BECKHOFF AL2000 Application - Manual (Page 13)

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13
Accuracy
Generally the accuracy of the motor system is influenced by
• the accuracy of the error detection and positioning system,
• the static stiffness of the system,
• the dynamic stiffnes of the system
The main restriction for the accuracy arises from the lack of stability of the mounting frame, as discussed be-
fore. Here we discuss the restrictions of the linear motor system.
The linear motor itsself is an electromagnetic device. It deals with resistance and selfinductance. This implies
that the current in the coils always lags behind the provided voltage signal. Since the magnetic force is directly
linked to the current this means that the motorforce also lags behind.
With a static load (constant velocity) the positioning error will be small. Static disturbance arises from cogging at
stand-still or friction. Only a constant force has to be compensated. The system must react with an appropriate
contant force to the disturbance. Generally this is done by an integrator action of the position controller. This
action takes some settling time. The more accurate the positioning, the higher the settling time. An accurate
positioning typically requires a settling time of 5 to 25 ms.
With a dynamic load (accelerating or decelerating) the system shouldn’t just react appropriately but also rapidly.
Now the disturbance is dynamic. It arises from cogging during move, acceleration forces, vabrations and con-
tact forces (for instance when milling). Depending on the stiffness of the whole system the accurcy can deterio-
rate because of this. As a motor system can be considered as a kind of mass-spring system, it is obvious that
the accuracy is worsened most by disturbances at the system’s natural frequency.
By the servo controller the returned information of the position, the velocity and the current is processed and
translated into an appropriate voltage signal to the linear motor. The controller has to deal with some delay
because of processor and update times. For a schematic overview, see the appendix ’Influences on accuracy’.
The voltage signal to the linear motor is provided as pulses with a fixed switching frequency. Here the pulse
width is the parameter to be regulated. Accuracy is limited by the fixed pulse frequency and the fixed pulse
voltage.
Needless to say that the accuracy of the positioning system also depends on the accuracy of the linear encoder
and the thermal stability of the system’s components.