostly stationary bicycle ergometers that are specially designed for this purpose.

For this purpose, angle sensors are normally used to determine the position of the crank. The measured values from the sensors on the moving pedal or rotating pedal crank are transmitted either via cables that are laid on the test subject's leg, via slip rings, radio, infrared or ultrasonic transmitters or only after the pedaling process has been completed by retrieving them from a data storage device. The force displayed does not correspond to the force exerted by muscle work, as this includes the weight and inertia of the legs. The force sensors and the amplifier circuit require a power supply that requires a relatively large battery or frequent replacement. The measuring system is only set up by a specialist.

Retrofitting any bicycle, even by a non-professional, is therefore not possible without restricting the use of the bicycle or the measuring device and causing high maintenance costs.

W. Petzke, Dipl.-lng. 2

(McLean Brian,Improving pedaling technique with "real time" biomechanical feedback, Excel, Belconnen, 5(1988), Vol. 1, pp.15-18 // Ericson MO, Efficiency of pedal forces during ergometer cycling, Int. J. of Sports Med.,Stuttgart, 9 (1988), 2, pp.118-122) Problem/Purpose of the invention:
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It is therefore the object of the invention to design a pedal force measuring arrangement of the type mentioned at the outset in such a way that the subsequent fitting of a bicycle can be carried out by a non-specialist while retaining all useful properties and that the use of the measuring and display device takes place during the normal use of the bicycle.

Solution:

This object is achieved according to the invention in that a microcontroller-controlled measuring transmitter is attached to the crank or a component rigidly connected to it and controls the measuring process, whereby the associated crank angle is calculated from a mark on the crank circle using angular velocity and time measurement and is transmitted together with the force measurement values to a fixed receiver in the area of the crank with the aid of a coupling between reference conductors.

Benefits achieved:

The combination of a transmission method using the physical effect of the coupling and an angle determination using a control program is advantageous, since both can be carried out by a microcontroller. This makes the entire measuring arrangement largely independent of the detailed design of the wheel in the area of the crank.

This makes subsequent installation easy. The principle of coupling (as described in the EMC law and commentary and normally considered as interference between two adjacent conductors) is chosen here because it is suitable for transmissions over distances of up to several cm and very low energy consumption.

Pulses are sent out by, for example, briefly changing the electrical voltage on a conductor. The very steep signal edges generated by the digital components are particularly suitable for this. A conductor located nearby receives these (interference) signals. With an amplifier circuit and filter,

W. Petzke, Dipi.-lng. 3

these are amplified to such an extent that they can be further processed as a data stream. The quality of the transmission also depends on the shape of the conductors; more energy is radiated at corners.

A particularly favorable arrangement for transmission is chosen according to claim 2. The transmission pulses are transmitted from a conductor on the inside of the pedal crank to the receiver. The conductor can be, for example, a thin strip of metal that is glued to the inside of the crank using double-sided adhesive tape and is bent up in the area of the receiver. This shortens the distance to the receiver and an arrangement that saves components and space is chosen.

Another particularly advantageous embodiment is achieved with claim 3. The information is sent digitally, so no accuracy is lost during data transmission. The signal is not distorted by external influences or changes and aging of the electrical components. For example, a data format such as that used in infrared remote controls (Pulse Code Modulation - PCM) is suitable.

A further advantageous embodiment of the invention is specified in claim 4.
The data to be sent is temporarily stored by the controller and sent in packets. This makes it possible to only define the coupling over part of the crank circle, e.g. the area between the rear fork and the down tube, and thus to make the design of the coupling conductor even simpler and smaller.

The transmission speed is set so high that at the highest pedaling frequency (150 revolutions/min) a crank angle of 120° is passed through in order to transmit all the data of one revolution.

A particularly favourable design is achieved with claim 5.

The measuring signal amplifier and the sensors are each supplied with power for only 50 us for the measurement carried out periodically or at specific crank angles.

During the much longer breaks in between, no electricity is consumed.

With 20 measurements per second, power consumption is reduced by a factor of 1000.

This means that the battery for the power supply of the measuring transmitter can be kept small, and so can the device. In addition, the battery life is increased, thus reducing maintenance effort.

W. Petzke, Dipl.-lng. 4

A further advantageous solution to the problem is characterized according to claim 6 in that

To determine the position, a permanent magnet is attached to the circumference of the crank circle as a signal transmitter. A reed switch is used as a signal receiver, which is housed in the measuring transmitter housing. The reed switch in combination with a permanent magnet allows a structure with minimal power consumption. The permissible tolerances for the magnet-reed switch arrangement make installation easier.

In the design according to claim 7, the forces are measured via strains in the pedal axles that result from transverse forces. The strain gauges are applied between the bearing and the screw thread and are arranged symmetrically around the circumference of the axle, offset by 90°. This achieves complete compensation of all disturbances, in particular strains from temperature and from bending moments introduced into the axle, as well as changes in the distance between the force application point and the measuring point.

