3. Construction

The 3D locator consists from these parts :

Moving handle with ultrasonic transmitter
Plane with four receivers (three of them are sufficient, the fourth is for maintaining some redundancy and improving accuracy).
Interface to the computer (PC)

All these parts are shown on the fig. 2. Schematical diagram

As ultrasonic receivers and transmitter are used crystal units with resonance frequency at cca 40 kHz. Each receiver has its own amplifier with amplification ratio 900:1. These receivers are multiplexed into one signal, which drives interrupt line of the micro-controller. Not shown, but important parts are driver and receiver of RS-232 line and power supply. Micro-controller used is an 8-bit type AT89C2051 from Atmel. It contains 128B RAM, 2kB Flash EPROM, 2 counters/timers, serial interface and analog comparator. Usage of this progressive part simplified construction a lot, because all the hard work is done inside the micro-controller. It has these main tasks :

Generates 40 kHz signal for the transmitter
One of the timers acts as an accurate "stop-watch"
Does first pre-filtering of measured data
Communicates with controlling PC via RS-232 interface (serial line)

Construction was inspired by article [7], where is described an ultrasonic motion detector for security applications. The digital part of the construction is an original work.

The simplified algorithm of the micro-controller can be described as follows :

After power-up or reset, micro-controller is awaiting configuration data and the starting command.
When starting command arrives, endless loop of measuring and sending data begins.
TOF is measured for all four receivers.
Status of the two buttons on the handle is detected and saved.
Test is performed, if there was some movement compared to previous measurement. If no, data are discarded.
Data are pre-filtered, most of largest errors are filtered out.
Pre-processed data are sent to the host-computer via serial line.
Process repeats from point 3.

Achieved parameters can be summarized in the following table :


Parameter	Value	Remark
Degrees of freedom	3	Mouse cannot determine pitch and roll
Resolution	cca 0.5 cm	Theoretically should be possible to achieve even 1 mm
Resolution	cca 0.5 cm	Theoretically should be possible to achieve even 1 mm
Working volume	cca 40x40x40 cm	Typical (desktop) installation
Max. working distance	cca 3 m	In z axis, distance of the trasmitter from the receivers plane
Max. working distance	cca 3 m
Max. angular error	cca 15^o	Angle, by which is possible to receive signal from the transmitters


Sampling frequency	cca 20 Hz	Depends on distance of the transmitter to the receivers
Sampling frequency	cca 20 Hz	Depends on distance of the transmitter to the receivers
Costs	cca 6000 Sk	Material + salary of one person