ECE 5320-Mechatronics Assignment 1: literature survey on Sensors and Actuators

1. ECE 5320-MechatronicsAssignment 1: literature survey on Sensors and Actuators Topic : Stepper motors (Actuators) Prepared by Vikas G Pai Dept of Electrical and computer engineering UTAH STATE UNIVERSITY Tel : (435)-753-4306 Email : vikasgpai@cc.usu.edu vikas@biology.usu.edu

2. Overview • The Step Motor is an electromagnetic device that converts digital pulses into mechanical shaft rotation • It has no brushes, or contacts. Basically it's a synchronous motor with the magnetic field electronically switched to rotate the armature magnet around • Most steppers can be stepped at audio frequencies, allowing them to spin quite quickly, and with an appropriate controller, they may be started and stopped "on a dime" at controlled orientations

3. Working • Stepper motors as the name suggests ‘step’ a little bit at a time. • Stepper motor consists of several field coils that are energized in a sequential fashion to rotate the motor’s armature in steps. • Stepper motors produce high torque at low speeds. • Stepper motors also have another characteristic, holding torque, Holding torque allows a stepper motor to hold its position firmly when not turning. • They have several windings which need to be energized in the correct sequence before the motor's shaft will rotate. • Most stepper motor control systems include a driver in addition to the translator, to handle the current drawn by the motor's windings.

4. Classification • Stepper motors types are: • Permanent magnet • Variable reluctance • Unipolar motors • Bipolar Motors • Bifilar motors • Multiphase motors

5. Working of variable reluctance motor • 3 windings typically connected as shown in the schematic below represent a variable reluctance motor. • The common wire typically goes to the positive supply and the windings are energized in sequence. • With winding number 1 energized, the rotor teeth marked X are attracted to this winding's poles. If the current through winding 1 is turned off and winding 2 is turned on, the rotor will rotate 30 degrees clockwise so that the poles marked Y line up with the poles marked 2

6. Working of variable reluctance motor • To rotate this motor continuously, we just apply power to the 3 windings in sequence. winding1 : 1001001 winding2 : 0100100 winding3 : 0010010 • There are also variable reluctance stepping motors with 4 and 5 windings, requiring 5 or 6 wires. • Toothed faces on each pole and a correspondingly finely toothed rotor allows for step angles as small as a few degrees.

7. Working of unipolar motors • These motors have a centre tap on each of the windings. • The center taps of the windings are typically wired to the positive supply, and the two ends of each winding are alternately grounded to reverse the direction of the field provided by that winding. • Motor winding number 1 is distributed between the top and bottom stator pole, while motor winding number 2 is distributed between the left and right motor poles

8. Working of unipolar motors • The rotor of unipolar motor a permanent magnet with 6 poles, 3 south and 3 north, arranged around its circumference. • For higher angular resolutions, the rotor must have proportionally more poles. • The current flowing from the center tap of winding 1 to terminal a causes the top stator pole to be a north pole while the bottom stator pole is a south pole. • This attracts the rotor into the position shown. If the power to winding 1 is removed and winding 2 is energized, the rotor will turn 30 degrees, or one step.

9. Working of unipolar motors • To rotate the motor continuously, we just apply power to the two windings in sequence. Winding 1a 1000100010001000100010001 Winding 1b 0010001000100010001000100 Winding 2a 0100010001000100010001000 Winding 2b 0001000100010001000100010

10. Working of Bipolar Motor • Bipolar permanent magnet are constructed with exactly the same mechanism as is used on unipolar motors. • Two windings are wired more simply, with no center taps. • The drive circuitry for such a motor requires an H-bridge control circuit for each winding. • some permanent magnet stepping motors have 4 independent windings, organized as two sets of two. Within each set, if the two windings are wired in series, the result can be used as a high voltage bipolar motor. If they are wired in parallel, the result can be used as a low voltage bipolar motor.

