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lect8

lect8 - Lecture 8 Lecture Motors Actuators and Power Drives...

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Lecture 8 Lecture 8 Motors, Actuators, and Power Drives Motors, Actuators, and Power Drives Forrest Brewer

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Motors, Actuators, Servos Motors, Actuators, Servos Actuators are the means for embedded systems to modify the physical world Macroscopic Currents and power levels Thermal Management Power Efficiency (often vs. Performance) Motor Types DC Brush/Brushless AC (shaded pole and induction) Stepper Motors Servo (variety of DC motor) Peisio-electric (Kynar, Canon ultra-sonic) Magnetic Solenoid Electro-static (MEMS)
DC Motor Model DC Motor Model Torque (force) ~ Current Max Current = V/R •Max RPM = V/B emf B emf = L dI/dt In general: Torque ~ (V – B emf )/R R + - Torque Bemf In-oz/Amp V/krpm L Output RPM Voltage

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torque τ speed ϖ k e V R k τ V max speed Linear mechanical power P m = F v Rotational version of P m = τ • ϖ power output speed vs. torque Jizhong Xiao speed vs. torque, fixed voltage stall torque
k τ k e Controlling speed with voltage DC motor model V e R The back emf depends only on the motor speed. The motor’s torque depends only on the current, I. e = k e ϖ τ = k τ I Consider this circuit’s V: V = IR + e I stall = V/R current when motor is stalled speed = 0 torque = max How is V related to ϖ ? V = + k e ϖ τ R k τ ϖ = - τ + R k e V Speed is proportional to voltage. Jizhong Xiao

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Electrostatic MEMS Actuation Electrostatic MEMS Actuation Electrostatic Drives (MEMS) Basic equations Rotation Drive Comb Drive
Electrostatic Actuator Analysis Electrostatic Actuator Analysis Consider the capacitance of the two figures: To good approximation, the capacitance is double in the second figure: Imagine that the charge is fixed in the top figure: The stored energy is not the same! The difference must be the work done by the motion of the plate: Plate-1 Plate-2 Plate-3 V (volts) d Plate-1 Plate-3 V (volts) d Plate-2 2 2 1 1 V C V C Q = = 2 / 2 1 C C = ) 2 1 ( 2 1 2 1 2 2 2 2 2 1 1 1 V C E V C E = = = 2 V dx dE F Fdx E = =

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Electrostatic Actuators Electrostatic Actuators Plate-1 Plate-2 Plate-3 V (volts) x y d Consider parallel plate 1 & 2 Force of attraction (along y direction) F p = (½ ε V 2 29( A/g 2 ) [email protected] dimensionless Consider plate 2 inserted between plate 1 and 3 (Popularly known as a COMB DRIVE) F c = (½ ε V 2 )(2t/g) Force of attraction (along x direction) Constant with x-directional translation
Paschen Curve Paschen Curve Distance*pressure (meter*atm) Breakdown voltage E bd ~100MV/m ~200V [email protected] 1atm

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