JLG 680S, Service And Maintenance Manual

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SECTION 4 - BOOM & PLATFORM
3121234 4-45
PLATFORM VALVES
The platform specific valves are located in a manifold at the plat-
form.
There are six valves that control various platform functions. Two
control Platform Level up and down for the leveling function,
two are used to rotate the platform, and two to control jib up and
down.
All platform valves are Pulse Width Modulated (PWM’d). PWM is a
method of setting the voltage across a valve, and therefore the
flow through it, by varying the On/Off duty cycle of the control
module output. PWM permits proportional flow control.
There is also a Platform Dump Valve, located in the platform
valve manifold, which is used to hydraulically isolate the control
valves and to improve hydraulic response.
The Ground Module controls this valve to enable automatic plat-
form leveling and to provide manual platform leveling in the
event that the Platform Module is inoperable.
In ground mode, the platform dump valve is turned on whenever
any platform or jib valve output is turned on. Whenever all plat-
form and jib valves are turned off, the platform dump valve is
turned off.
In platform mode, the platform dump valve is turned on when-
ever the footswitch is depressed.
Normal Operation
AUTOMATIC PLATFORM ANGLE CONTROL
The level system will assume a new fixed set point (fixed incline
of the platform with respect to gravity) each time the control sys-
tem is powered up (cycling of the EMS) and each time the
footswitch is engaged.
Automatic platform angle control only functions while operating
drive, telescope, lift or swing. It does not adjust the platform
angle while operating any other function (e.g. rotate, jib, or
steer). Furthermore, machines equipped with control system
software P5.0 and later, automatic platform angle control for
drive and swing may be disabled by using the analyzer. For this
case, the platform angle setpoint is taken when the joystick
moves from a non-leveling function (drive/swing) to a leveling
function (lift/tele).
The machine controls attempt to maintain the angle of the plat-
form to setpoint by providing a command proportional to the
angular error from setpoint. Since the sensors used to measure
the platform angle are fluid-filled, gravity-based sensors, reading
the sensors in real time would cause constant correction of the
platform position due to machine vibration and inertial changes
of the boom. Therefore, the sensor readings are averaged over
time, or filtered, in order to achieve a more uniform reading. This
filtering has the advantage of providing smoother operation, but
has the disadvantage of causing a lag (or sluggishness) in the
system response. This lag may cause the platform to be several
degrees from setpoint.
In order to provide a better system response, the controls also
compute the rate of angular change of the platform position and
set the leveling valve positions to achieve a matching velocity.
The measured velocity is the average platform speed over the
last 0.5 seconds. The desired valve command is computed by
comparing the measured velocity to the desired velocity and set-
ting the valve opening to correspond to the required amount of
make-up angle. The amount the valve opens when making an
automatic correction is proportional to and directly affected by:
• Crackpoint setting
• Velocity error (proportional factor)
• Sum of velocity errors over time (integral factor)
These three factors are summed together with appropriate gain
factors to compute the resulting current to the valves. The opera-
tor does not have control over the latter two factors, but can
affect the resulting current by adjusting the crackpoint. Increas-
ing the crackpoint makes the valve current higher, resulting in
quicker more aggressive control and larger amounts of over-
shoot. Decreasing the crackpoint will result in smoother opera-
tion but may not permit enough platform velocity to keep up
with the boom (i.e., may get platform timeout alarms) in some
multi-function operations. The platform controls are set up to
provide smooth leveling operations for the majority of condi-
tions and will perform best for steady operator command, as
opposed to command values for function, that change fre-
quently.
In order to obtain acceptable performance while performing all
hydraulic functions, five sets of parameters are used. These
“zones” allow compensation for differences in how the basket
level changes when doing different functions. These zones are as
follows:
1. Lift up
2. Lift down
3. Other boom functions
4. Drive
5. Auxiliary
The other boom functions zone includes Swing, Telescope, Jib
swing (It is not necessary to level with jib lift, since the mechani-
cal linkage keeps the basket level).