Parker EME
Setting up Compax3
192-120101 N11 C3I11T11 November 2007
145
Stiffness
In this chapter you can read about:
Static stiffness..................................................................................................................................145
Dynamic stiffness.............................................................................................................................145
Correlation between the terms introduced.......................................................................................147
The stiffness of a drive represents an important characteristic. The faster the
disturbance variable can be compensated in the velocity control path and the
smaller the oscillation caused, the higher the stiffness of the drive. With regard to
stiffness, we distinguish static and dynamic stiffness.
Static stiffness
The static stiffness of a direct drive is comparable with the spring rate D of a
mechanical spring, and indicates the excursion of the spring in the event of a
constant interference force. It is the ratio between the constant force FDmax of the
motor and a position difference. Due to the I term in the velocity controller, the
static stiffness is therefore infinitely high in theory, as the I term is integrated until
the control difference vanishes. In a digital control the static stiffness is above all
limited by the finite resolution of the position signal (the error must be at least one
quantization step, so that it can be detected by the reading system) and by
numerical resolution. Additional effects are for instance mechanical stiffness of the
mechanic components in the control path (e.g. load connection, guiding system) as
well as measurement errors of the measurement system.
Dynamic stiffness
In this chapter you can read about:
Traditional generation of a disturbance torque/force jerk ................................................................145
Electronic simulation of a disturbance torque jerk with the disturbance current jerk .......................146
Disturbance jerk response ...............................................................................................................146
The dynamic stiffness is described by the ratio between the change in load torque
or in load force and the resulting position deviation (following error):
x
M
L
Δ
Δ
−
The higher this ratio (=dynamic stiffness), the higher the necessary change is loard
torque in order to generate a defined following error.
The dynamic stiffness can be acquired from the disturbance jerk response.
Traditional generation of a disturbance torque/force jerk
FG
FM
m
In settled state of the control, the motor force FM corresponds exactly to the load
force FG=m×g.
If the cord is cut through, the load force is eliminated abruptly and the controller
must first of all settle to the new situation.
In order to simulate this load jerk electronically, a disturbance current jerk is fed to
the Compax3 as a variable proportional to the disturbance torque at the velocity
controller output.
