7.11.3.3 Example of application for volume/mass sensors
Image 7.60: Example of application for volume/mass sensors
7.11.4 Hydraulic Limitations
7.11.4.1 Pressure limit
It is usually necessary to limit the pressure to a maximum permissible value. This
can be done either mechanically using an overpressure valve or using the pressure
limitation of the servo drive. The pressure limitation allows both the maximum and
the minimum permissible reference value of the pressure control to be limited. If
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ID No.: CB40859-001 Date: 11/2020
MSD Servo Drive - Device Help
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7 Control
"negative" pressures can also be present in an application with pressure difference
measurement, a negative value must also be entered or permitted in the HYD_
PressCon.pMin parameter. If these pressure limits are reached, the pump speed is
limited accordingly dynamically. In addition, the source of the limiting pressure value
can be selected using P 2840[5] - pMaxSrc and P 2840[6] - pMinSrc.
It is possible to toggle between parameter P 2850[6] - pMax and the reference value
P 2843[0] - pRef by selecting PressCon(0), Pos_pRef(1) and Neg_pRef(2). This
allows the pressure limitation to be specified via the analog input of the pressure
reference value, for example.
The pressure limitation control uses the same controller parameters P 2850[0] Kp
and P 2850[2] Tn as the standard pressure control. However, the control structure
differs from the standard pressure controller. The pressure limiting controller tends to
oscillate when the control is set very dynamically. Consequently, we recommend
choosing a setting with low gain factors and then increasing it according to the
requirements. In addition, the setting of the integral-action time (the integral part) is
absolutely necessary for the function of the limiting controller. Our recommendation
is values in the high two-digit or lower three-digit range. However, these values are
only a guideline. Every application will, of course, have its own requirements for
dynamics and accuracy. The optimum setting must be determined during
commissioning.
For the purpose of pressure limitation during the process or in individual motion
blocks, a scaling (P 2872 HYD_Tab_Limit) can be used to influence the preset
limitation. (
see also section "Hydraulic motion block table" on page 193 and see also
section "Scaling of the hydraulic limits " on page 195)
Furthermore, the limitation can be switched to inactive by means of a higher-level
control system. The five lower bits of the hydraulic control word are used for this
purpose, P 2841 HYD_Ctrl bit 11 to bit 15. Setting bit 11 is deactivates the pressure
limitation. Bits 12 to 15 only switch off the limitation in the respective quadrant of the
P/Q coordinate system. (Bit 12 = Q1, bit 13 = Q2, bit 14 = Q3, bit 15 = Q4).
