What is the difference between pursuit tracking and compensatory tracking? In our computer demos (as elsewhere), the difference comes down to where the disturbance acts. In compensatory tracking, the unseen disturbance acts on the cursor’s position, causing the cursor to wander away from the (fixed) target position. In pursuit tracking, the disturbance acts on the target’s position, causing the target to wander.
In both tasks, the participant’s mouse affects the cursor’s position. Thus, in compensatory tracking, cursor position is jointly determined by disturbance and mouse movements; the participant learns to move the mouse in such a way a to compensate for the disturbance’s effect on cursor position. This allows the participant to keep the cursor close to the target position despite those disturbances. In pursuit tracking, cursor position is determined by mouse position and target position is determined by the disturbance. The participant learns to move the mouse in such a way as to make the cursor pursue the target (attempting to keep the cursor on the target).
In engineering, compensatory tracking mechanisms are called regulators; their job is to keep the controlled quantity close to a fixed reference value. For example, many electronic devices contain voltage regulators, which take as input a variable voltage and output a regulated voltage that is fixed at, say, 5 Volts. Pursuit tracking mechanisms are called servo-mechanisms, or servos. These are designed to receive a variable reference value and produce an output that matches that value. Robot actuators typically are servos, receiving reference inputs specifying actuator positions and varying the actuator positions so as to track the changing reference values. The control systems we employ to move and position our skeletal-musculature are servos. When you sign your name you are sending a specific, coordinated pattern of changes to the references of the systems that control the positions and rates of contraction of the several muscles driving the position of the pen point.
BA: In engineering, compensatory tracking mechanisms are called regulators; their job is to keep the controlled quantity close to a fixed reference value. For example, many electronic devices contain voltage regulators, which take as input a variable voltage and output a regulated voltage that is fixed at, say, 5 Volts. Pursuit tracking mechanisms are called servo-mechanisms, or servos. These are designed to receive a variable reference value and produce an output that matches that value. Robot actuators typically are servos, receiving reference inputs specifying actuator positions and varying the actuator positions so as to track the changing reference values. The control systems we employ to move and position our skeletal-musculature are servos. When you sign your name you are sending a specific, coordinated pattern of changes to the references of the systems that control the positions and rates of contraction of the several muscles driving the position of the pen point.
RM: I think the distinction between regulation (disturbance resistance) and servo control (reference variation) is unnecessary when we are talking about the controlling done by living systems. In a hierarchical control organization -- such as the one that seems to be the basis of the behavior of living systems -- both of these processes are going on simultaneously. Signing your name is, indeed, a good example of what you call "servo control", since you are secularly varying the reference for the position of the pen to "create" a signature. But what you call "regulation" is also involved, since it is necessary to compensate for disturbances to the position of the pen -- such as variations in the texture and angle of the surface being written on -- in order to keep the pen in the varying reference positions that create the signature.
BestÂ
Rick
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Richard S. MarkenÂ
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