Page 190 - Mechatronics with Experiments

176 MECHATRONICS
When the driver wants to change gear,
1. First, the clutch is disengaged so the engine does not power the counter-shaft.
2. Then the gear selector lever is moved to the desired position which then moves the
synchronizer into contact with the selected gear on the output shaft. Hence, that gear
is now connected to the output shaft rigidly, instead of free-spinning. The process
of moving the synchronizer to engage a selected gear takes some time. Before the
clutch is re-engaged, the synchronizer couples with the selected gear, and then brings
its speed to the same speed of the output shaft, and locks it to the output shaft.
3. After that, the clutch should be re-engaged. If this speed synchronization does not
happen well with proper sequence and timing (i.e., the clutch is re-engaged too early,
before the synchronizer brings the selected gear to output shaft speed and locks it in),
then there will be some slip and audible noise in the gear shift until the selected gear
is fully coupled with the output shaft. On the other hand, if the clutch is re-engaged
too late, the vehicle will lose speed due to too operating without a power source for
too long.
The automatic shift versions of this type of transmission are commonly used in
agricultural equipment and automotive applications. The difference in the design is in the
mechanism by which the gears are engaged and disengaged. In the manual version, this is
done by the driver using a shift lever mechanism which physically moves the synchronizers
with a fork mechanism to engage/disengage desired gears. In the automatic version, there
are two main components,

1. An electronic control module (ECM) makes the decisions on which gear to shift, based
on various sensor signals (i.e., operator pedal position, engine speed, transmission
output speed), and sends the corresponding command signals (i.e., current) to the
actuation devices (i.e., valve solenoids).
2. An actuation mechanism that would include an electro-hydraulic (or electro-
pneumatic or electric) solenoid actuated valves, cylinders, and hydraulic supply
pressure in order to move the selected synchronizer to the desired gear engage-
ment position. Cylinders in this type of applications are so small and have a very
short stroke, they are often referred as “pistons.”
In other words, the decision to shift the gear is made by the ECM, the actual motion to make
the shift is accomplished by a motion control system (which is controlled by the ECM)
which may be an electro-hydraulic, electro-pneumatic, or all electric circuit. A common
feature of any automatic shift transmission is that the components of the shift motion
mechanism are typically an electro-hydraulic system which has:

hydraulic supply/return: hydraulic supply pressure line from a pump and return line
to oil reservoir,
valves: solenoid actuated valves to control the flow or pressurized fluid into the
“actuators,”
actuators: small pistons/cylinders which are moved by the flow from the valves and

then the pistons are used to engage/disengage the selected clutch or brakes or move
synchronizers,
clutch/brakes or synchronizers: highly integrated compact disc clutches and brakes

are built into the transmission frame.
At any given time, the transmission is in either neutral position (none of the gears are
engaged) or in one of the six gear positions (5 forward, 1 reverse). The gear ratio at any