Page 1 1 CS 5780 School of Computing University of Utah CS/ECE 6780/5780 Al Davis Today’s topics: • Relays & Motors • prelude to 5780 Lab 9 2 CS 5780 School of Computing University of Utah Relays • Common embedded system problem digital control: relatively small I & V levels controlled device requires significantly higher power • Solution amplify the control power use the control signal to activate a switch » switch turns on/off bigger power source • Electrically controlled switches transistor » can be used as a switch but it’s really an amplifier since it has gain • MOS – voltage controlled, BIPOLAR – current controlled relay » control induces magnetic field in coil » magnetic field moves a mechanical switch • bounce problem? – usually not a concern for outputs to non-digital gizmos like motorsPage 2 3 CS 5780 School of Computing University of Utah Poles and Throws • Terminology used for switches relay is just an electrically controlled switch » pole – controlled » throw – contact point » relay difference – magnetic movement of pole • difference in where the switch is when switch/magnet is off – off state usually controlled by a spring 4 CS 5780 School of Computing University of Utah Relay Types • Basic issue is size control power » reed relays – smallish power • common in ES designs » general purpose – large’ish power • you have lots of them in your carPage 3 5 CS 5780 School of Computing University of Utah Mechanical DPDT Illustrated 6 CS 5780 School of Computing University of Utah Solid State Relays • Improvement on mechanical relay problems contact bounce and arcing limit lifetime sensitive to vibrations, EMI issues slow movement of large mechanical pole • Optocoupler provides electrical isolation between input (pseudocoil) and output triac (pseudocontact) » particularly important in driving large inductive loads zero-voltage detector triggers triac » reduces surge currents when triac is switched once triggered » triac conducts until next zero crossingPage 4 7 CS 5780 School of Computing University of Utah Reed Relays 8 CS 5780 School of Computing University of Utah SolenoidsPage 5 9 CS 5780 School of Computing University of Utah Interfacing to Inductive Loads • Interface circuit must provide sufficient current and voltage to activate the device » common error • “my microcontroller puts out 5v but at the device it’s only 200 mV” • what’s the problem? 10 CS 5780 School of Computing University of Utah Interfacing to Inductive Loads • Interface circuit must provide sufficient current and voltage to activate the device » common error • “my microcontroller puts out 5v but at the device it’s only 200 mV” • what’s the problem? • Ohm’s law – current, impedance and voltage are related – microcontroller can’t provide enough current so voltage is similarly low in off state current should be zero BEWARE » large L huge back EMF when coil is turned off • fast digital switch causes large di/dt • 50 – 200V back is common » it will destroy your controller • isolation or buffering is required – optoisolator – or snub diode – etc.Page 6 11 CS 5780 School of Computing University of Utah Relay Control Examples 12 CS 5780 School of Computing University of Utah Relay & Motor InterfacesPage 7 13 CS 5780 School of Computing University of Utah IRF 540 Power Transistor 14 CS 5780 School of Computing University of Utah Isolated Interfaces split Darlington photodetector .5 mA in 1 A out 2000% current transfer ratio 5V logic compatible (TTL, CMOS)Page 8 15 CS 5780 School of Computing University of Utah Typical H-Bridge Motor Control 16 CS 5780 School of Computing University of Utah Isolated H-Bridge w/ Direction ControlPage 9 17 CS 5780 School of Computing University of Utah Stepper Motors • Popular due to inherent digital interface easy to control both position and velocity in an open-loop fashion more expensive than simple DC motor » still not too bad since may not require feedback sensors can be used as shaft encoders » measure both position and speed 18 CS 5780 School of Computing University of Utah Stepper Motor Basics • Stator stationary frame with electromagnet poles • Rotor teeth are permanent magnets alternating south and north pole teeth 360 degrees/(4 poles * 5 teeth) = 18 degrees per stepPage 10 19 CS 5780 School of Computing University of Utah 2 Phase Operation stable state 20 CS 5780 School of Computing University of Utah 2 Phase Operation reverse phase 1 polarity – unstable state – closest stable state?Page 11 21 CS 5780 School of Computing University of Utah 2 Phase Operation next stable state 22 CS 5780 School of Computing University of Utah 2 Phase Operation reverse polarity of phase 2 and movement continuesPage 12 23 CS 5780 School of Computing University of Utah Continue by Reversing Phase 1 24 CS 5780 School of Computing University of Utah Simple Interface note this motor has 200 steps – hence 1.8 degrees Port B output is (10, 9, 5, 6)* reverse direction? (6, 5, 9, 10)*Page 13 25 CS 5780 School of Computing University of Utah Slip & Torque Issues • Slip command issued but motor doesn’t move causes » motor torque insufficient to drive mechanical load » or if computer change is too fast • magnetic field is too weak • IF no slip can be guaranteed then computer knows the shaft position » and doesn’t need a sensor 26 CS 5780 School of Computing University of Utah Stepper Motor SequencePage 14 27 CS 5780 School of Computing University of Utah Control Data Structures (FSM) 28 CS 5780 School of Computing University of Utah Init RitualPage 15 29 CS 5780 School of Computing University of Utah Helper Functions 30 CS 5780 School of Computing University of Utah High Level ControlPage 16 31 CS 5780 School of Computing University of Utah Concluding Remarks • Lots of types of electrical motors stepper & DC are most common in inexpensive ES’s • Beware when driving inductive loads back EMF has to be controlled » snub diode is cheap » optical isolation
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