Page 208 - Mechatronics with Experiments

194 MECHATRONICS
controlled, the pressure at the piston control chamber is regulated proportionally to the
desired torque transmission. Such multi disc clutch/brake components are commonly used
in automotive transmissions to engage or disengage desired gear ratios.
The actuation mechanism may also be based on a direct electrical coil actuation. When
the current is applied to the coil, it acts as an electro-magnet, and is used to engage/disengage
the multi disc brake/clutch. If the coil current is proportionally controlled, then the multi
disc brake is capable of proportional operation.
The mechanical dimensions and the number of disc-plates used determines the max-
imum torque capacity of the multi disc clutch/brake. The dynamic response depends on
the type of actuation method, that is mechanical linkage, pneumatic, hydraulic, electro-
hydraulic, or direct electro-magnetic. Typical response times vary from 15 ms to 500 ms
for electro-magnetically controlled types.

3.8.6 Example: An Automatic Transmission
Control Algorithm
Figure 3.32a shows the basic logical blocks of an automatic transmission control algorithm.
In automatic transmission control, the real-time control algorithm in the electronic control
module (ECM) essentially tries to do what a good professional driver tries to do with a
manual shift transmission; accomplish a “smooth and yet fast enough” gear shift using
various sensory information about the condition of the vehicle.
The algorithm monitors three main sensory signals:
1. transmission gear selection lever, operated by the driver (i.e., P, R, N, D for park,
reverse, neutral, and direct automatic mode),
2. engine speed sensor (input speed to transmission),
3. transmission output speed sensor.

Based on the operator gear selection, the algorithm selects a desired gear ratio. If the
operator selects P or N, the corresponding gear ratio selection is directly made based on
that selection of the operator. If the operator selects R or D, the gear ratio is selected based
on the engine speed and transmission output speed sensor. The algorithm estimates the
vehicle speed based on the transmission output speed (neglecting the effects of wheel slip),
and decides on the appropriate gear. This decision is based on the principle illustrated in
Figure 3.32.
Given current gear and current engine speed, from the gear shift table we can deter-
mine up-shift speed and down-shift speed for the transmission output speed (Figure 3.32b).
If the current transmission output speed is between these two values, no gear change is
made. If it is below the down-shift speed value, the gear is shifted down one gear. If it is
larger than the up-shift speed value, the gear is shifted up one gear. In order to make sure
the gear shift does not occur due to noise in the signal, the measured transmission output
speed is verified to be out of the range defined by up-shift and down-shift speed values for
a period of time, that is 100 ms, before an actual up-shift or down-shift decision is made.
Next, using a look-up table, the algorithm determines which clutch/brake combination
should be engaged and disengaged (Figure 3.32a). An example of such a table is shown
in Figure 3.27. Once the decision of which clutch/brake combination needs to be engaged
or disengaged is made, the next block on the control algorithm implements a pressure
control algorithm to achieve the desired pressures in those clutch/brakes as a function
of time as shown in Figure 3.32c. This is to control the transient response of the gear
shift. The figure shows the typical desired pressure profile for smooth gear shifting in
automatic transmissions. In large equipment applications, typical clutch/brake engagement