Figure 1 shows a diagram of the entire system. The method begins with the wheel sensor (shown in Fig. 2). This sensor works similarly to a bicycle odometer in that it merely registers when the wheel completes a revolution.

Figure 1.

Figure 2.

The valve stem cap on the car wheel is replaced with a special magnetic one. It consists of a tiny neodymium magnet (diameter 0.25 × 0.125 in, Edmund K35-429)placed inside a brass valve stem cap. A small washer put in the cap after the magnet ensures that when the cap is screwed on tightly, the valve pin is not depressed.

A special clamp holds a standard magnetic field sensor (Vernier MG-DIN) near the wheel. The clamp was designed to be able to be connected rapidly to almost any car. The probe tip is positioned so that the magnet passes within about 1 cm of it. One difficulty we have encountered is that the suspension is compressed when the car accelerates. This changes the relative position of the probe to the tire. In some cases the probe moved far enough that the signal was lost during the acceleration. We solve this problem by mounting the probe near the rear of the tire instead of near the top. This makes the relative motion of the probe tangent to the circle described by the moving magnet rather than perpendicular to it. Rather than losing the signal, the relative motion introduces a small timing error that we ignore.

The magnetic field sensor creates a small peak in its output voltage when the magnet passes near it. For our system this was about 1 V in height above the baseline. To make data collection more straightforward, we have designed a simple circuit to convert this signal into a digital signal similar to the type produced by digital pulleys (Fig. 3). The circuit (Fig. 4) employs an analog comparator to change the analog input into a digital output. The threshold is controlled by a potentiometer that the students adjust. There is a small amount of positive feedback to give it some hysteresis. The circuit also has an LED indicator that flashes with the digital output signal so that the students can easily adjust the threshold to the optimum level.

Figure 3.

Figure 4.

The output of the conversion circuit is connected to the laptop computer through a standard laboratory interface (we use the ULI interface from Vernier powered by an external 9-V battery). The interface software (Logger Pro) is set to treat the signal as if it were from a digital pulley. To calibrate the system, we measure the distance traveled during one wheel revolution and enter this as the pulley calibration factor.

We have a simple template to configure Logger Pro to display real-time graphs of position and velocity versus time. The saved data file consists of many rows of time intervals and computed positions and velocities. The time intervals correspond to each revolution of the wheel. We then transfer the data into a spreadsheet program (Excel) for analysis.