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Technical Documents - Documentos Técnicos: Stepper Motors

STEPPER MOTOR – an electromagnetic actuator. It is an incremental drive (digital) actuator and is driven in fixed angular steps.

This mean that a digital signal is used to drive the motor and every time it receives a digital pulse it rotates a specific number of degrees in rotation.

Stepping motors can be viewed as electric motors without commutators. Typically, all windings in the motor are part of the stator, and the rotor is either a permanent magnet or, in the case of variable reluctance motors, a toothed block of some magnetically soft material. All of the commutation must be handled externally by the motor controller, and typically, the motors and controllers are designed so that the motor may be held in any fixed position as well as being rotated one way or the other.

Most steppers, as they are also known, 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.

For some applications, there is a choice between using servomotors and stepping motors. Both types of motors offer similar opportunities for precise positioning, but they differ in a number of ways.

Servomotors require analog feedback control systems of some type. Typically, this involves a potentiometer to provide feedback about the rotor position, and some mix of circuitry to drive a current through the motor inversely proportional to the difference between the desired position and the current position.

Stepping motors fill a unique niche in the motor control world. These motors are commonly used in measurement and control applications. Sample applications include ink jet printers, CNC machines and volumetric pumps. Several features common to all stepper motors make them ideally suited for these types of applications. These features are as follows:

  • Brushless – Stepper motors are brushless. The commutator and brushes of conventional motors are some of the most failure-prone components, and they create electrical arcs that are undesirable or dangerous in some environments.
  • Load Independent – Stepper motors will turn at a set speed regardless of load as long as the load does not exceed the torque rating for the motor.
  • Open Loop Positioning – Stepper motors move in quantified increments or steps. As long as the motor runs within its torque specification, the position of the shaft is known at all times without the need for a feedback mechanism.
  • Holding Torque – Stepper motors are able to hold the shaft stationary.
  • Excellent response to start-up, stopping and reverse.
  • Each step of rotation is the response of the motor to an input pulse (or digital command).
  • Step-wise rotation of the rotor can be synchronized with pulses in a command-pulse train, assuming that no steps are missed, thereby making the motor respond faithfully to the pulse signal in an open-loop manner.
  • Stepper motors have emerged as cost-effective alternatives for DC servomotors in high-speed, motion-control applications (except the high torque-speed range) with the improvements in permanent magnets and the incorporation of solid-state circuitry and logic devices in their drive systems.
  • Today stepper motors can be found in computer peripherals, machine tools, medical equipment, automotive devices, and small business machines, to name a few applications.

Stepper motors are usually operated in open loop mode.

•Stepper motors are operated open loop, while most DC motors are operated closed loop.
•Stepper motors are easily controlled with microprocessors, however logic and drive electronics are more complex.
•Stepper motors are brushless and brushes contribute several problems, e.g., wear, sparks, electrical transients.
•DC motors have a continuous displacement and can be accurately positioned, whereas stepper motor motion is incremental and its resolution is limited to the step size.
•Stepper motors can slip if overloaded and the error can go undetected. (A few stepper motors use closed-loop control.)
•Feedback control with DC motors gives a much faster response time compared to stepper motors.

ADVANTAGES OF STEPPER MOTORS

•Position error is noncumulative. A high accuracy of motion is possible, even under open-loop control.
•Large savings in sensor (measurement system) and controller costs are possible when the open-loop mode is used.
•Because of the incremental nature of command and motion, stepper motors are easily adaptable to digital control applications.
•No serious stability problems exist, even under open-loop control.
•Torque capacity and power requirements can be optimized and the response can be controlled by electronic switching.
•Brushless construction has obvious advantages.

DISADVANTAGES OF STEPPER MOTORS

•They have low torque capacity (typically less than 2,000 oz-in) compared to DC motors.
•They have limited speed (limited by torque capacity and by pulse-missing problems due to faulty switching systems and drive circuits).
•They have high vibration levels due to stepwise motion.
•Large errors and oscillations can result when a pulse is missed under open-loop control.

STEPPER MOTOR BASICS

STEPPER MOTOR STATES FOR MOTION

The above figure is the cross-section view of a single-stack variable-reluctance motor. The stator core is the outer structure and has six poles or teeth. The inner device is called the rotor and has four poles. Both the stator and rotor are made of soft steel. The stator has three sets of windings as shown in the figure. Each set has two coils connected in series. A set of windings is called a “phase”. The motor above, using this designation, is a three-phase motor. Current is supplied from the DC power source to the windings via the switches I, II, and, III.

Starting with state (1) in the upper left diagram, note that in state (1), the winding of Phase I is supplied with current through switch I. This is called in technical terms, “phase I is excited”. Arrows on the coil windings indicate the magnetic flux, which occurs in the air-gap due to the excitation. In state I, the two stator poles on phase I being excited are in alignment with two of the four rotor teeth. This is an equilibrium state.

