Page 598 - Mechatronics with Experiments

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auto-code generation tools, and compiled and linked to generate the executable code for
the target ECM. The executable code would then be downloaded to the flash memory of
the ECM, and tested on the vehicle with the actual I/O hardware. The conditions of the
control variables and control logic parameters can be continuously monitored and recorded
for debugging purposes using direct memory access tools into the ECM memory map as
well as by monitoring a common communication bus, that is CAN bus monitor software.
Basic operator I/O devices include two proportional command devices, such as a
joystick connected to a position sensor for each degree of freedom (Figure 7.114). One
joystick-position sensor combination is used to define the forward (positive) speed and
reverse (negative) speed and neutral speed (zero speed) of the vehicle, a second joystick-
position sensor combination is used to define the left and right turn steering command
signal. Steering is achieved by controlling the speed of the two tracks to different levels. If
the machine is commanded to turn left, then the left track is slowed down while the right
track is speeded up proportional to the steering angle command as well as the speed of the
machine.
The basic machine I/O includes six solenoids (two solenoids for bidirectional pump
displacement swashpalte angle control, one solenoid for motor displacement swash-plate
angle control per track), and two angular speed sensors (one per track).
An open loop speed control system for the hydrostatic drives is shown in Figure 7.114.
The commanded speed of each track is determined from two joystick sensors: F/N/R speed
command and the steering command,
V cmd1 = 1,cmd ⋅ (1 + 2,cmd ) (7.759)
V cmd2 = 1,cmd ⋅ (1 − 2,cmd ) (7.760)

Furthermore, for a realistic operator controlled system, there is always the following desired
components in the control logic:

1. Deadband between the joystick position sensor and output signal in control logic so
that small unintended movements of the joystick (due to vibrations and resolution of
the operator control ability) do not cause motion in the hydrostatic drive.
2. Lowpass filter or rate limiter functions to limit the maximum accelerations and remove
the sensor noise.
3. Gain (or look-up table for variable gain) to determine the commanded speed based
on joystick sensors. In practice, this is generally a variable gain, which is a curve or
multiple lines with different slopes or a look-up table.
4. Offset signal to solenoids, since there are always variations in the manufacturing
tolerances of the solenoids and valves they actuate, the initial offset current command
needed to just start the motion of the solenoid needs to be determined for each solenoid
as part of a calibration process on the machine during manufacturing.

Remark 1 If the vehicle it to travel in a straight line, the steering command would
be zero and both tracks would receive the same speed command (forward or reverse).
The solenoids for both track’s pumps and motors would also receive the same current
commands, where the only difference might be if their offset currents are different, in order
to obtain the same speed for the tracks. However, due to small variations in components
(i.e., between pumps in two tracks), the actual speed of each track cannot be exactly equal
in practical conditions. As a result, the vehicle travel path would not be a straight line. This
can be corrected by the operator periodically by corrective steering commands. A better
solution is to implement a closed loop control on the track speeds to make sure that both
tracks run at the same speed, and hence assure a better straight path motion. The only