BECKHOFF EL7037 - Manual (Page 20)

Product overview
• Cogging torque
In many cases the stepper motors design results in high cogging torque, which can lead to relatively strong
natural resonance in a motor- and load-dependent speed range. In relation to the cogging torque, increased
inertia often leads to a less strong resonance and smoother operation.
• Mass moment of inertia
In standard mode, the key parameter of the mechanical system is the mass moment of inertia J
Σ
. It is
essentially composed of the mass moment of inertia of the stepper motor rotor J
M
and the mass moment of
inertia of the connected load J
L
. The friction moment J
fric
and the moment of inertia of the encoder J
Enc
can be
neglected in a first approximation.
J
Ʃ
≈ J
M
+ J
L
The ratio between the load torque and the motor torque is defined by the constant k
J
.
k
J
≈ J
L
/ J
M
Fig. 10: Simplified representation of the mass moments of inertia
As a first approximation, the coupling of the individual masses over the rotor shaft can be modelled as two-
mass oscillator. The resonance frequency between the motor and the encoder lies in a relatively high
frequency range, which is usually not relevant for stepper motor drives and is suppressed within the drive by
low-pass filtering. The resonance frequency between the motor and the load is frequently in the range
between 20 and 500 Hz. It is therefore often in the operating range of the drive control. Design measures to
reduce the influence of the load resonance include a small load ratio k
J
and a rigid coupling of the motor
shaft to the connected load.
• Resonance
At certain speeds, stepper motors run less smoothly. This phenomenon is particularly pronounced when the
motor runs without coupled load, in which case it may even stop (in standard mode). This is caused by
resonance. A distinction can roughly be made between
• resonances in the lower frequency range up to approx. 250Hz; and
• resonances in the medium to upper frequency range.
Resonances in the medium to upper frequency range essentially result from electrical parameters such as
inductance of the motor winding and supply line capacity. They can be controlled relatively easily through
high pulsing of the control system.
Resonances in the lower range essentially result from the mechanical motor parameters. Apart from their
impact on smooth running, such resonances can lead to significant loss of torque, or even loss of step of the
motor, and are therefore particularly undesirable.
In principle, the stepper motor represents an oscillatory system (comparable to a mass/spring system),
consisting of the moving rotor with a moment of inertia and a magnetic field that creates a restoring force that
acts on the rotor. Moving and releasing the rotor creates a damped oscillation. If the control frequency
corresponds to the resonance frequency, the oscillation is amplified, so that in the worst case the rotor will
no longer follow the steps, but oscillate between two positions.
The EL70x7 EtherCAT Terminals prevent this effect thanks to their field-oriented control (Extended
Operation Modes) for all Beckhoff stepper motors.
EL70x7 20 Version 1.0