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Technical Reference 227 Step Motors & Drives

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Technical Reference 227 Step Motors & Drives Step Motors & Drives The typical step motor system consists Step Motor Construction Permanent Magnet Step of a step motor and a drive package The three most popular types of step Motors that contains the control electronics motors are the variable reluctance and a power supply. The drive receives The permanent magnet (PM) step (VR), permanent magnet and the hy- step and direction signals from an in- motor is shown in Figure 14. Perma- brid. The hybrid step motor is by far dexer or programmable motion con- nent magnet motors differ from VR’s the most popular. While a variety of troller, which may be an integrated part by having a permanent magnet rotor step angles are available, the most of the drive. The drive logic determines with no teeth, which is magnetized common and popular is the 200 step the correct current level for each phase perpendicular to its axis. The stator per revolution, which is commonly re- of the motor and the power amplifiers consists of two stamped steel cups ferred to as a 1.8 degree step motor. generate and maintain the proper cur- with diagonally shaped teeth facing the rent loads. The power supply provides rotor. Coils of wire are wrapped the necessary voltages from the AC Variable Reluctance Step around each of the stator cups. or DC power source. Motors By energizing the phases in sequence, The general construction for a variable the rotor will rotate as it follows the Advantages of step motor reluctance step motor is shown in Fig- changing magnetic field. In this ex- ure 13. There is a stator assembly con- systems include: ample, the motor’s step will be 90 de- sisting of an insulated lamination stack • Inherently a digital device, number of grees. Typical steps for PM motors are with coils wound around the teeth. The pulses determine distance, frequency 45 and 90 degrees. They step at rela- stator assembly is positioned within a determines speed tively low rates, but exhibit high torque housing, or main frame, so that its lo- and good damping characteristics. • Cost efficient, simple designs cation is secured. The rotor assembly • Drift free consists of a steel magnetic core, a A steel shaft, and bearings. The rotor A1 • Non-cumulative error assembly is positioned in the center • Brushless design for reliability and of the stator, both radially and axially. simplicity The rotor is supported by end frames or bearing supports. B • Maintenance free, bearings lubricated D1 for life N S B1 When a stator phase is energized, the D • High torque per package size, high soft-iron rotor is electro-magnetically acceleration and power rates attracted to the stator poles. A rotor • Stable at zero speed tooth attempts to align with the near- • Holding torque at standstill (high stiff- est energized stator pole. A rotor step ness) takes place when one stator phase is C1 de-energized and the next phase in C • Can be stalled repeatedly without the sequence is energized. To reverse damage Figure 14: Permanent Magnet the direction of rotation, the stator Step Motor • No extra feedback components re- poles would be energized in the re- quired (encoders can be added for verse sequence. Hybrid Step Motors additional advantages) Combining the qualities of both the VR • Bi-directional operation A B and PM, the hybrid step motor has multi-toothed stator poles and a mag- Step Motor Technology C net encased within a multi-toothed ro- tor. These types of motors exhibit high Step motors are electromechanical, detent torque, excellent static and dy- rotary, incremental actuators that con- namic torque, and can achieve high A1 vert digital pulses into mechanical shaft stepping rates. Normally, hybrids are rotation. The amount of shaft rotation designed for 0.9, 1.8, and 3.6 degrees is directly proportional to the input per step. pulses generated and the speed of the rotation is relative to the frequency of Refer to Figure 15. The motor shown the pulses. Most microprocessors, tim- has two windings on each stator pole ing/counting logic, or even switch or so that the pole can be either a mag- relay closures can generate the pulses Figure 13: Variable Reluctance netic north or south, depending on the used in a stepper system. The step Step Motor direction of current flow. Hybrid step driver acts to convert the logic pulses motors can achieve a wide range of by sequencing power to the stepper torque values by altering the length windings; generally, one supplied pulse Technical and diameter of the motor. Reference will yield one rotational step of the motor. MotionSELECTsm Motion Control Products & Systems Technical Reference 227 Step Motors & Drives C A bifilar. the two phases of the stator windings B1 creates a rotating