Basics of the Bode plot

A Bode plot is a graphical toolbox for displaying the frequency characteristics of a system. It consists of two plots, the gain response and the phase response.

Basics of the Bode plot 1:

The specific properties of a mechanical drive system can be graphically displayed
and determined using the Bode plot. It also provides information about the system stability and the dynamics of the system.

In the following, the Bode plot for mechanical control systems is explained in such a way that the meaning of the variables and parameters used in the user interface with regard to dynamics and stability is made clear and a basic understanding of how to read a Bode plot is taught.

To describe the system characteristics using the Bode plot, it is necessary to divide the control drive system into open and closed control loops.

Basics of the Bode plot 2:

In a control system with a cascade structure, consisting of a current, speed and position control loop from inside to outside, the speed control loop is the one in which the mechanical system properties act.


Control loop without feedback consisting of controller, controlled system (mechanical system in this case) and feedback (sensors).

Open loop (stability analysis)

System stability is determined on the basis of the phase margin and the gain margin.

Basics of the Bode plot 5:

Closed loop

The Bode plot of the closed loop shows the physical effect of the frequency-dependent gains and phase shifts.

The ideal system would be one for which the output signal has the same gain as the input signal for all frequencies and has no delay in relation to the input signal. Such a system has zero values for gain and phase in the Bode plot.
In practice, this is not possible in a real system for energy and runtime reasons. As the frequency increases, the output signal becomes weaker than the input signal over time and exhibits an increasing phase shift compared to the input signal.
Furthermore, resonance points and the parameterization of the controller can cause the gain of the output signal to exceed that of the input signal locally for certain frequencies. In this context, the terms bandwidth and maximum gain are defined.

Basics of the Bode plot 6:

Optimization goals

In principle, short machine throughput times are required for high productivity.
These require correspondingly high bandwidths of the drive controllers used for fast processing while guaranteeing the requirements for accuracy and precision.
The demands for a high bandwidth and high stability are usually contradictory. An increase in bandwidth generally leads to lower stability and vice versa. Another optimization goal can be requirements for the smooth running of the drive. Moderate controller gains are often advantageous for smooth running.