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Using the Precision Navigation Vector 2X with the indoor location system.

[datasheet]

The PNI Vector 2X is used in conjunction with the location system to provide heading data. It is used in Master mode sending two bytes to the PIC. These are then passed on to the RS232 output port along with the location data.

Wiring Diagram

(Click here for full circuit diagram integrating Pinger Rx, Ultrasonics and Compass and 4 transducer PIC code too).

Description

Two magnetometers are used to measure the magnetic field strengths in the X-Y plane of the circuit board. Once it has been calibrated to remove the effects of local static magnetic fields, it is able to determine heading with an accuracy of a few degrees and a resolution of one degree. Accuracy deteriorates rapidly if the board is not level.

The compass is calibrated by:

  1. closing the switch on P5;

  2. pressing the switch on P6 (the red led on P13 illuminates);

  3. turning the board through 180deg, and pressing the switch on P6 again (the red led extinguishes); and

  4. opening the switch on P5 returning the board to it's normal mode in which it outputs data on P2 (the green led on P12 flashes). 

The switches on P10 and P11 invert the signal from the two magnetometers allowing the unit to be used upside down(!). P10 is normally closed.

The data is normally output as two bytes which can be read from P2. The clock frequency tends to be slightly erratic (why?!), so it is useful to synchronise reading the data with the clock signal. Consistent results are then obtained.

 

PIC subroutines to read compass.


;******
;    PULSE waits for -ve pulse, then returns on +ve edge

PULSE

                    MACRO
                    LOCAL     EDGE_P
                    LOCAL     EDGE_M

EDGE_P

                    BTFSC     A_PORT,SCLK        ; Test receive clock
                    GOTO     EDGE_P                     ; Clock is still high so loop until pulse (low) is seen.

EDGE_M

                    BTFSS     A_PORT,SCLK        ; Now wait for +ve edge
                    GOTO     EDGE_M                     ; Clock is still low
   
                    ENDM    


;-----------RXHEADG--------------------
;
;    reads and stores the two bytes of compass data

;-------v2.0 uses clock signal into RXD4 for timing data

RXHEADG

                    MOVLW      .7                 ; Load seven clock pulses
                    MOVWF     GP1

                    CLRF         DIR1                 ; Clear Direction byte 1

                    MOVLW     b'00000001'        ; Bit 0 is shifted with data and flags 8 rx'd.
                    MOVWF     DIR2                     ; Initialise Direction byte 2

HDGINI

                    PULSE                                     ; Read 7 pulses
                    DECFSZ     GP1,1       
                    GOTO     HDGINI

                    PULSE
                    BTFSC     A_PORT,SO             ; +ve edge so now receive data
                    BSF     DIR1,0                             ; Set Serial bit 0 - data was high i.e. a '1'   
   
                             ; Now get the second byte

HDG_RD

                    PULSE
                    RLF     DIR2,1                     ; Shift data left to make way for next bit
                    BCF     DIR2,0                     ; Clear Serial bit 0
                    BTFSC     A_PORT,SO    ; Test receive data
                    BSF     DIR2,0                     ; Set Serial bit 7 - data was high i.e. a '1'   
                    BNC     HDG_RD             ; Loop until the 'carry' bit appears

                            ; Received character is in DIR2, now dump the stop bit

                    WAITEM             ; Dump stop bit ???
                    RETURN             ; Exit from RXHEADG
;----------------------------------------------


This page last updated April 23, 2002
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