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Issue 1.0 16/12/99
Fluxgate (Compass).Two anti-phase signals are provided from microprocessor
Ports P1.7 and P1.6. These signals are buffered by TR15 and TR16 to provide a high-
er current drive to the excitation coil of the fluxgate. Areference voltage level of +2
volts is provided by R46 and R53 decoupled by C53. The 2 coils, mounted at right
angles, provide output signals proportional to the sine and cosine of the Earth’s mag-
netic field. These signals are fed via the electronic switch IC9, to 2 dual slope inte-
grating analogue to digital converters IC7 and IC 8 plus associated components. The
outputs of the comparator IC7 are fed to the microprocessor Ports P1.4 and P1.5
which provide input capture facilities. Accurate timing of the conversion is kept by the
microprocessor to provide simultaneous precision analogue to digital conversion of
both sine and cosine signals to avoid errors created by multiplexing the inputs.
Motor Drive.The motor drive signals (MDRIVE_Aand MDRIVE_B) are generated
from the microprocessor at Ports P0.7 and P0.6, these lines being LOW when there
is no drive and HIGH to drive. The drive outputs control IC10 and IC11 which are
comparators set at threshold levels of +4V and +1V produced by the resistor network
R71, R72 and R73. The comparators invert the signals and provide the current to
switch the MOSFETs in stages which prevents both Pand N channel MOSFETs on
the same side of the “H-bridge” configuration, (i.e. TR10 and TR11 or TR12 and
TR13), being partially switched on at the same time. When there is no drive, both N-
channel MOSFETs TR11 and TR13 are switched on giving a direct short across the
motor to the 0V line thus providing active braking
Feedback. Mechanical movement is sensed by 2 Hall Effect devices mounted on the
Hall Effect PCB (Drawing E03182refers). The devices are triggered by rotating mag-
nets mounted in the pulleywheel of the Wheelpilot drive assembly to produce quad-
rature style feedback. The 2 feedback signals, FB1 and FB2 which are at logic lev-
els, are connected to microprocessor ports P1.1 and P1.2.
EMC.Capacitors with values of 100pF, 100nF and 1nF are extensively used to
decouple noise from switched data lines. Two A.C. coupled connections labelled
“CHASSIS” , routed via capacitors C63 and C64, were provided for interconnection to
the internal metalwork for EMC purposes. Tests have confirmed that connection is not
required.
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