3. Construction

    The 3D locator consists from these parts :     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 :     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 :

  1. After power-up or reset, micro-controller is awaiting configuration data and the starting command.
  2. When starting command arrives, endless loop of measuring and sending data begins.
  3. TOF is measured for all four receivers.
  4. Status of the two buttons on the handle is detected and saved.
  5. Test is performed, if there was some movement compared to previous measurement. If no, data are discarded.
  6. Data are pre-filtered, most of largest errors are filtered out.
  7. Pre-processed data are sent to the host-computer via serial line.
  8. Process repeats from point  3.
    Achieved parameters can be summarized in the following table :
Parameter  Value  Remark 
Degrees of freedom 
Mouse cannot determine pitch and roll 
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. angular error 
cca 15o 
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 
cca 6000 Sk 
Material + salary of one person