SmartWheel

The SmartWheel is used for measuring velocity and with additional processing produces position and acceleration data. In addition it can be used to measure angular rotation such as that produced by a pendulum. It uses a cdrom with a simple bearing on the hub and an extended shaft to either hold or attach to a moving object. We measure the rotation using optical quadrature methods.

Front of SmartWheel prototype with probe card

Back of SmartWheel prototype with probe card

Operation:

The encoder has 164 lines which results in a resolution of 0.55 (360/656) degrees resolution. Accounting for the circumference of the plastic disc this gives a linear resolution of about 1 mm. The current microcode in the interface accumulates counts over 10 ms and reports the results. The output can be thought of as a measure of velocity of the wheel with a resolution of 100 mm/s.

The optical encoder resolves one complete 360 degree rotation into 656 parts. Taking into account the circumference of the wheel this gives a linear resolution of approximately 0.55 mm.

Hardware Specifications:

- an angular resolution of 0.55 degrees.
- current used when measuring is approximately 30 mA at 5V
- the shaft/handle can be attached to a small cart, or extended so that it can be held by a person

Since this probe uses 30 mA the interface is designed so that the 5V power to probes like this can be switched on and off.

cdrom diameter (in)	4.675
circumference (in)	14.69
circumference (mm)	373
encoder lines	164
resolution	656
resolution (deg.)	0.55
linear resolution (mm)	0.57
sampling rate (Hz)	100
velocity resolution (mm/s)	56.9
maximum linear rate (m/s)	7.17
maximum angular rate (rps)	19.2
maximum angular rate (rpm)	1152
specified max linear rate (m/s)	5
specified max angular rate (rps)	13.4
specified max angular rate (rpm)	804

Interface Firmware:

- measuring of encoder rates to 10,000 transitions/s
- reporting accumulated encoder counts every 10ms (100 Hz) -- after queuing the data for transmission, the internal accumulator is zeroed.

High-level Software:

- display of distance, velocity and acceleration -- one or more than one on the screen at the same time
- velocity and accel. use digital filtering to reduce noise.

The following graph shows an initial data collection with an earlier prototype:

I turned the wheel slowly with my hand approximately 360 degrees over a 1.5 period. Please note that the graph uses two different Y axis scales for velocity and distance. The distance is the integration of the velocity data.