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This is information on the basics of stepper motors and step motion control.
The following should give you a brief yet overall understanding of the operation of stepper motors.
Stepper Motors and Stepper Motion - IntroductionA stepper motor system is an electro-mechanical rotary actuator that converts electrical pulses into unique shaft rotations. This rotation is directly related to the number of pulses.
The speed is synchronous to the rate of pulsing.
The result is absolute speed and position.
Stepper motors feature bi-directional control, built-in braking, variable torque, power control, precision accuracy, high resolution, open-loop control, and direct interface to digital systems.
Compared to other servo systems, steppers exhibit an excellent power to weight ratio, minimum rotor inertia, no drift, no starting surge, and no cumulative errors. Note: the following descriptions start from the motor and progress to the control electronics.
Stepper Motors - General DescriptionA step motor converts electrical energy into discrete motions or steps. The motor consists of multiple electrical windings wrapped in pairs (phases) around the outer stationary portion of the motor (stator).
The inner portion (rotor) consists of iron or magnetic disks mounted on a shaft and suspended on bearings. The rotor has projecting teeth which align with the magnetic fields of the windings. When the coils are energized in sequence by direct current, the teeth follow the sequence and rotate a discrete distance necessary to re-align with the magnetic field.
The number of coil combinations (phases) and the number of teeth determine the number of steps (resolution) of the motor. For example, a 200 step per rev (spr) motor has 50 rotor teeth times 4 coil combinations to equal 200 spr.
There are no brushes between the rotor and stator assembly; a stepper motor is a multipole (polyphase) brushless DC motor. These multiple coil pairs can be connected either positive or negative resulting in four unique full steps. When the coils are sequenced correctly, the motor rotates for- ward. When the sequence is reversed, the motor rotates in reverse.
When the sequence is held, the rotor locks (brakes) in place.
The amount of torque required to force the rotor from position is the holding or static torque. If the rotor slips (step loss), it will align with the next available coil combination; either four steps forward or four steps backwards.
Steppers can be stalled or held indefinitely without damage.If the sequencing is faster than the rotor can move, the rotor will slip until sequencing is slowed enough for the rotor to again lock-in to the sequence. The rotor requires a minimum settling time (ringing) to stop when held. This limits the minimum time for the motor to change direction successfully.
PM motors settle faster than hybrids. If the sequencing frequency (step rate) is close to the natural frequency Of the coils, the motor will attempt to resonate at sub- multiples of this period; resulting in step loss and unusual noise (growling).
The low-frequency resonant point of a typical motor is 100 full steps/second or slower; the mid-frequency point is between 900-1200 spr. Resonant behavior (electro-mechanical feedback phenomena) can be minimized by reducing the current (gain reduction), isolating the mechanical connection (de-coupling), reducing the step angle to half or micro step, and not operating the motor, continuously, in the resonant bands (ramping)
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Electrical Types of Stepper Motors VR, PM, HybridVR (Variable Reluctance) motors have soft iron rotors with teeth and are mostly used for specialty applications.
Permanent magnet motors have magnet rotors with no teeth.
Hybrid motors are hybridization of both VR and PM and have magnetic rotors with teeth. mostly used for
Mechanical Types of Stepper Motors -Pressed Case (PC) and Machined Case (MC) Pressed Case (tin can) motors are stamped, mated shells with sleeve bearings. Machined case motors have cast aluminum end-bells with ball hearings; the bodies are stacks of laminations held together with screws. A PC is generally the permanent magnet type and has a 7.5 or 15 degree step angle.
MC motors typically are 1.8, 0.9 or 0.45 degree and exhibit positional accuracies of 3 to 5% Also the air gap (the space between the motor and the stator) is tighter and therefor pro- duces more torque. PC motors because of their thin cases are more limited in the amount of heat they can handle. The torque of a stepper is a function of the magnetic field (guassian strength) produced by the direct current flowing in the coils. The subsequent heating of the coils limits the motor case to a wattage that the case can dissipate before the insulation is damaged (temperature rise).
Motor Case Sizes - Size 17, 23, 34, 42 There are NEMA standards for the MC case front view and the mounting flange holes. The most common are;
size 17 (1.7 sq. or 40 mm) with a 5 mm shaft,
size 23 (2.3" dia. or 56mm) with a 1/4" shaft,
size 34 (3.4" dia. or 86 mm) with a 3/8" shaft,
and size 42 (4.2" dia. or 107 mm).
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Step Motor Windings -2-Phase, 4-Phase, 5-Phase
Common step motors have 2 sets of windings. Each can be energized positive or negative; therefore the minimum number of connections (lead wires) is four.
Four wire motors are 2-phase (bipolar).
For electrical convenience (L/R driver) each winding is center tapped into two coils (Bifilar winding).
The result is a six wire or 4/1 phase motor (unipolar).
Unipolar can also have the center taps in common creating a 5-wire motor.
An 8-wire, the most versatile configuration, has 8 leads available.
