Lecture3 - Position and Velocity Measurements Important in...

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Position and Velocity Measurements Important in many embedded applications, especially those in which the microprocessor must interface to mechanical hardware. Question: How many examples can you name? - - - Many microcontrollers have special features that expedite such measurements. The MPC5553 has an enhanced Time Processing Unit (eTPU) that is essentially a special purpose microcomputer that operates simultaneously with the CPU. The eTPU performs special functions that otherwise would require CPU interrupt service. - Quadrature Decoding - many others... Analog vs. Digital Measurement Technology - Signal/Noise ratio (S/N) - Dynamic Range (largest vs. smallest measurements) Information about technology for position and velocity measurements can be found in many textbooks, including [1], [2], and [4]. Reference [1] was used primarily in developing this set of notes. EECS461, Lecture 3, updated September 12, 2007 1
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Analog Velocity Measurement Tachometer: produces a voltage proportional to velocity (usually rotational velocity) same operating principle as DC motor; in fact, it is possible to use a DC motor as a tachometer. (will describe when we discuss DC motors) tachometer performance is often limited by noise (e.g., brush noise) at low velocities S/N ratio is poor, and it is difficult to determine when velocity is zero (i.e., when the mechanical system is stopped) EECS461, Lecture 3, updated September 12, 2007 2
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Analog Position Measurement Potentiometer: produces a voltage proportional to distance from a reference point Example: Measure distance of a cart from one end of a track Since there is a sliding mechanical contact, noise, dirt, humidity, and mechanical wear limit precision. The maximum range is also limited! EECS461, Lecture 3, updated September 12, 2007 3
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Optical Encoder Often used for digital position and velocity measurement Two types - absolute encoders: gives actual position - incremental encoders: gives change in position Usually encoders measure angular displacement - can be used to measure rectilinear position - computer mouse (2-dimensional position!) Will first discuss incremental encoder, because we will use in lab. EECS461, Lecture 3, updated September 12, 2007 4
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Incremental Encoder a wheel with little windows (front and side views): . LED light sensor As wheel rotates, the photocell generates a digital signal: 0 1 If windows and dividers are of equal width, then the “on” and “off” times are of equal duration for constant rotation rate.
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