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Appendix A: Hardware Reference Manual
10.4.4 VRS Conditioning
10.4.4.1
Design Criteria
The variable reluctance sensor (VRS) conditioning circuit is used to convert the analog signal from the
crankshaft variable reluctance sensor into a logic level signal that the MCU can measure and extract
timing information from. A large AC differential signal from the sensor is conditioned to be a pulse where
edges represent the edges of the toothed-wheel on a rotating engine crankshaft. Since the amplitude of
the VRS signal ranges from less than 1.0 V over 70 V in most applications, the large gain of the
conditioning circuit must also reject noise.
10.4.4.2
Implementation Recommendations
The circuit chosen for the reference design uses the Maxim MAX9924. This device was selected because
it contains both a precision differential amplifier and a comparator with selectable adaptive peak threshold
and zero-crossing circuit block, all in a small 10 pin μMax package. The circuit is capable of operating in
four different modes noted A1, A2, B and C. The data sheet for the MAX9924 explains the pros and cons
of each mode and for the reference Design we opted for mode A2 because it was the simplest mode to
implement and required the fewest components. Custom implementations may incorporate the external
threshold voltage and provide interaction from the MCU based on the operating point of the engine. This
would allow the detection voltage of the VRS signal to be changed based on operating conditions. This
specifically benefits startup timing precision.
Additionally, the reference design can accommodate the use of a Hall Effect Sensor. Since Hall Effect is
essentially a low side switch that grounds a 5V signal on tooth edges, it provides good detection with
much less sensitivity than a VRS. To use a Hall Effect Sensor with the reference design, a short must be
placed from the VRSP input pin to the VRSOUT signal of the VRS conditioning circuit. This is provided
through R15, which is not populated in production. While Hall Effect Sensors provide cleaner signals and
simpler connection to the MCU, they potentially add system cost due to the sensor design itself.
10.4.5 Dual H-Bridge
10.4.5.1
Design Criteria
The idle bypass valve directly addresses issues with associated with running a cold engine. This is the
replacement for a choke found on carbureted engines and is electronically controlled by the MCU. Two
architectures are known for controlling an idle bypass valve. The difference is the type of electric motor
that is used to vary the amount of air flow in the bypass valve. A DC motor with a position feedback sensor
and a bi-phase stepper motor are the two possible solutions. Stepper motors require more complex
control techniques but allow for a simpler mechanical design and less calibration. A small stepper motor
used as an idle air speed motor typically has less than 1.0 A peak current and requires a dual H-bridge
circuit to control. The stepper motor is also an inductive load and requires suppression of the flyback
voltage. Since the idle speed motor provides a key role in the start and idle of an engine, it also plays an
important part in the start up and low speed emissions. This then requires diagnostic information about
the health of the idle speed motor to maintain emissions performance.
10.4.5.2
Implementation Recommendations
To create the dual H-bridge circuit used to control a stepper motor, the MC33879 is used. Early versions
of the reference design may use the pin for pin compatible MC33880. The MC33879 is a configurable
output device capable of delivering a minimum of 0.6A for each leg of the dual H-bride. Each output is
52
Freescale Semiconductor
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相关代理商/技术参数
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