The design is characterized by relative simplicity and low part count. In particular,
it should be noted that the dog actuators do
not include a synchronizer mechanism, which typically is used in manual transmis-
sions. Thus, the dog actuator essentially cre-
ates a rigid link or engagement, and, as such,
it tolerates very small speed differentials at
the time of the engagement. This is an im-
portant constraint for the shift controller, to
be discussed later.
As with the conventional automatic trans-
mission, the example ECT has the capability
of executing power-on shifts such that pos- itive driving force is supplied throughout the
duration of the shift. Manual transmissions,
on the other hand, are characterized by a
(short) period of zero-output torque during the neutral phase of a shift. Among the
power-on shifts with simultaneous dog ac-
tuator engagement, the 1-2 upshift was judged as the most difficult in view of the large torque levels and speed ratio change involved. Consequently, the main emphasis
of the modeling and control work was on the
1-2 upshift as described next.
The 1-2 upshift strategy is illustrated in
Fig. 8. The shift consists of three phases:
torque, inertia, and level holding phases.
During the torque phase, the engine com-
bustion torque is transferred from the low to the high clutch, as can be seen from the cor-
responding pressure traces in Fig. 8. This transfer is necessary to reduce the turbine speed to the second-gear synchronous level. It should be pointed out that, due to the large speed ratio difference between the low and high clutch power paths, the torque phase may result in a relatively large torque
"hole." The corresponding drivability ef-
fects can be minimized through a fast torque transfer, however. Once the low clutch has
been unloaded, the high clutch controls the turbine speed to the new synchronous level
by following the speed ratio ramp as shown
in Fig. 8. This constitutes the inertia phase
of the shift. Subsequent to the inertia phase, the turbine speed is held at the second-gear
synchronous level. This level holding phase
facilitates the dog actuator engagement,
which is the most critical phase of the shift
for the present ECT in view of the precise
speed ratio control requirements. Once the
dog actuator has been engaged, the shift is
completed by releasing (or venting) the high
clutch.
Closed-loop implementation of speed ratio
control is essentially mandatory in order to
meet the stringent requirements of the inertia
and level holding phases. An important start-
ing point in control algorithm design is the development of a suitable power train model.
A model used for the present ECT control
design consists of submodels of an engine,
torque converter, and an automatic clutch with its associated electrohydraulic control
valve. This model is suitable for the design
of controllers that are active during the in-
ertia and level holding phases of a shift,
where only one clutch is used as a control-
ling actuator. Here the torque converter tur-
bine speed is controlled through the clutch
pressure modulation, which itself is con-
trolled via duty-cycle variations of a PWM solenoid.
The overall model block diagram is shown
in Fig. 9, and includes the torque converter,
engine, control valve-clutch actuator, and a
zero-order hold, which reflects the digital-
to-analog conversion process. The torque converter has been modeled as a two-port device, with the turbine and impeller torques as inputs and the corresponding speeds as
outputs. The details of the mathematical
model derivation can be found in [7]. The
resulting nonlinear differential equations
contain four states (impeller, turbine and re-
actor speeds, and torus flow) and two inputs (impeller and turbine torques).