Tensile forces of up to 7000 N at a pulse frequency of 3 Hz and 100 000 load changes per test run. These are the stringent requirements that the fluid muscle, a unique actuator from Festo that closely resembles a real human muscle, had to measure up to for a project team testing bicycle frames at Pforzheim University in Germany.
The interaction of all the stresses and strains which mountain and racing bikes are exposed to were to be simulated, starting with pedalling forces realistically resembling uphill cycling, and followed by the braking force through to the saddle and transition force.
The fluidic muscle, used several times throughout a test rig constructed by Canyon and Bergwerk bicycle manufacturers, provided a convincing solution for the mechanical engineering students and professors involved. The main advantage of the fluidic muscle in this application is its reduced control complexity. In contrast with pneumatic cylinders used in conventional test procedures, it also permits the simultaneous generation of several loads, eliminating the need for retooling during testing.
The bicycle test rig constructed by Canyon and Bergwerk using Festo’s fluidic muscle technology
A total of six fluidic muscles, each with a diameter of 40 mm and nominal lengths of 400 to 700 mm, are used as tensile actuators in the computer controlled test rig. Attached to the clamped cycle frames, they produce the desired loads on the frames in any combination and magnitude. The testing force is controlled via a control chain consisting of tensile force sensors, measuring force amplifiers and restrictor or non-return valves. Depending on the test programme, these fluidic muscles operate at up to 100 000 cycles, using a tensile force of between 1200 and 7000 N to simulate Alpine ascents, downhill rides or mountain bike tours across impassable terrain. During testing, the measuring technique used continually compares the progression of force of the muscle, using a setpoint progression, and switches off the test as soon as a deviation points to a possible crack in the bicycle frame.
The team of experts working on the rig is convinced that, with the help of findings established in this way, it has been possible to drastically reduce frame fractures due to faulty construction or faulty machining. "Medium to long term, we would like to jointly develop test units on a scientific basis for different types of bicycle and areas of use, which will only be possible with the new testing technique using the Festo fluidic muscle," concludes departmental head, Dr Peter Kohmann.
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