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SCA PRINCIPLES
COMPUTERIZED CROWNER RETROFIT SYSTEM
With a conventional mechanical crown system, the adjustable eccentric rotates a
fixed angle either side of top dead center. The angle of rotation is controlled
by change gears, which are selected from a chart to produce the desired
truncated portion of a sinusoidal curve. The cam eccentricity is adjusted for
each roll to give the desired crown height. The RGB Engineering Servo Cam
Actuator (SCA) utilizes the same cam to control the wheel motion. However, the
drive train is interrupted (usually at the change gear interface), so that the
cam rotation can be controlled with a DC servo drive. The cam is offset a
specific amount based on the customers maximum crown requirements. Instead of
the cam rotating either side of TDC, the cam rotation is restricted to only one
half of a full revolution - i.e.: 0 to 180 degrees. The graph below shows the
relationship of the wheel infeed as a function of the cam angle, for a typical
crowner installation. Note that this is a nonlinear curve, due to the sinusoidal
characteristics of the r
otating
eccentric. However, this does not matter, since the LVDT feedback transducer is
measuring the tilting wheelhead directly. In fact, the non-linearity of the cam
actuator actually works to improve system accuracy: For low crowns or concaves,
the system is operating near the 0 degree point where the slope of the curve is
less. As the crown heights increase, more rotation of the cam is used in the
steeper slope portion. This natural "variable gain" feature assures maximum
resolution for the grinding of straight rolls and small crowns where maximum
accuracy is required, and increased gain for the rolls with larger crown heights
where faster response is required. The cam eccentricity is set initially at the
time of the crowner installation, and does not have to be adjusted by the
operator. The eccentricity must be large enough so the throw of the cam is
sufficiently greater than the maximum infeed requirements. The 0 to 10 volt
working range of the LVDT is established so that there is a safe distance to the
cam's TDC and BDC critical rotation points. One of the primary advantages of
this SCA philosophy is that the existing cam gear drive remains essentially
intact ready to be used as a standby spare if ever needed.