ECGR-6185 Advanced Embedded Systems INTRODUCTION TO DIGITAL ACTUATORS (Stepper and Servo Motors) Gurunath Athalye University of North Carolina-Charlotte 02/21/20 Gurunath Athalye 1 Classification:
Variable Reluctance Permanent Magnet Hybrid Unifilar Bifilar Winding Configurations Full Step Half Step Micro Step 02/21/20 Construction
Drive Mode Gurunath Athalye 2 Variable Reluctance: Salient pole stator No windings on the rotor Stator and rotor pole numbers are different
Torque is developed by the tendency for the magnetic circuit to adopt the configuration of minimum reluctance 02/21/20 Gurunath Athalye 3 Permanent Magnet: Permanent magnet in the rotor
Stator is similar to single stack variable reluctance High inertia and lower torque to inertia ratio Restricted to large step size 02/21/20 Gurunath Athalye 4 Hybrid Motor:
Operated with the combined principles of the PM and VR motors Small step angle and a high torque from a small size 02/21/20 Gurunath Athalye 5 Unifilar Motor: Only one winding per stator pole
To change direction requires reversing the current in the same winding. 02/21/20 Gurunath Athalye 6 Bifilar Winding: Two identical sets of windings on each stator pole
Winding configuration simplifies the drive circuitry 02/21/20 Gurunath Athalye 7 Unipolar Motor: Current flow is limited in one direction To rotate the motor, just apply power to the two
windings in sequence 02/21/20 Gurunath Athalye 8 Modes of Stepper Motor: Full Step Half Step Micro Step
02/21/20 Gurunath Athalye 9 Conceptual Model of Unipolar Stepper Motor: Full Step The center taps of the windings are wired to the positive supply The two ends of each winding are alternately grounded to reverse the direction of the field
provided by that winding 02/21/20 Gurunath Athalye 10 Full Step Mode: Full step sequence showing how binary numbers can control the motor 02/21/20 Gurunath Athalye 11
Conceptual Model of Unipolar Stepper Motor: Half Step Same circuitry with different winding sequence Two windings are energized at the same instance 02/21/20 Gurunath Athalye 12 Half Step Mode:-
Half step sequence showing how binary numbers can control the motor 02/21/20 Gurunath Athalye 13 Micro Stepping Mode: Positional resolution is limited because of the mechanical design of the unit It allows a stepping motor to stop and hold a position between the full or half-step positions
The jerky character of low speed stepping motor operation and the noise at intermediate speeds 02/21/20 Gurunath Athalye 14 Principle of Micro Stepping (contd) Two sine waves in 'quadrture' (90 degrees out of phase) form the ideal current drive. 02/21/20
Gurunath Athalye 15 Principle of Micro Stepping: Phasor 02/21/20 Gurunath Athalye 16 Identifying a stepper motor:- 02/21/20 Gurunath Athalye 17
Identifying a stepper motor: Stepper motors have numerous wires, 4, 5, 6, or 8. When you turn the shaft you will usually feel a "notched" movement. Motors with 4 wires are probably Bipolar motors and will not work with a Unipolar control circuit. The most common configurations are pictured above. You can use an ohm-meter to find the center tap - the resistance between the center and a leg is 1/2 that from leg to leg. Measuring from one coil to the other will show an open circuit, since the 2 coils are not connected. (Notice that if you touch all the wires together, with power off, the shaft is difficult to turn!) Shortcut for finding the proper wiring sequence Connect the center tap(s) to the power source (or current-Limiting resistor.) Connect the remaining 4 wires in any pattern. If it doesn't work, you only need try these 2 swaps... 1 2 4 8 - (arbitrary first wiring order) 1 2 8 4 - switch end pair 1 8 2 4
- switch middle pair You're finished when the motor turns smoothly in either direction. If the motor turns in the opposite direction from desired, reverse the wires so that ABCD would become DCBA. 02/21/20 Gurunath Athalye 18 Servo Motors: Servo motors has an output shaft which can be positioned to specific angular positions by sending the servo a coded signal
As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. As the coded signal changes, the angular position of the shaft changes 02/21/20 Gurunath Athalye 19 FUNCTIONAL SERVO LOOPS Feedback causes the amplitude of the error signal to decrease, slowing the speed at which the load is moving
Servo systems are also classified according to their functions: POSITION, VELOCITY, and ACCELERATION . 02/21/20 Gurunath Athalye 20 References: Dr. James M. Conrad http://www.coe.uncc.edu/~jmconrad accessed : February 2006 Douglas W. Jones, Control of Stepping Motors A Tutorial, http:// www.cs.uiowa.edu/~jones/step/
accessed : February 2006 http://www.stepperworld.com accessed : February 2006 http://hibp.ecse.rpi.edu/~connor/education/IEE/IEE-Lec8.ppt accessed : February 2006 http://www.tpub.com/content/neets/14187/css/14187_95.htm accessed : February 2006 02/21/20 Gurunath Athalye
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