Servo Frequency Response

Servo frequency response is the range of frequencies to which the system is able to respond in moving the load. It is a characteristic of the system, chosen by the designers so the system will be able to respond to whatever frequencies are expected to be present in the input signal for the particular application.

Oscillating Input Signal

At first, we considered the input order to a servo as being suddenly put at a fixed desired value. Later, we studied the case where the order slowly increased to the desired value. Actually, the input order to a servo in a given application may accelerate, start, stop, or oscillate about a fixed point. We will now consider the actions of a servo while the order oscillates. When the order is constant, oscillations of the load are undesirable. When the order oscillates, the load must oscillate in a similar manner.

Let's assume that an oscillating input signal (order) is applied to a servo. The load may behave in several ways. Ideally, it would respond in perfect sync with the order. Actually, the amplitude and phase of the load are different from those of the order, shown in the figure below. As we noted above, the frequency response of the system is normally designed so the load is able to respond to the order.


Servo frequency response .

A servo frequency response may follow the order in amplitude and differ in phase; it may follow the order in phase, and differ in amplitude; or it may differ in both phase and amplitude.

Bandpass Frequencies in a Servo system

Servos are plagued by noise signals that ride through the system on desired electrical signals. These noise signals cause roughness in the servo system and must be eliminated to obtain smooth servo operation.

By examining the different signals in a servo system, we can determine which frequencies are related to the movement of the load and which ones are from noise sources, such as static, motors, harmonics, and mechanical resonances.

Filters in the signal circuit can be used to shunt some of the unwanted frequencies away from the amplifier, and allow only those frequencies that represent load movement to enter the amplifier. This can also be accomplished by designing the BANDWIDTH of the servo amplifier to accept only the range of frequencies that represents valid servo signals and to reject all others. This smooths servo response, but has the drawback of reducing amplifier gain. Reduced amplifier bandwidth is another compromise in achieving optimum servo operation.

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