Feature

Of Bicycles, Strains
and Designs

A lightweight unit will let researchers and trainers monitor the performance of cyclists.

Alexis Orange

It is the final lap of the 10,000-metre velodrome sprint and the two competing cyclists are head to head until one attempts to break the deadlock and pumps harder on the pedals. Two factors will now determine the outcome of the race -- the athlete's physical ability and his or her cycling technique. Assuming the cyclists are matched in physical power and stamina, it is perfected technique that will make the difference in such a competition.

Mounted on the leading cyclist's high-tech bicycle frame is a small red box. This is the "Bicycle Strain Gauge" (or BSG) remote performance monitoring unit.

The remote unit contains electronic circuits connected to several small, light transducers. One measures the bicycle crank position, two others measure the foot pressure on each pedal. Each transducer output is digitally encoded by a microprocessor and broadcast via a low power FM transmitter.

Connected to the trainer's laptop PC is another box -- the BSG data receiver. The FM broadcast is recovered and the transducer data extracted. This data is input to the PC where it are displayed as graphical functions over time.

At present I am designing and building this project at the University of Otago Physics Department as a contribution towards a Masters degree in electronics. Post-graduate degrees in electronics seek to train students in one of New Zealand's fastest growing industries emphasising practical aspects of the field as well as providing formal academic training in communications, microprocessors, and signal processing, as well as the newer technology of opto-electronics (or photonics). The methodology of electronic design, and manufacture is indirectly addressed through a largely practical thesis project.

The initial concept of a cycling technique (or performance) monitoring device, as originally proposed by the University of Otago Physical Education Department, is not new. However, with advances in modern electronics it has become more viable, largely because small, robust designs can be achieved with minimal impact on the cyclist and bicycle. This fits with our basic design criteria; to monitor key aspects of cycling technique remotely without inhibiting athletic performance.

The first step in the design process after conceptualisation is to identify the operational limits -- the maximum positive and negative pedal force -- and the maximum crank rotation rate that an athlete can conceivably achieve

A maximum force one and a half times greater than that due to gravity on a 100 kilogram human (ie. 1,500 newtons) and a rotation rate two and half times faster than that for comfortable cruising at eighty revolutions per minute (ie. 200rpm) have been chosen so that the device's usefulness can cover many human builds and cycling techniques.

Important considerations for the remote unit are size, weight, power and range. Obviously the device must not get in the user's way, nor be so large that it contributes to wind drag. Weight must be kept to a minimum. Range of the radio transmitter/receiver is a consideration and line of sight transmission up to five hundred metres has been settled upon, requiring only a low power UHF transmitter similar to those used in cordless telephones. Power consumption is quite closely related to weight (the main contribution being the battery pack), and must be traded off against useful life of the power source, ie. a light, short-lived unit versus a heavy, long lasting unit. Four D-size dry cells provide adequate power for a few hours' use, and the overall dimensions of the remote unit are similar to a family size block of cheese.

Operational limitations of the receiver are much less of a concern, the major consideration being that the life of the receiver power source should match that of the remote transmitter.

Once the concept and limitations of the Bicycle Strain Gauge were settled upon, designing could begin in its proper form. Starting with a basic block diagram, each block (or module) was designed separately but in parallel, undergoing several revisions. On the surface, this approach appears to be complicated it but gives flexibility by allowing the individual modules to evolve during design whilst still ensuring overall compatibility.

During this phase the main components are selected and general circuit operation decided upon. Having a clear and definite idea of each circuit's purpose and its operation greatly reduces the possibility of complete circuit redesign -- a time consuming process.

Time involved in circuit design is greatly reduced by using modern CAD (computer aided design) techniques, primarily in the form of integrated schematic capture and PCB layout packages. Advantages of CAD include reducing the possibility of schematic circuit errors and error propagation into the PCB design. Accurate PCB artwork is also easily created using industry standard printers and plotters.

Each separate module is assembled, tested and possibly adjusted or reworked and retested until the required level of operation is achieved. Finally all modules can be linked and the overall operation tested. This method of prototyping enables circuit deficiencies to be isolated and easily fixed within each module, and problems linking the modules appear early and can be easily eliminated.

In in this prototyping phase any unforseen environmental and ergonomic problems can be eliminated. For example, if we mount transducers on the pedals and the main unit on the frame, how do we wire transducer output to the main unit without inhibiting the crank rotation or the athlete's leg movement?

Closely related to the hardware design is the underlying software design, especially the firmware (in ROM) of the remote unit microprocessor. Software design is commonly treated as a subassembly of the parent module but essentially follows the same design and prototyping process.

Once software and hardware prototyping have been completed we reach the final design stage: operational testing, where the entire product is assembled.

The remote unit, transducers and power supply are mounted on the bicycle. The local receiver, PC input circuitry and power supply are assembled and connected to the laptop. Both units are powered up and the PC software executed. The device is tested in such a way to ensure that it operates within the original conceptual limits. This is the last stage at which unforseen problems can be identified and fixed by reverting back to previous processes in the design cycle.

When creating electronic products, a systematic and planned approach is often the most successful and this may be directly attributed to the logical nature of the field. However there is always room for creative flair, originality and flexibility and such skills should not be disregarded in order to fit a mould of the ideal.

A consequence of a formal course of study often is training in areas outside the documented curriculum -- training that is usually as beneficial in the long term as that initially embarked upon.

Alexis Orange is presently working towards his Masters degree in Engineering at Otago University.