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motors080904

# motors080904 - Motors and Position Determination...

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Motors and Position Determination Motors and Position Determination L17 6.111 Fall 2003 – Introductory Digital Systems Laboratory

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x Controlling Position Controlling Position Feedback is used to control position. Measure the position, subtract a function of it from the desired position and then use this resulting signal to drive the system towards the desired position. This is negative feedback. The natural frequencies of the feedback system are the “zeros” of 1 + G(s)H(s). z The total system is unstable if these “zeros” are in the right half plane (RHP). With 180 degrees phase shift, “negative” feedback becomes “positive” feedback. z So we want these “zeros” to be in the left half plane (LHP). z Putting an integrator into H(s) drives steady state error to zero. z But high order systems are more likely to have RHP zeros. z Time delay and high gain lead to RHP zeros. H(s) G(s) + + y Y(s) H(s) = X(s) 1 + G(s) H(s) L17 6.111 Fall 2003 – Introductory Digital Systems Laboratory
Servos Servos We can control parts of the servo, but the system dynamics is often a part we can’t control. The system dynamics results from masses. springs, losses, etc. Likely, we will implement servos as digital systems. Digital systems are more flexible to design. z They are more repeatable; they are not subject to gain drift. z We can use as many bits as we like so we can keep the computation noise small. Digital systems can have significant delays. z These delays are sometimes fixed, but are sometimes stochastic. Position + + x y Output Position G(s) C(s) A(s) Measurement System Dynamics Controls Likely boundary of Desired digital system L17 6.111 Fall 2003 – Introductory Digital Systems Laboratory

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Analog Position Measurements Analog Position Measurements Voltage is proportional to position.
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