With the revolution of bicycle culture seen in recent years, cycling has become an increasingly popular hobby worldwide. Many of these novice to professional cyclists are interested in monitoring and improving their pedaling form, but find the high costs of power meters systems currently on the market to be prohibitive.
To meet the needs of a growing bicycle community, NEVADAtude Cycling of University of Nevada’s Mechanical Engineering Department decided to develop the VeloDAQ, a self-contained data acquisition system used to analyze a cyclist’s pedal stroke and wirelessly transmit that information to the rider’s smartphone (Fig. A & B). The VeloDAQ is comparable to other pedal based power measurement devices on the market with the exception of two profound characteristics. First, the VeloDAQ transmits its data via Bluetooth to the user’s smart phone or other Bluetooth device; this reduces cost and avoids proprietary communication protocols by using a device a cyclist is likely to already own. Second, the VeloDAQ pedal is the only product that provides a display of force as a function of crank angle, a new but important metric to analyze a cyclist’s pedal stroke (as opposed to only power measurements). These two differences allow the VeloDAQ to be less expensive, and therefore more accessible, while still providing greater functionality than similar products already on the market.
The design is built around already existing pedal technology in order to minimize the cost of production while maintaining a traditional clipless pedal form factor. To measure applied pedaling force, the VeloDAQ uses strain gauges in half Wheatstone bridge configurations which are mounted on the pedal spindle. Within the pedal, angular position and velocity are measured using a Hall Effect sensor at a greatly reduced cost when compared to other technologies such as encoders and accelerometers. The outputs of the Wheatstone bridges and the Hall Effect sensor are then sent to an on-board microprocessor and transmitted via Bluetooth to the smartphone which displays the final data for the user. Most of these added components are housed inside a custom designed enclosure that closely follows the style of the original pedal. The video below describes how each of these components work together to form the complete VeloDAQ system:
Engineering analysis of the design shows that the pedal is capable of measuring applied forces within ±5 pounds and angle within ±3 degrees during steady cadence. These two specifications, when coupled with high data transmission rates and low power consumption standard for Bluetooth (300Kbs and 30mW respectively), create a system that can provide a cyclist with many hours of high quality data collection. When this information is applied to a training regimen, it can be used to improve the efficiency and power generation of a cyclist when compared to more traditional systems that only provide basic power production and cadence data to the rider. The installed prototype is shown in Fig. C below.
As a baseline, our team wanted to design the VeloDAQ to perform on par with current power meters in terms of measurement accuracy, battery life and durability, while minimizing the total system costs whenever possible. The VeloDAQ has also been designed to communicate with standard Bluetooth-capable user interface devices (or UIDs), including smartphones, tablets, and laptops. These types of UIDs offer a significant advantage over current power meters, as they do not require the purchase of expensive dedicated cycling computers. This approach also offers economical advantages in large-scale manufacturing, as the complete system requires fewer components and materials.
To meet these goals, it was decided that several key components were required regardless of the final prototype design:
- Pedal force measurement sensors
- Crank angle measurement sensors
- Signal conditioning equipment
- Microcontroller with Bluetooth for data packaging and transmission
- Compact battery to power electronics and sensors
Figure A shows an exploded view of the complete VeloDAQ prototype. So how do these components work together to map a user’s pedaling force?
While riding, the VeloDAQ’s on-board sensors continuously measure applied pedaling force and crank arm angle. These signals are conditioned prior to being packaged and transmitted from the VeloDAQ to the selected UID. On the receiving end, the UID gathers and processes the raw data prior to displaying and archiving the results. This entire process takes less than a second to execute, which will provide cyclists with a large amount of performance data over the course of a typical ride. Users are also given information about their power, cadence, velocity, and GPS location on-demand.
Prior to selecting the design shown in Fig. A, we considered alternative form factors and sensing methods. Criteria were generated where each alternative could be roughly evaluated using a modified decision matrix (Fig. B).
We built three proof of concepts to verify the performance of the proposed force measurement sensors (Fig. C & D: strain gauges), angle measurement sensors (Fig. E & F: Hall effect sensors and encoders) and slip ring electrical interface equipment (Fig. G & H: various slip rings). The components for the VeloDAQ prototype design were selected based on proof of concept test results.
Performance metrics were established for each component to optimize the overall performance of the prototype. To ensure a robust prototype and simplified design process, we considered mitigation techniques for real-world factors that could be affected by the final design selection. Several key “Design for X” (DFX) factors have been described below:
The VeloDAQ was designed to be a high quality pedal-based performance meter. Maximum effort has been put into ensuring it is capable of providing reliable and accurate data to the user while not obstructing their ability to perform while riding. The VeloDAQ is designed to accurately measure two variables – force and crank orientation – and transmit the data to a secondary device. To accurately measure crank orientation, multiple methods were tested. Of these methods, the Hall Effect sensor was selected due to superior test results. To measure force, two half Wheatstone bridges were selected for their ability to measure force applied at any orientation by the rider. These sensors were selected for their accuracy and small footprint preventing interference with the rider’s natural pedal stroke. Baseline performance metrics were established for each component to optimize the overall performance of the prototype.
