National Instruments NI 9751, Руководство пользователя, Страница: 18 / 30

NI 9751 User Manual
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© National Instruments
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17
Piezo Injector Operation
The NI 9751 can of drive unipolar piezo injectors. Unipolar means the piezo injector is charged
to a positive voltage and discharged to ground. Bipolar drive means the piezo injector can be
charged to a positive or negative voltage. Channels 1 and 2 are viable for driving piezo
injectors. You must connect Channel 3 with a shorting jumper between the Inj+ and Inj-
terminals.Electrically, piezo injectors are a capacitive load, instead of an inductive load from
solenoid injectors. Solenoid injectors require current to be driven through the solenoid to create
the proper magnetic field within the injector body to mechanically open a valve. Piezo injectors
contain a stack of piezo crystal elements that respond mechanically to a voltage applied across
the stack. The two pins of the piezo injector are connected internally across the stack. As the
voltage across the stack increases, the stack expands, and exerts a force to open a fluid valve.
The piezo injectors respond more quickly than a solenoid operated injector. It is difficult to
estimate the fuel quantity injected while the injector valve is opening. By achieving a shorter
time from energizing to valve opening, the fuel injection quantity can be metered more
precisely.
Electrically, a piezo stack is like a capacitor because it requires current in one direction to charge
up to a specified voltage, and current in the opposite direction to discharge
.
The typical charge
voltage required for piezo injectors ranges from 100 V to 200 V. Most piezo injectors in
production today require up to 165 V, but vary depending on desired fuel rail pressure. A piezo
stack charges up to a specified voltage at a rate that does not damage the stack. The charge and
discharge current must be controlled. You can control charge and discharge current with an
inductive element in series with the piezo stack and treating the element similar to a current
control inductive load. The NI 9751 provides a special piezo mode of operation that utilizes the
same IPhaseArray interface to control current to the piezo stack for charge and discharge rates.
When charging the piezo stack, the final charge is a few volts less than the HVTarget setpoint.
When discharging the piezo stack, the final discharge state is 0 V.
The IPhaseArray behaves differently in piezo mode. The first four elements of the IPhaseArray
are used for charge and the last four elements are used for discharge. The BackBoostTime
parameter is not used for piezo injector operation. The DriveSrc Boolean for each IPhaseArray
element is not applicable for piezo injector operation because the high-voltage driver circuit is
always used. Figure 16 shows a representative simplified schematic that drives two piezo
injectors.