JLG 800S Service Manual - Manual (Page 244)

SECTION 4 - BOOM & PLATFORM
4-28 – JLG Lift – 3121139
PLATFORM VALVES
The platform specific valves are located in a manifold at
the platform.
There are six valves that control various platform func-
tions. Two control Platform Level up and down for the lev-
eling 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. PWMing permits
proportional flow control.
There is also a Platform Dump Valve, located in the plat-
form valve manifold, which is used to hydraulically isolate
the control valves and to improve hydraulic response.
The Ground Module controls this valve to enable auto-
matic platform leveling and to provide manual platform
leveling in the event that the Platform Module is inopera-
ble.
In ground mode, the platform dump valve is turned on
whenever any platform or jib valve output is turned on.
Whenever all platform and jib valves are turned off, the
platform dump valve is turned off.
In platform mode, the platform dump valve is turned on
whenever 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 system 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 set-
point 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
platform 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 opera-
tion, but has the disadvantage of causing a lag (or slug-
gishness) 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 setting the valve open-
ing to correspond to the required amount of make-up
angle. The amount the valve opens when making an auto-
matic 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 operator does not have control over the latter two fac-
tors, but can affect the resulting current by adjusting the
crackpoint. Increasing the crackpoint makes the valve cur-
rent higher, resulting in quicker more aggressive control
and larger amounts of overshoot. Decreasing the crack-
point will result in smoother operation 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 conditions
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 perform-
ing 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, Tele-
scope, Jib swing (It is not necessary to level with jib lift,
since the mechanical linkage keeps the basket level).
These zones are prioritized when multiple functions are
active. The priorities are as follows.
1. Auxiliary power and any other function, zone = aux-
iliary power
2. Drive and any other function, zone = Drive