LOAD CONTROL OPTION
COMPUTERIZED CROWNER RETROFIT SYSTEM
In
addition to producing the normal crown profiles, the FFG/SCA (or SLA) control
can also be used to servo on the wheel load during finish grinding. Essentially,
the same SCA/SLA control is made to servo on the wheel load instead of the
position reference used during crowning.
The standard procedure is to rough and semi-finish grind in the conventional
manner, using the appropriate crown profile and bed correction curve if required
to compensate for wheel path deviations. The neckrests still have to be adjusted
(either manually or via the RGB taper control) to correct for roll taper. Once
the roll shape (flatness, crown, taper, etc.) is within tolerance, as determined
by caliper or saddle mic measurements, the load control mode can be employed.
Generally, this would be during the latter stages of semi-finish grinding, or in
the finish grind cycle. But it is important to understand that the roll shape
must be obtained before switching to load control, since theoretically load
control will only preserve the shape and not improve it. The advantage of load
control is that the wheel pressure across the roll face will be controlled by
the computer instead of the operator. It is possible for the operator to
continuously feed the wheel in and out while watching the ammeter. But this can
be a very tedious process, and the degree of control is a function of the
operator’s skill and endurance.
A wheel load threshold pot is provided which the operator uses to set the wheel
load. A separate program (see below) is used to calibrate the “no-load” curve –
i.e.: the amount of power required with the wheel running but not grinding, as a
function of wheel speed. The wheel load threshold is the increment above the
no-load power. In practice, the operator will probably simply adjust the pot
while watching the ammeter (or KW meter); in other words, the pot setting will
be increased or decreased until the desired amp load is maintained. The normal
procedure is to back off a little on each successive pass gradually reducing the
load to the minimum required for the desired surface finish.
The actual wheel load reference signal is input to the FFG, as well as to the
summing board in the SCA control station. The normal position signal (crown,
taper, precision infeed commands, etc.) is used as a “feed forward”
approximation of the true position, but during load control, the system is
servoing on wheel load, not LVDT position.
If unavailable from the wheel drive, the actual wheel motor horsepower can be
measured using a “Load Controls Incorporated” power transducer. This will
also be used as a wheel overload sensor tied in directly to the SCA/SLA to
inhibit the drive should an overload occur.
During the load control mode, the FFG display will be configured to reflect the
critical parameters. The “RUN” screen format will be changed to “RUNNING – LOAD
CONTROL”. When load control is called for, crowning and taper control are not
commanding the position directly, but provide the basis for a position
reference, so that the load control doesn’t have to work as hard. The computer
monitor will display information pertinent to the load control mode in the
left-hand corner of the screen.
Load control is only active between TSF and TSH. Beyond these points, the servo
motor will be inhibited, since load control has no meaning beyond the ends of
the roll face. The operator normally returns to the crown control mode by
deactivating the load control signal. Returning to the crown control is fail
safe, since the wheel will always pull away from the roll depending on how much
the wheel has worn.
Since the referenced load settings are referred to the wheel “no-load” current,
and the no-load current is a function of wheel speed, it will be necessary to
construct a no-load curve. This is really a lookup table that is developed using
a specialized motor calibration program “NO_LOAD”. The software is set up so
that the values of wheel speed and corresponding motor current will
automatically be read into the tables as the wheel speed is varied from minimum
to maximum. The FFG accepts an analog input from the wheel motor tach. During
operation, the FFG uses the actual wheel speed to look up the normal idle
current in the no-load table. This idle current is then added to the threshold
current to give the load control output to the SCA/SLA.