11. Working of Bipolar Motor • The control sequences for single stepping such a motor are shown below, using + and - symbols to indicate the polarity of the power applied to each motor terminal Terminal 1a +---+---+---+--- ++--++--++--++-- Terminal 1b --+---+---+---+- --++--++--++--++ Terminal 2a -+---+---+---+-- -++--++--++--++- Terminal 2b ---+---+---+---+ +--++--++--++--+

12. Working of Bifilar motors • Bifilar windings on a stepping motor are applied to the same rotor and stator geometry as a bipolar motor, but instead of winding each coil in the stator with a single wire, two wires are wound in parallel with each other. • Motors with bifilar windings are always powered as either unipolar or bipolar motors. • To use a bifilar motor as a unipolar motor, the two wires of each winding are connected in series and the point of connection is used as a center-tap.

13. Working of Bifilar motors • To use a bifilar motor as a bipolar motor, the two wires of each winding are connected either in parallel or in series.

14. Working of Multiphase motors • A less common class of permanent magnet or hybrid stepping motor is wired with all windings of the motor in a cyclic series, with one tap between each pair of windings in the cycle, or with only one end of each motor winding exposed while the other ends of each winding are tied together to an inaccessible internal connection. • These configurations would be described as Delta and Y configurations. • Some multiphase motors expose all ends of all motor windings, leaving it to the user to decide between the Delta and Y configurations, or alternatively, allowing each winding to be driven independently.

15. Working of Multiphase motors • Control of either one of these multiphase motors in either the Delta or Y configuration requires 1/2 of an H-bridge for each motor terminal. • Many automotive alternators are built using a 3-phase hybrid geometry with either a permanent magnet rotor or an electromagnet rotor powered through a pair of slip-rings.

16. Specifications • Step angles: More poles are required on both the rotor and stator for more steps. • Accuracy: The accuracy for can-stack style steppers is 6 - 7% per step, non-cumulative.. The incremental errors are non-cumulative because the mechanical design of the motor dictates a 360° movement for each fullrevolution. The physical position of the pole plates and magnetic pattern of the rotor result in a repeatable pattern through every 360° rotation (under no load conditions).

17. Specifications • Resonance: Stepper motors have a natural resonant frequency as a result of the motor being a spring-mass system. When the step rate equals the motor’s natural frequency, there may be an audible change in noise made by the motor, as well as an increase in vibration. The resonant point will vary with the application and load, but typically occurs somewhere between 70 and 120 steps per second • Torque The torque produced by a specific rotary stepper motor is a function of:• The step rate• The current through the windings• The type of drive used

18. Application • Computer peripherals • Floppy drives • CD Rom’s • Hard drives • Plotters • Business Machines • Card Reader • Copy Machine • Type Writers

19. Application • Process controls • Carburetor Adjusting • Valve Control Valve Control • Silicon Processing • Liquid Gasket Dispensing • Machine tools • Milling machines • Drilling machines • Grinding machines • Laser cutting • lathes

20. Advantages • Advantages of Stepper MotorsSome of the advantages of stepper motors over servo motors are as follows: • Low cost • Can work in an open loop (no feedback required) • Excellent holding torque (eliminated brakes/clutches) • Excellent torque at low speeds • Low maintenance (brushless) • Very rugged - any environment • Excellent for precise positioning control • No tuning required

21. Disadvantages • Disadvantages of Stepper MotorsSome of the disadvantages of stepper motors in comparison with servo motors are as follows: • Rough performance at low speeds unless you use micro stepping For more information about micro stepping, see the Stepper Motor Switching Sequence link below) • Consume current regardless of load • Limited sizes available • Noisy • Torque decreases with speed (you need an oversized motor for higher torque at higher speeds) • Stepper motors can stall or lose position running without a control loop

22. Selection, cost ,buying info • Selection: Depending on the number of steps required stepper motors with appropriate • Average to small size stepper motors cost around $5 and above, with more high performance stepper motors running into hundreds of dollars. • www.digikey.com • www.73.com • http://www.orientalmotor.com/products/ac-dc-step-motors/index.htm

23. References • http://zone.ni.com/devzone/conceptd.nsf/webmain/A18266D91803B4D18625685D006EC4E8 • http://www.cs.uiowa.edu/~jones/step/

24. Thank You ! Vikas Pai