The sensing systems’ conceptual solution is based on both the product design specification defined in the previous section, as well as the overall system conceptual solution defined in Section 4.4.
The sensing solution acts as a supplementary means of protection for the mechanical frame presented in Section 5. The mechanical frame provides physical protection to the bicycle’s critical components by physically securing the parts. In order for a thief to remove the bicycle or any part of the bicycle from the frame, physical interaction between the thief (or the tools used by the thief), and the bicycle, is required. The principle on which the sensing solution functions, is derived from Newton’s 3rd Law of Motion, which states: “For every action, there
is an equal and opposite reaction” [36]. The law implies that in any interaction between two
bodies, there is a pair of equal (in magnitude) and opposite (in direction) forces acting on the two interacting objects. The conclusion drawn is therefore that in order to remove the bicycle or any part of it from the frame that protects it, a force will have to be applied to the bicycle or its frame, and therewith an opposite force will be applied by the bicycle on the object where the force originates from, as well as the frame that supports it. Figure 17 illustrates how the different elements involved in the solution utilises this insight to create the principle on which the concept is based.
Figure 17: Sensing System Functional Concept
In Figure 17, forces F1 and F2 represents any force or combination of forces that are applied to the bicycle or components on the bicycle. These forces may be intentional (caused by an attempted theft) or unintentional (caused by a natural disturbance eg. wind or pedestrians bumping into the bicycle). According to Newton’s 3rd law, these forces will be countered by
the bicycle with equal and opposite forces – forces F3 and F4. Since the bicycle cannot accelerate in a vertical direction, the sum of the vertical components of forces F5.1 and F5.2 will be equal in magnitude to the sum of the vertical component of forces F3 and F4. Forces F5.1 and F5.2 acts on a force bed which the bicycle rests on, causing the two force-transducers that the force bed rests on to experience forces F6.1 and F6.2 – with the sum of forces F5.1 and F5.2 being equal in magnitude to the sum of forces F6.1 and F6.2 in a vertical direction. Forces F6.1 and F6.2 are therefore equal to the vertical components of any forces (F1 and F2 in this case) acting on the bicycle, and are converted to a corresponding electric signal that can be
further processed. Since only the forces acting on the bicycle or frame are measured, the sensors’ measurement scope is limited to the bicycle and frame – conforming to PDS 3.4. The electric signal produced by the transducers is amplified and converted to a time-discrete signal that can be interpreted by the software signal-processing part of the solution. The software processes the signal by applying the required filters and extracts certain characteristics from the signal. The signal characteristics are then analysed to determine whether the signal is deemed to be of an intentional or unintentional nature. A binary YES or NO is produced by the signal processing software if the signal is intentional or unintentional, respectively.
6.2.1 Technical Conceptual Validation
Due to the novelty of the proposed concept, a proof-of-principle experiment was required to first validate the critical assumptions of the concept’s technical elements before the detailed design and development phases were engaged. The experiment also helped to gather data and therefrom gain insights into the design and development of the solution.
At the conceptual stage, the concept’s success is dependent on two critical assumptions that are made; these are (1) that there is an adequate and capturable characteristics differences between intentional disturbances (steeling the bicycle) and unintentional disturbances (noise) executed on the bicycle, when the disturbances are converted into an electrical signal format, and (2) the converted signal that is received from the hardware is of high enough quality and resolution to allow for accurate identification and processing of the signal characteristics. An experimental bicycle docking setup was developed in an attempt to gather data. The experimental setup used stock load cells, a basic Wheatstone-Bridge, and an operational amplifier. The resulting signals were captured with an oscilloscope. Some of these unintentional and intentional signal traces, as well as certain extracted signal characteristics are presented below in Table 12.
Table 12: Oscilloscope-Captured Signal Traces
UnintentionalDisturbance Signals Signal
Signal Description
Disturbance: Impact to bicycle saddle from above (x2)
Resolution: 712 mV Duration: 1 sec Frequency: 8.0 Hz
Disturbance: Wind causing bicycle oscillation Resolution: 145 mV Duration: n.a. Frequency: 2.0 Hz
Disturbance: Pedestrian walking into the bicycle’s rear wheel
Resolution: 552 mV Duration: 0.75 sec Frequency: 8.2 Hz
Intentional Disturbance Signals Signal
Signal Description
Disturbance:
Loosening a wheel nut
Disturbance: Loosening handle bars
Disturbance: Removing the saddle bolt
Resolution: 900 mV Duration: 3.5 sec Frequency: none Resolution: 380 mV Duration: 3.8 sec Frequency: none Resolution: 280 mV Duration: 3.6 sec Frequency: none
The results obtained from the experiment validated the assumptions that there are clear and definable characteristics (amplitude, frequency, energy, duration, etc.) that can be used to distinguish the intentional and unintentional signals from one another. The signals are also of adequate amplitude to enable the signal processing process to accurately extract the characteristics from the signals produced. The insights gained from the experiment also show that the hardware used in the experiment is adequate, and can be incorporated into the detailed design of the solution. The resolution of the signal also provides as a guideline for the type of A/D converter and processing capacity that is required to extract the signal characteristics.