Most cyclists care a great deal about what their bicycles and attached gear look like. When designing the VeloDAQ, it was determined that adding functionality should not take away from the bicycle’s appearance in any way. Rather than using a dongle that houses components outside of the pedal, as several of our competitors have done (Fig. I), it was decided that all components would be housed inside the pedal. Therefore the pedal must be modified and a components housing must be added to protect the components while in use. This housing is designed to match the shape of the pedal so that the VeloDAQ pedal is indistinguishable from other pedals except for added functionality (Fig. J).
Because the VeloDAQ is intended for use on performance bicycles capable of speeds exceeding 40 mph, it is very important that the system be a safe device that neither directly nor indirectly causes a riding accident. For this reason, the VeloDAQ is designed to be as safe to use as possible. Since material is being removed from an existing pedal, the highest priority is for the pedal to not have a mechanical failure. To strengthen the pedal, a steel sleeve is fitted over contact points between the pedal and the spindle (Fig. K). This sleeve will both shield the added components and restore the strength of the pedal shaft, thus providing additional safety for the rider.
With these and other design considerations in mind, we began the process of building our prototype VeloDAQ confidently.
PROTOTYPE FABRICATION & TESTING
Despite the complexity of the VeloDAQ system, all of the major components are capable of fitting in a modified LOOK pedal with minimal impacts on pedal weight and performance. A basic LOOK pedal was selected to be the platform for the prototype due to its simple form factor and near universality with common cycling cleat bolt patterns (Fig. A).
First, the pedal spindle dimensions were modified to accommodate the necessary sensors and electronics (Fig. B). Due to the fact that the pedal interfaces with the spindle via needle bearings on each end of the shaft, a collar was shrink-fitted to retain necessary bearing surfaces (Fig. C). A section of the pedal also needed to be removed to allow for installation of the slip ring, which carries electrical signals and power between the rotating pedal and spindle interface (Fig. D).
With the majority of the machining complete, the sensors and signal conditioning components were then installed on the spindle and routed to the slip ring stator (Fig. E). To complete the prototype build, a custom 3D printed enclosure was mounted on the front surface of the pedal to house the slip ring rotor, battery, microcontroller, and Bluetooth transmitter (Fig. F). Small gaskets are used to dampen vibrations of the electronics while riding, and a weather barrier prevents the components from getting wet during inclement weather. The completed VeloDAQ prototype is given in Fig. G & H below.
One observation noted during the manufacturing process that is worth mentioning is that the completed VeloDAQ actually weighs less than the unmodified pedal (Fig. I & J), which is certainly beneficial in the world of performance cycling and a welcome addition to all of the added functionality our system offers.
Once the prototype build was complete, we began extensive calibration and testing procedures to ensure all electronics were performing as expected. The prototype was installed on a road bike and mounted on a trainer in the lab (Fig. K). We paired the VeloDAQ with a laptop computer running LabVIEW to simplify the data analysis process (Fig. L). In the LabVIEW plots shown, the left two show the raw voltages from the strain gauges, the top right shows crank position over time, and the bottom right show the force over time. To calibrate the VeloDAQ force measurement sensors, weights of known value were applied and the responses at various angles recorded. The results of calibration and tests will show that the VeloDAQ is capable of measuring applied pedaling force, crank angle, and power at design-specified levels while decreasing the overall weight of the pedal.
In conclusion, the VeloDAQ project has offered us an opportunity to apply technical engineering knowledge gained through the bulk of our undergraduate studies in a fun and interesting way. We also learned quite a bit from our teamwork experience, project redesigns, and troubleshooting over the past year.
ABOUT THE TEAM
Behind any great project is a hardworking, brilliant, and fun group of people making it happen. Meet the members that make up NEVADAtude Cycling.
Milan Heninger – Team Captain (Center)
Milan Heninger is responsible for team activities and leads with his impressive history of executive management and technical experience. Milans diverse professional background includes experience as a security management executive, as well as electrical startup technician in the renewable energy field. Milans personal interests are as diverse as his professional background, including classic cars, riding motorcycles, training in historical european martial arts and reconstructing ancient technologies. A very accomplished student, he has worked for several years in a university research lab and looks forward to returning to industry upon graduation.
Nick Baker (Lower Left)
Nick Baker is an exceptional student preparing to enter graduate school at UNR in pursuit of advanced degrees in mechanical engineering. His research interests include renewable energy technologies and applications, smart materials, microelectromechanical systems (MEMS), and mechatronics. Nick’s hobbies include hiking, cycling, playing guitar, and enjoying the company of his young daughter.
Crystal Harvey (Lower Right)
Crystal Harvey is a dedicated team member and student. She will be graduating Magna Cum Laude in the Spring of 2013 and looks forward to entering the Reno’s engineering industry the following summer. She, like many others on the team, is a cycling enthusiast and spends any spare time involved in various outdoor and/or acrobatic hobbies.
Andy Smith (Upper Right)
Andy Smith is also an outstanding student who has multiple published papers as an undergraduate student. He is also preparing to enter graduate school at UNR in pursuit of advanced degrees in mechanical engineering.
Gene Lengdorfer (Upper Left)
Gene Lengdorfer is an extremely accomplished student who also plans on entering industry after graduating. Work interests include thermodynamic cycle power generation, controls theory, programming, and their integration. Raised near the Tahoe area, he especially enjoys alpine skiing and fly fishing in the Sierras.