5-phase motors have 5 coils (10 wires) and require a 5-phase driver.
Stepper Motor Driver Electronics -Unipolar and Bipolar
A stepper motor requires an electrical sequencer called a driver, a specialized type of DC power supply. If the driver can reverse the polarity of its outputs, it is bipolar (4 leads). Simple, less expensive (but less effective and efficient) are drivers that cannot reverse polarity, called unipolar (6 leads). Bipolars can drive 4, 5, 6 or 8-lead motors.
To correctly connect a 5-wire to a bipolar drive, the center tap must be connected to the motor supply.
To convert 6-wire to bipolar only the center tap and one leg are connected. Half of each winding is not used.
8-lead motors are connected as 6-wire in unipolar and in parallel for bipolar (more torque and efficiency).
Bipolars use 8 transistors arrayed in 2 H-bridge arrays; unipolars use 4.
Driver Types -L/R Driver
A stepper motor is nameplate rated to a voltage and current based on the resistance of the winding and the maximum power (torque) the case can dissipate.
The resistance and inductance produce a time constant for the charging of the coils to full torque. The voltage and the time constant deter- mine the top speed of the stepper motor. lf the step rate is faster than charge time, then the torque will diminish. lf the rated voltage is applied, the step motor is said to be configured in L/R.
If dropping resistors, equal to the stepper motor resistance, are inserted in series with the coils, then the configuration is L/2R. Also twice the voltage is applied across the stepper motor. This will decrease the charge time (faster) and increase the torque at specific step rates. However, the resistors dissipate wasted energy equal to one stepper motor.
Bi-level unipolar drivers use two voltages. The higher voltage is turned on for a burst (kicker pulse) at the start of each step. The integrated bipolar driver circuit obsoleted the L/R and bi-level.
Switch Mode Stepper Motor DriversIn switching drivers, current control circuits (sensing resistor and comparator) are inserted in series with the step motor coils and a higher than rated voltage is applied. The sense the coil and then rapidly turns the power circuit on/off continuously to regulate coil current (constant current). This technique is called chopping or switch mode. Two to fifty times the rated voltage is applied across the stepper motor. The resulting performance (speed) of the driver is the equivalent of L/2R to L/50R.
Switchers have a second preset for reduced current when the stepping motor is not rotating (parking); otherwise the motor rises to maximum temperature at standstill. Parking allows stepper motor running current (torque) to be increased (overdrive) based on the reduced duty-cycle of the system. The current of a switcher is easily adjusted by varying the reference voltage to the comparator.
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Step Motor Angle Reduction - Full, Half and Micro-StepIf both coils are equally energized (full-step), the rotor rests at the resultant vector between two intersecting fields in the neutral (dead band) region. If one coil is de-energized (zero current), the rotor sweeps to a position in the center of the energized field.
Alternately inserting this condition (one coil off) into the stepping sequence (half step) steps the motor a total of eight unique positions; four half and four full. By controlling the reference voltage, in a chop- per drive, to both coil drivers with a dual D/A converter and subsequently stepping the output coil current from 1OO (full) to 0 (half) percent, the stepper motor microsteps between these two pole positions.
Microstep typically increases the resolutions of the stepper motor 4 to 64 times. The D/A table must match the guassian distributor of the step motor, a function of stepper motor quality and magnet style.
Microstep based simply on a sine/cosine function does not take equally spaced steps as a sine function is not a guassian curve. Also, microstep does not improve the base accuracy of a stepping motor; a function of the number of rotor teeth and the manufacturing quality.
Translators - Step and Direction
The phase polarity signals (step sequence) between driver and coils is based on the step function which can take the form of logic gates or memory (step table). If this logic is configured to increment from a single pulse (clock), the circuit is called a translated driver or step/ direction driver. The direction input controls the direc- tion of sequencing. A translated driver is easily connected to a source of clock pulses (pulse generator) called an indexer or controller.
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Intelligent Motion Control - Indexers and Controllers
Controllers are logic or processor circuits (programmable motion) which accept command or switch inputs. The specific distance of rotation (number of steps), the speed at which the pulses are issued (rate), and a function of speed increments (slope) is preset or input to the controller. The slope function (ramping) allows the stepper motor to accelerate to a speed greater than the instantaneous stop/start speed. In this case, a starting speed (first rate) and a top speed (slew rate) is input. The controller accelerates the stepper motor through the motion and decelerates to a stop after the present number of steps (target). Ramping allows a stepping motor to advance through the resonance bands. Open Loop Control - Homing and Slip Detection In typical operation of the motion system, the stepping motor initially steps backwards until a reference position (home) is detected and the position counter in the controller is set to zero. The step motor is then moved to positions by incriminating or decrementing the position counter (absolute motion) or repeatedly cycling the counter a fixed amount (incremental). In open loop control, it is conditional that the load is within the motor speed and torque range. A positioning system, when successfully returned to the home sensor under command (slip detection), is said to operate plus or minus zero steps (no error). Top of page