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Servo Tuning

Setpoin

Distance

( D )

Acceleration

Constant

Velocity

Deceleration

Profile

Complete

Position

Time

Commanded

Position

Actual Position

The other type of time-varying position information is the actual position; that is, the

actual position of the motor/load measured with the encoder. Since this is the position

achieved when the motor responds to the commanded position, we call the overall picture of

the actual position over time the position response (see further discussion under Servo

Response Terminology).

To view the actual position, use the TPE (Transfer Position of Encoder) command; the

response represents the actual position at the instant the command is received. When the servo

system is not properly tuned, the actual position is often not the same as the commanded

position at any given point of time.

The difference between the commanded position and actual position is the position error.

To view the position error, use the TPER (Transfer Position Error) command; the response

represents the position error at the instant the command is received. When the motor is not

moving, the position error at that time is called the steady-state position error (see

definition of steady-state under Servo Response Terminology). If a position error occurs when

the motor is moving, it is called the position tracking error, or position following

error.

In some cases, even when the system is properly tuned, the position error can still be quite

significant due to a combination of factors such as the desired profile, the motor limitation,

the dynamic characteristics of the system, etc. For example, if the value of the velocity (V)

command is higher than the maximum velocity the motor can physically achieve, then when

the motor is commanded to travel at this velocity, the actual position will always lag behind

the commanded position and a position error will accumulate, no matter how high the gains

are.

Servo Response Terminology

Stability

The first objective of tuning is to stabilize the system. The formal definition of system

stability is that when a bounded input is introduced to the system, the output of the system

is also bounded. What this means to a motion control system is that if the system is stable,

then when the position setpoint is a finite value, the final actual position of the system is also

a finite value.

On the other hand, if the system is unstable, then no matter how small the position setpoint

or how little a disturbance (motor torque variation, load change, encoder noise, etc.) the system

receives, the position error will increase continuously, and exponentially in almost all cases.

In practice, when the system experiences instability, the actual position will oscillate in an

exponentially diverging fashion as shown in the drawing below. The definition here might

contradict what some might perceive. One common perception shared by many is that

whenever there is oscillation, the system is unstable. However, if the oscillation finally

diminishes (damps out), even if it takes a long time, the system is still considered stable. The

reason for this clarification is to avoid misinterpretation of what this user guide describes in

the following sections.

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Calculated Industries 6250 Guide de l'utilisateur Page 32

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