The base of the foot is made of 1/8" thick aluminum plate. Each of the four toes on a foot is 3/8" wide and has a rubber pad on the bottom side that contacts the ground. When weight is placed on a toe, the aluminum plate bends proportional to the amount of force applied. A small magnet is mounted over the pad on each toe. Hall effect sensors are mounted rigidly to the foot directly above the magnets. Therefore, when force is exerted on a toe, the base of the foot bends and moves the magnet closer to the hall sensor. Figure 1 shows the construction of the toes. The strength of the magnetic field at the hall sensor increases as weight is applied and the magnet gets closer. The sensor then converts this change in flux to a corresponding change in voltage.

Figure 2 shows the circuit used to condition the hall sensor output. The hall sensor output is nominally half of the supply voltage in the absence of any magnetic field. As connected, this translates to 0V with respect to the op-amp supply. The output voltage of the sensor is increased or decreased depending on the strength and polarity of the magnetic field. The magnet is oriented so that the output voltage drops as the magnet gets closer (when force is applied to the toe). This signal is applied to the inverting input of the op-amp and amplified so that the output of the op-amp increases with increasing force. An offset adjust is included to trim the sensor output to zero with zero force. This output signal is then low pass filtered to reduce noise. Due to a lack of information on the magnets used and a convenient way to measure flux levels, the required gain of the amplifier was determined experimentally.

The use of the hall effect sensors required only a 100:1 gain to attain a useable output level. These sensors were also less expensive to implement than strain gages. Instrumentation amplifiers would have been required with strain gages due to the extremely high gain requirement. With the chosen design, the much lower cost LM324 op-amps could be used successfully. Hall effect sensors themselves are also fairly inexpensive. The picture below shows the current implementation of the force sensors.