electromagnetic field. D1 D Unifilar Winding The number of times that switching B takes place controls the shaft position A unifilar winding refers to the winding of the motor. The frequency of the configuration of a step motor where A1 switching controls the velocity of the each stator pole has one set of wind- C1 motor shaft. The permanent magnet ings; the motor will only have four lead rotor will rotate in fixed mechanical in- wires. See Figure 16 for winding con- crements to maintain rotor and stator figurations. This winding configuration tooth alignment. Reversing the basic can only be driven by a bipolar drive switching sequence causes the motor design. shaft to reverse direction. Maintaining current flow at standstill results in zero Bifilar Winding speed stability and shaft stiffness. Figure 15: Hybrid Step Motor A bifilar winding refers to the winding configuration of a step motor where There are three types of step modes Hybrid step motors consist of just five each stator pole has a pair of wind- a stepping motor can operate in: full- basic components, with the bearings ings, and the motor will have either 6 step, half-step, and microstep. being the only contacting part. The or 8 lead wires, depending on the ter- rotor consists of a permanent magnet, mination, see Figures 17 and 18 re- often with rare earth properties, located Full-Step Mode spectively. This type of winding can between the top and bottom half of The term full step means that for each be driven by either a unipolar or bipo- the rotor stacks. The two stacks are digital pulse received from the drive, lar drive design. similar to the VR step motor, with the the shaft will rotate 1.8 mechanical magnet being placed between the two degrees. For one complete shaft revo- stacks. The two stacks are offset from Stepping Modes lution, the driver must give 200 digital each other by one-half tooth pitch. The In both the unipolar and bipolar drive, pulses. stacks are made up of a series of lami- current for both phases is controlled nations, each with 50 teeth on the cir- in a four-state sequence. This se- In full-step, the motors have a ten- cumference. This creates 50 natural quence, multiplied by the 50 rotor dency to oscillate with each step. This detent positions on the rotor. Each of teeth, creates the 1.8 degree step condition is most visible at low speeds, the detent positions will be maintained angle or 200 step per revolution mo- below one revolution per second. A even without power being applied. Di- tor. Switching the current on and off to resonant condition (loss of torque) may viding 360 degrees by the 50 natural positions on the rotor yields a mechani- cal step angle of 7.2 degrees. For this reason, motion will be seen + 3.6 de- grees when energizing or de-energiz- ing the motor. By changing the num- ber of stator and rotor teeth, the step angle of the rotor will change. Typical step angles are 0.9°, 1.8°, and 3.6°. Step motors are manufactured with single, double, and triple stack rotors on a single shaft. The number of stacks on a motor will be a determin- ing factor in the maximum torque out- put of the motor. Typically, a three Figure 16: 4-Lead Unifilar Step Motor Wiring for Bipolar Driver stack rotor will have approximately 2.5 to 3 times the static torque of a single stack rotor motor. Step Motor Windings There are two phases, or windings, in a typical step motor. The windings of the motor are located in the stator. This allows for optimum heat dissipation and lower inertia than DC brush type motors. There are two types of wind- ings used in step motors, unifilar and Figure 17: 6-Lead Step Motor Wiring 228 Technical Reference © API Motion Step Motors & Drives operation of a full-step system fre- quently jars the mechanical system to which it is attached, and therefore has limited application on machinery requir- ing smooth operation. Forbidden operating speeds below 1 rev/sec force system designers to ac- celerate the motor instantly to some “base” speed to avoid this velocity re- gion. This instantaneous acceleration also prohibits smooth mechanical mo- tion. The limited resolution of these systems (usually 200 or 400 steps/ revolution) may also necessitate gear- boxes or reduction transmissions to achieve the desired position resolution. There is a technique, however, that overcomes the low speed and limited resolution problems associated with step motors. It is known as microstepping. Microstepping systems use the same hybrid step motor (usually 200 full steps/rev) and precise current control to position the motor’s rotor at a loca- tion in-between the normal full-step po- sitions. While full-step drives produce Figure 18: 8-Lead Step Motor Wiring coil currents that are either “on” or “off”, microstepping drives proportion the current smoothly between the motor’s be observed.
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