# How to understand Bode plot or Bode diagram

I am having difficulty understanding the Bode diagram, which I have understood so far: that depending on the angle of the phase, where the system becomes unstable or stable.

I just don't know what the practical applications are when using this system in a closed loop.

I mean a practical example how to change the control, such as a robotic arm, or a mechanism that can be more sensitive (control)?

• The phase margin, obtained from the open loop Bode diagram, tells you the relative stability of the closed loop system, and also what you might do to the system in order to obtain the phase margin that you require.
– Chu
Commented Nov 1, 2020 at 0:47

The BODE plot is a graphical representation of the frequency-dependent gain and phase of a circuit (amplifier, filter, control system,...). In this context, there are two fundamental cases as far as a system with feedback is concerned:

(1) BODE-Diagram for the closed-loop system. Here you can see if the sysytem can meet your requirements concerning gain, bandwidth, phase shift, attenuation (for filters),...

(2) BODE-Diagram for the open-loop system (open the loop at a suitable point and find the gain and the phase for all pars/blocks within the whole loop in series).

This function is called "loop gain". Now you can apply the well-known stability criterion. For this purpose, you must find the phase shift for the frequency where the magnitude of the loop gain is 0 dB. From this analysis you can derive the stability margin, which tells you if the closed-loop will be stable and how many "margin" the closed-loop system has until it will become unstable.,

• When you said: From this analysis, you can derive the stability margin, which tells you whether the closed circuit will be stable and how many "margins" the closed circuit system has until it becomes unstable. Okay, thank you very much. But I understood the practical application concept in the area. Example a robotic arm with sensor, when an anomaly occurs in the system because of the signal that is unstable (the operation is out of control), when it is working normally the system is stable.Is the example you said about the robotic arm correct?In practice, I don't know where it is applied. Commented Nov 1, 2020 at 11:51
• @LUFER. I must admit that it is not clear to me what you are asking and what the problem is.
– LvW
Commented Nov 1, 2020 at 14:32

In analog signal processing, the Bode plots show the gain response of a filter at various frequencies. This is helpful for confirming whether or not filters have suitable gain at pass frequencies and the proper attenuation at frequencies you don't want.