In addition, due to the symmetry, it is possible to record the forces evenly over the entire circumference of the pedal or at any angle around the pedal axis, in particular regardless of the angle at which the pedal is aligned. (If the forces are recorded on the pedal body, it would be possible to send the measurement data to the display unit via a cable, since the pedal body only performs small rocking movements and is otherwise only moved in a translational manner, but then the rocking angle would also have to be determined).

In the embodiment according to claim 8, the device has the option of recording measured values, e.g. from one or more periods, and storing them in a RAM or EEPROM for at least several weeks. This creates the prerequisite for displaying the difference between a current force curve and a previously recorded one.

This is very useful for filtering out information that is not essential for the rider and that remains constant over the pedaling periods. Since all constant values are eliminated when the difference is formed and only the differences are retained, measurement variables that are independent of the current muscle work are eliminated: the weight force and the forces from dynamic processes such as deceleration and acceleration of the legs, as well as centrifugal forces.
In particular, the considerable weight of the leg (and clothing/shoes) would overlay the representation of the forces acting tangentially and normally to the crank circle and obscure the actual information content.

W. Petzke, Dipl.-lng.

Description of an example:

An embodiment of the invention is explained with reference to Figures 1 to 2.

Figures 1 and 2 together show the arrangement of the device when installed on a bicycle
Show it:

Fig. 1 the view of the measuring transmitters on the left and right pedal cranks for the left and right pedals, the pedals with pedal axles, the transmitting antenna for the left side on the inside of the left pedal crank, the receiver above the bottom bracket between the seat tube and down tube, the connecting cable to the display unit. Fig. 2 the display unit on the handlebars in the driver's field of vision near the stem.

In the figures

1 measuring transmitter, 2 antenna left side,

3 recipients,

4 fastening tabs of the measuring transmitter on the left crank

5 Connection cable to the display unit (ground, power supply, data line) 6 Permanent magnet on the left side of the rear fork

7 Display unit on the handlebar

8 Pedal body, 9 Pedal axle, 10 Crank left side

Example of a design of the device and its components.

The complete unit consists, for example, of the pedals with the strain gauges applied to the pedal axles, the measuring transmitters connected to them via short connecting cables, the insulated metal strips covered with adhesive film for transmission, the receiver on the frame, which is connected to the display unit via a connecting cable, the magnets on the rear fork as a mark on the pedal circle.

Standard strain gauges are used as force transducers for shear stress analysis. These are applied rotationally symmetrically at 90° between the pedal bearing and screw thread and are connected together to form two bridges offset by 90°. This arrangement makes it possible to record the angle and the amount for any force application angle transverse to the axis direction.

W. Petzke, Dipl.-lng. 6

The connecting cables to the amplifier circuit in the measuring transmitter are twisted to suppress external interference and can be short due to the invention.

The two DMS bridge signals are amplified on two separate channels with operational amplifiers in a bridge circuit. For a higher degree of amplification, several amplification stages can be connected in series. A typical amplification is around 5000. The voltage supply for the DMS bridges, the amplifier and the A/D converter is switched on by a microcontroller at the time of measurement via an intermediate transistor.

After a short settling time (e.g. 5 us), the two amplifier output signals are read in via one channel of an A/D converter. The measurement time is buffered by the microcontroller in a RAM together with the two measured values. After the conversion time (e.g. approx. 45 us), the voltage supply to the bridges and amplifiers is interrupted again. In order to protect the battery, e.g. a lithium cell, from the drop in resistance during the switch-on time, a 1000 ohm resistor is connected in series and a 50 uF capacitor is connected in parallel, which mainly supplies the current during the switch-on time and is recharged via the resistor in the pause times in between. The resistor limits the current from the battery.

With 20 measurements per second, this results in a switch-on time of 20*50us = 1 ms, and a power consumption reduced by a factor of 1000.

This means that the battery of the circuit can be kept small. This has a positive effect on the usefulness of the device, as a large device would be a hindrance, especially in rough racing conditions, and would be more likely to break due to the limited space in the crank area.

With each revolution of the pedal crank, the measuring transmitter is moved past the permanent magnet, which is attached to the frame (e.g. the rear fork) at a distance of 1-5 mm. The reed switch built into the measuring transmitter is switched on and sends a pulse to the microcontroller. This starts a new time measurement and evaluates the last revolution.

If the period duration does not deviate from the value of the previous revolutions or only slightly, this means that the pedaling process was uniform and of constant angular velocity.

The angular speed of the crank can be determined with sufficient accuracy from the period duration, since due to the mass inertia speed changes in the range of a single pedaling period at

W. Petzke, Dipl.-!ng. 7

continuous operation are low. The case of an interruption in pedaling, slowing down or acceleration can be recognized by the changed period duration. An angle calculation or assignment of the measuring points can therefore be carried out via the time measurement. The time values for the measurements of the last crank revolution are then converted into the corresponding angle values. The collected and temporarily stored data of the last pedaling period are then sent to the receiver.

