Page 44 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c01
30 MECHATRONICS Printer: Yet to Come October 9, 2014 7:39 254mm×178mm
ECM
w cmd Electronic governor
control algorithm Torque
+ Droop regulator
Governor governor "Full range"
– governor
Accelerator pedal
position sensor Low idle
speed limit
High idle
Droop speed limit
Fuel
injectors Speed
sensor
Engine Low Intermediate High Speed
idle speed idle
speed Speed
(~600 rpm) (~2100 rpm)
FIGURE 1.24: Electronic governors: steady-state operation and performance description on
lug curve.

controller is suppose to control the fuel rate from zero to maximum fuel rate range in
order to maintain the engine speed equal to the low idle speed. As the load torque on the
engine changes (i.e., due to friction, temperature condition, or other parasitic loads such as
heating and air-conditioner load), the controller will increase or decrease the fuel rate along
a vertical line on the low idle side, from zero (or minimum) fuel rate to maximum fuel
rate range.
On the high idle speed, that is when throttle pedal is pressed to maximum position,
the electronic governor again tries to maintain the maximum desired high idle speed by
selecting the proper fuel rate from zero to maximum fuel rate. Let us consider that the engine
is at the high idle speed (throttle is pressed to maximum position) and the load torque is
zero. After some time, let us assume the engine load torque increases to a value that is less
than the maximum torque capacity of the engine at the high idle speed. This will result in
a drop in engine speed in transient response. But, in steady-state the engine speed should
recover to the high idle speed. In other words, engine speed is regulated along a vertical
line at high idle speed. Mechanical governors are able to operate along a line that has a
finite slope instead of being vertical line. If we desired to emulate mechanical governor
behavior in our electronic governor, then the control algorithm for the electronic governor
may be modified as follows. The commanded (desired) speed (w cmd ) is determined first
from the throttle position sensor, then it is modified (w ∗ ) based on the estimated load
cmd
torque. Let us assume that we have estimated load torque (T ) information or measurement
l
in real-time.
w cmd = f( throttle ); throttle or accelerator pedal sensor position (1.9)
= w max ; for high idle condition (1.10)
1
∗
w = w − ⋅ T (1.11)
cmd cmd l
K tw
With this modified command signal to the closed loop control algorithm, the net result of
the high idle speed regulation turns into a line with slope of −K instead of being a vertical
tw
line.
Likewise, at any intermediate speed between low idle and high idle, the engine can be
controlled to maintain a desired speed exactly, regardless of the load torque amount as long
as it is less than the maximum torque the engine can provide at that speed using maximum
fuel rate. This torque is defined by the lug curve for each speed. If we want to achieve a
vertical line of speed regulation (ability to maintain speed despite load torque as long as