TwinCAT characteristic curves - Introduction
When hydraulic cylinders are used together with the appropriate valves, the way in which the cylinder and valves are constructed often results in a non-linear transmission behaviour of the axis, and thus of the system that is to be controlled. The speed of the hydraulic cylinder is not, in other words, proportional to the valve's drive signal. This can occur, for instance, if a hydraulic cylinder is used in which the two faces to which pressure is applied to not have a 1:1 area ratio. Non-linear valve curves are another typical reason for this behaviour. The following diagram illustrates a possible form for such a characteristic curve.
Characteristic Curve Linearisation
Problems can arise in association with such non-linear controlled systems, in that a linear controller design based on this non-linear segment is found to be inadequate, because the controller can only be set up to operate optimally over one part of its working range. The consequence is a loss in the quality of regulation in many applications, or even that the behaviour of the controller is unacceptable.
The purpose of the curve linearisation module presented here is to facilitate continued use of the familiar and proven procedure for designing linear control loops, but to improve the regulation quality. The superposition of a characteristic curve module compensates for the non-linearity of the control system, resulting in approximately linear behaviour. This procedure is illustrated in the following functional diagram.
The curve employed in this procedure must describe the inverse of the transmission behaviour of the particular combination of valve and hydraulic cylinder being used as exactly as possible; the net result of the inclusion of this characteristic curve module in series with the physically existing controlled system then results in an approximation to a linear curve.
Emphasis is particularly to be placed on achieving the greatest possible accuracy of the curve where it bends, since these regions are particularly critical for the linearisation.
The following functional diagram illustrates the use of the characteristic curve linearisation to the TwinCAT axis control loop:
The curve required for the linearisation process can be created and edited with the Valve Diagram Editor. After the curve has been created and loaded into the real-time environment, it can be activated within the axis control loop. This is done in the System Manager on the drive's analog tab, or generally by means of anADS command.
The unique table-ID for the valve curve must be entered in the "Valve Diagram: Table Id" line. It is possible to choose between the "Linear" and "Spline" types in the "Valve Diagram: Interpolation type" line. (A table with equidistant reference points is created in the real-time environment, and this is interpolated at run-time using either a linear or a spline function.)
It is also possible to insert an output offset before and after the characteristic curve module.
The parameter "Drift Compensation (DAC offset)' operates in the signal flow before the characteristic curve. An offset correction in the form of a velocity (in mm/s, for instance) can be added here.
An offset can be inserted in the signal flow after the characteristic curve with the parameter 'Valve Diagram: Output offset [-1.0 ... 1.0]'. At this point in the signal flow the offset is presented as a percentage value relative to the maximum output magnitude.
 | Using the hydraulic characteristic curve The hydraulic characteristic curve can only be activated through entry of the table ID when
|
The parameters described on the drive's analog tab can also be specified by means of ADS commands (sent, for example, from the PLC).
Drive types:
The characteristic curve linearisation described is supported by the following drive types:
- M2400 DAC 1/ DAC 2 / DAC 3 / DAC 4
- KL4XXX/KL2503-30K/KL2521
- Drive (universal)