To do this, the controller sends short pulses to the transmitting conductor via a capacitor that serves as DC protection. In doing so, it briefly pulls the signal level, which would otherwise be at the supply voltage, to ground. The steep edges of the pulse are transmitted as interference to distant conductors. Since the transmitting conductor is routed over the inner surface of the crank to the vicinity of the receiver, and corners increase the radiation, a signal is generated in the receiver on a conductor that is also placed near the crank. A solid conductor is better for reception than a small, thin one. The transmission distance is about 2 cm. The metal strip that acts as the transmitting conductor is slightly wider at the level of the receiver conductor, so that reception with sufficient interference immunity is achieved over a crank angle of 120°.

The received pulses are fed back to a microcontroller or another component suitable for further processing via an amplifier circuit made up of transistors. Due to the short range, the pulses from the left and right pedals or measuring transmitters do not interfere with each other and can be received separately, both individually or simultaneously.

The receiver housing is mounted between the seat tube and down tube above the bottom bracket.

Since the measuring transmitters only transmit during a maximum crank circumference angle of 120°, they can be arranged at different times. Since both measuring transmitters are synchronized by their own brand on the pedal circumference and the cranks are arranged at a different time by 180°, a temporal separation is automatic if both are constructed in the same way. This is precisely what is sought for because of its simplicity and cost-effectiveness.
If the information about which measuring transmitter the data originates from is also transmitted, both data streams can be transmitted to the display unit via a data line without losing any information.

W. Petzke, Dipl.-lng. 8

The Pulse Code Modulation (PCM) method used in infrared remote controls is suitable as a data format for sending by coupling from the measuring transmitter to the receiver. The two states 0 and 1 are characterized by different distances, e.g. 100 and 200us, between the pulses. The bit values can be recovered using a timer. An additionally sent protocol helps to detect transmission errors.

The forces can be displayed on the display unit, for example, after conversion into normal and tangential components.

The angle of the force with respect to the pedal axis and the magnitude of the force can be determined from the signals recorded together by the strain gauges offset by 90° according to the rules of vector calculus (calculation of the main axes).

In order to be able to display the values generated by active muscle power, i.e. to separate them from those caused by the weight of the legs, a force curve in which the pedal is pedaled with as little force as possible and with a low gear ratio is stored in an EEPROM.
This is done by a microcontroller in the display unit, which has a suitable control program for this purpose.

Current measured values are corrected by the control program in the microcontroller using the stored values, i.e. they are subtracted from each other. The microcontroller is used both to read the signals serially, to convert them, and to control the display.

Claims (8)

Protection claims 1. Electronic device for measuring the forces applied to a continuously rotating bicycle crank or a bicycle pedal during the biomechanical process of pedaling and which vary in direction and magnitude. characterized, that a microcontroller-controlled measuring transmitter is attached to the crank or a component rigidly connected to it and controls the measuring process, whereby the corresponding crank angle is calculated from a mark on the crank circle using angular velocity and time measurement and is transmitted together with the force measurement values to a fixed receiver in the area of the crank using a coupling between reference conductors. 2. Electronic device according to claim 1. characterized,
20 that the coupling is made by means of a conductor on the inside of the crank to a fixed conductor in the area of the crank circle. 3. Electronic device according to claim 1. 25 characterized, that the measurement signal is sent digitally, i.e. pulses are sent to the transmission line for coupling, the time interval between which transmits the information bit by bit (Pulse Code Modulation PCM). 4. Electronic device according to claim 1. characterized, that intermediate storage of the data to be sent makes it possible to transmit the measured values only within a certain angle range of the crank, so that the coupling does not have to be equally pronounced over the entire crank circle. W. Petzke, Dipl.-lng. 2 5. Electronic device according to claim 1. characterized,
. · that the measuring signal amplifier and the sensors for the measurement carried out periodically or at specific crank angles are each only supplied with power for about 50 us and that there are much longer pauses in between during which no power is consumed.
10 6. Electronic device according to claim 1. characterized, that the determination of the crank angle in relation to a measured force is carried out by placing a signal generator at a location on the cyclically traversed circumference, e.g. a magnet, whose signal is used to determine the period length, and as a reference point for calculating the angle, by a control program calculating the period length in advance and relating it to the current time period. 7. Electronic device according to claim 1. characterized,
25 that the force is measured biaxially using strain gauges on the pedal axis in the direction of the transverse force. 8. Electronic device according to claim 1.
30 characterized, that measured values of several pedaling periods are stored over a period of several weeks in a RAM or EEPROM or similar, and the difference to currently measured values is calculated and displayed.