Page 289 - Understanding Automotive Electronics

2735 | CH 8 Page 276 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

a specific purpose. Op amp 1 is used as an error amplifier. The output of op
amp 1 is proportional to the difference between the command speed and the
actual speed. The error signal is then used as an input to op amps 2 and 3. Op
amp 2 is a proportional amplifier with a gain of K = –R /R . Notice that R is
1
1
2
P
variable so that the proportional amplifier gain can be adjusted. Op amp 3 is an
integrator with a gain of K = –1/R C. Resistor R is variable to permit
3
3
I
adjustment of the gain. The op amp causes a current to flow into capacitor C
that is equal to the current flowing into R . The voltage across R is the error
3
3
amplifier output voltage, V . The current in R is found from Ohm’s law to be
e
3
V e
I = -----
R 3
which is identical to the current flowing into the capacitor. If the error signal V
e
is constant, the current I will be constant and the voltage across the capacitor
will steadily change at a rate proportional to the current flow. That is, the
capacitor voltage is proportional to the integral of the error signal:
1
V = – --------- V td
I ∫ e
R C
3
The output of the integral amplifier, V , increases or decreases with time
I
depending on whether V is above or below zero volts. The voltage V is steady
I
e
or unchanging only when the error is exactly zero; this is why the integral gain
block in the diagram in Figure 8.10 can reduce the system steady-state error to
zero. Even a small error causes V to change to correct for the error.
I
The outputs of the proportional and integral amplifiers are added using a
summing amplifier, op amp 4. The summing amplifier adds voltages V and V
I
P
and inverts the resulting sum. The inversion is necessary because both the
proportional and integral amplifiers invert their input signals while providing
amplification. Inverting the sum restores the correct sense, or polarity, to the
control signal.
Because the output of The summing amplifier op amp produces an analog voltage, V , that must
s
the summing amplifier is be converted to a duty-cycle signal before it can drive the throttle actuator. A
an analog signal, it must voltage-to-duty-cycle converter is used whose output directly drives the throttle
be converted into a actuator solenoid.
duty-cycle signal to Two switches, S and S , are shown in Figure 8.10. Switch S is operated
2
1
1
pulse the throttle by the driver to set the desired speed. It signals the sample-and-hold electronics
actuator. (Figure 8.11) to sample the present vehicle speed and hold that value. Voltage
V , representing the vehicle speed at which the driver wishes to set the cruise
1
controller, is sampled and it charges capacitor C. A very high input impedance
amplifier detects the voltage on the capacitor without causing the charge on the
capacitor to “leak’’ off. The output from this amplifier is a voltage, V ,
s
proportional to the command speed that is sent to the error amplifier.
276 UNDERSTANDING AUTOMOTIVE ELECTRONICS