Eurotherm Drives 590 DRV Digital, Product Manual

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590 DRV Digital DC Drive Product Manual
App. M - 27
Appendix M Special Blocks and Application Notes
M
Control Modes
SC provides the drive with control features required for a line drive section. SC uses drive terminals for all inputs and
outputs, both analog and digital. These terminals may need to be re-configured depending on the functions required for
the particular application.
A typical web process line will have a master or lead drive that sets the speed of the line and other slave drives which
follow in a variety of ways. The primary control requirement is usually to maintain a reasonable tension between sec-
tions. Section control may be achieved in a number of ways.
Draw
Draw mode is the nearest approximation to an old fashioned line shaft
with its mechanical variators. It works well even when the section has
little or no grip on the web although in those applications it will not
control tension. In draw mode, the slave drive is speed controlled with
a percentage difference from the line speed reference. The speed dif-
ference or draw, which is usually small, stretches the web if it is ex-
tensible and the speed difference is positive, or permits the web to
contract if the speed difference is negative.
When draw is used with non-extensible webs, very accurate speed
control is required to provide acceptable tension control. With these
materials, a very small speed difference either causes the web to be
too loose or too tight. This application uses the Digital Draw feature
discussed later in this chapter.
Ratio
Ratio control is speed control. It is similar to draw except that it is
specified as a ratio of the line speed instead of a percentage differ-
ence. Ratio control generally varies over a much wider operating
range than draw. Ratio is typically used by sections that have no effect
on web tension but is required by the manufacturing process, for ex-
ample on coater applicators and metering rolls.
Torque
Torque control is an open loop method of controlling tension because
there is no direct tension measurement. It assumes that the process
losses are small so the web tension will be proportional to torque at
the motor shaft. The drive section torque is proportional to motor ar-
mature current and the speed is determined by the web speed assum-
ing the drive section does not slip.
NOTE. To achieve constant tension throughout the speed
range, use a constant torque motor. A motor with a field range
will produce less torque as the speed increases above its base
speed.
Figure M.19 - Draw
Figure M.20 - Ratio
Figure M.21 - Torque