Electrical Design
Power Supply
Power supply and distribution could arguably be the most
important part of the design. With many of the integrated
circuits (ICs) chosen for the ECU requiring different
operating voltages, it was necessary to develop a method
of power conversion. A 1.9V supply is needed to power the
core of the microcontroller; 3.3V is required to set the
maximum I/O voltage of the controller and to power
several ICs; 5 V to power multiple ICs and most car
sensors; 8V to power the car's Hall Effect sensors and
two ICs. With the only source of voltage coming from the
car's 12V battery, DC-DC buck converters were chosen to
reduce the 12V down to usable 8V, 5V, and 1.9V levels. To
generate the 3.3V, a linear regulator was chosen instead
of another buck converter. Using the linear regulator,
the 5V rail was reduced to 3.3V. Generally, the low
efficiency associated with this type of a regulator is a
serious draw back for power applications. In this case,
however, the voltage drop is relatively small, therefore
causing minimal loss in efficiency. Aside from
efficiency, the small amount of current that the 3.3V
rail is predicted to draw made the linear regulator a
logical choice.
DC-DC Converters
Typical LT1933 Circuit
The original design team chose a component from
Linear Technology based on its wide input range, ability to
handle the 200mA loads (which were determined through a
power analysis), and the ability to simulate the circuit
prior to building. However, the part chosen fell into the
category of a DC-DC controller, giving the user control
over several circuit specifications (e.g. switching
frequency, current mode, sense voltage, etc). The current
design team found the part to be unnecessarily complicated
and instead chose to use a standard DC-DC buck converter,
also from Linear Technology, which allowed less overall
user control but met all requirements to successfully
regulate the voltages on the ECU. The chosen part was the
LT1933. With its high efficiency operation, 500 kHz fixed
switching frequency, wide input range (3.6V - 36V), current
sourcing up to 600mA, simple voltage divider feedback
network, and its tiny 6-lead package, the LT1933 was an
ideal fit to out circuit.
Injector Drivers
Injection Driver Circuit
Two common types of injectors exist: peak-and-hold
and saturated. Saturated injectors are of high impedance
(10 - 15 ohm) and require the driver circuit to supply a
constant voltage for the duration of time the injector is
to be opened. These are very inexpensive, simple, and
reliable. A downfall to the saturated injection system is
that it has a slower response time than a peak-and-hold
type. Peak-and-hold injection systems are more expensive
and complex than saturated circuit systems and are
primarily used in aftermarket high performance systems due
to their faster response time. Peak-and-hold injectors are
of lower impedance (1-4 ohms) and use a peak and hold
driver to control them. The peak current (~ 4A) is required
to open the injector, while the hold current (~ 1A) is used
to keep it open for the remainder of the injection cycle.
It is the driver's job to detect when the peak current has
been reached. It will then reduce the injector current to
the appropriate hold level. RIT's formula car utilizes the
peak-and-hold injector system.
Nevertheless, the injector driver of choice, National
Semiconductor's LM1949, is capable of controlling both
types of injectors. With the formula car utilizing
peak-and-hold injectors, the driver circuit was
configured accordingly. There were numerous flaws in the
initial design of this circuit which were caught during
testing; incorrect zener diode placement and timer
configuration, specifically. The errors were fixed and
the new design was successfully verified during testing.
Ignition Drivers
RIT's formula car uses an ignition box to fire the
ignition coils. The ignition box requires an input signal
of 5V peak-to-peak for every instance that the ignition
coils are to be fired. The microprocessor generates these
signal based on the crank and cam inputs signals from the
car's Hall Effect sensors. The resulting ignition signal
is output from the microcontroller to a digital buffer
(SN74AHCT125), which increases the voltage from the
processor's 3.3V peak to the desired 5V peak. A series
resistance is added to the output of the line to minimize
any ringing that may occur. The original design of the
ignition drivers was successfully verified during
testing.
Hall Effect Comparator Circuit
The Hall Effect sensors operate off of an 8V supply
and produce an inverted pulse that occurs each time a
magnet passes by the surface of the sensors. To minimize
errors, the interface circuitry in the ECU utilizes
comparators with an adjustable threshold. When the input
voltage from the sensors steps below a desired voltage a
transition in the output of the comparators occurs, sending
a digital pulse to the microcontroller. A standard quad op
amp (AD8604) chip was used and configured as a comparator.
The original design of this circuit was successfully
verified during testing and therefore left untouched.
Analog Input Circuit
Analog inputs for the ECU include sensors such as the
manifold absolute pressure sensor (MAP) and the throttle
position sensor (TPS). All analog sensors on the car
operate off of a 5V supply rail and so their output signals
are capable of a 5 V swing. Since the analog to digital
converter (ADC) on the microcontroller requires that
signals have no larger than a 3 V swing there must be
circuitry that exists to drop the voltage. Because analog
signals are extremely susceptible to noise interference,
the signal is sent through a low pass filer, serving to rid
the signal of any high frequency noise. To achieve this, a
simple 2nd order op-amp filter is implemented with a cutoff
frequency of 1 kHz. The microcontroller uses the MAP and
TPS inputs to determine fueling and timing parameters. For
the most part, these analog input circuits were left alone
since the original circuit was designed correctly. This was
verified during testing.
Temperature Sensor Interface
Temperature Sensor Circuit
Temperature sensors are utilized on the FSAE car to
monitor intake air temperature and water temperature. These
sensors use a NTC thermistor, which means that as the
temperature of the sensor increases, the resistance
decreases. The temperature sensors on the car are measured
with a single stage op-amp circuit on the ECU, which serves
to supply voltage to the sensor and to amplify the
circuit's input voltage. The resulting voltage at the
output of this analog circuit is fed to the
microcontroller's ADC.
The original design of the temperature sensor interface
was modified slightly to include a "safety resistor" in
series with the thermistor. This safety resistor ensures
that the current through the thermistor never exceeds its
maximum current rating of 1mA through a temperature range
of 0 to 100 degrees celsius.
Relay Circuits
Relay Circuit
The fuel pump and fan are activated using single pole
single throw (SPST) relays in the car. When the relays are
energized, the fan and fuel pump draw their power from the
car battery. The relays require a minimum of 6V to switch,
and draw a maximum of 100mA (measured through testing).
The original design of this relay circuit was incorrect
and had to be completely redesigned. The microcontroller
will determine when the fan or fuel pump need to be
turned on and output a dc voltage accordingly. Since the
maximum voltage that the microcontroller can source is
3.3V, an op amp was chosen to boost it to the desired 6V.
Most op amps, however, cannot source the 100mA required
for the relay to switch so it was important to find one
that could (AD8397). This system was successfully
verified during testing.