Next, switch II is closed to excite phase II in addition to phase I. Magnetic flux is built up at the stator poles of phase II in the manner shown in state (2), the upper right diagram. A counter-clockwise torque is created due to the “tension” in the inclined magnetic flux lines. The rotor will begin to move and achieve state (3), the lower left diagram. In state (3) the rotor has moved 15°.

When switch I is opened to de-energize phase I, the rotor will travel another 15° and reach state (4). The angular position of the rotor can thus be controlled in units of the step angle by a switching process. If the switching is carried out in sequence, the rotor will rotate with a stepped motion; the switching process can also control the average speed.

STEP ANGLE

The step angle, the number of degrees a rotor will turn per step, is calculated as follows:

BASIC WIRING DIAGRAM

The above motor is a two-phase motor. This is sometimes called UNIPOLAR. The two-phase coils are center-tapped and in this case they the center-taps are connected to ground. The coils are wound so that current is reversed when the drive signal is applied to either coil at a time. The north and south poles of the stator phases reverse depending upon whether the drive signal is applied to coil 1 as opposed to coil 2.

STEP SEQUENCING

There are three modes of operation when using a stepper motor. The mode of operation is determined by the step sequence applied. The three step sequences are:

WAVE STEPPING

The wave stepping sequence is shown below.

Wave stepping has less torque then full stepping. It is the least stable at higher speeds and has low power consumption.

FULL STEPPING

The full stepping sequence is shown below.

Full stepping has the lowest resolution and is the strongest at holding its position. Clock-wise and counter clockwise rotation is accomplished by reversing the step sequence.

HALF-STEPPING – A COMBINATION OF WAVE AND FULL STEPPING

The half-step sequence is shown below.

The half-step sequence has the most torque and is the most stable at higher speeds. It also has the highest resolution of the main stepping methods. It is a combination of full and wave stepping.

TYPES OF STEPPING MOTORS

There are three basic types of stepping motors: permanent magnet, variable reluctance and hybrid.

Permanent magnet motors have a magnetized rotor, while variable reluctance motors have toothed soft-iron rotors. Hybrid stepping motors combine aspects of both permanent magnet and variable reluctance technology.

The stator, or stationary part of the stepping motor holds multiple windings. The arrangement of these windings is the primary factor that distinguishes different types of stepping motors from an electrical point of view. From the electrical and control system perspective, variable reluctance motors are distant from the other types. Both permanent magnet and hybrid motors may be wound using either unipolar windings, bipolar windings or bifilar windings.

Variable Reluctance Motors: Variable Reluctance Motors (also called variable switched reluctance motors) have three to five windings connected to a common terminal.

Unipolar Motors : Unipolar stepping motors are composed of two windings, each with a center tap. The center taps are either brought outside the motor as two separate wires or connected to each other internally and brought outside the motor as one wire.

As a result, unipolar motors have 5 or 6 wires. Regardless of the number of wires, unipolar motors are driven in the same way. The center tap wire(s) is tied to a power supply and the ends of the coils are alternately grounded.

Unipolar stepping motors, like all permanent magnet and hybrid motors, operate differently from variable reluctance motors. Rather than operating by minimizing the length of the flux path between the stator poles and the rotor teeth, where the direction of current flow through the stator windings is irrelevant, these motors operate by attracting the north or south poles of the permanently magnetized rotor to the stator poles.

Thus, in these motors, the direction of the current through the stator windings determines which rotorpoles will be attracted to which stator poles. Current direction in unipolar motors is dependent on which half of a winding is energized. Physically, the halves of the windings are wound parallel to one another. Therefore, one winding acts as either a north or south pole depending on which half is powered.

Bipolar Motors : Bipolar stepping motors are composed of two windings and have four wires. Unlike unipolar motors, bipolar motors have no center taps. The advantage to not having center taps is that current runs through an entire winding at a time instead of just half of the winding. As a result, bipolar motors produce more torque than unipolar motors of the same size. The draw back of bipolar motors, compared to unipolar motors, is that more complex control circuitry is required by bipolar motors. Current flow in the winding of a bipolar motor is bidirectional. This requires changing the polarity of each end of the windings.

Bifilar Motors : The term bifilar literally means “two threaded.” Motors with bifilar windings are identical in rotor and stator to bipolar motors with one exception – each winding is made up of two wires wound parallel to each other. As a result, common bifilar motors have eight wires instead of the four wires of a comparable bipolar motor.

Bifilar motors are driven as either bipolar or unipolar 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.

Hybrid Motors : Hybrid motors share the operating principles of both permanent magnet and variable reluctance stepping motors. The rotor for a hybrid stepping motor is multitoothed, like the variable reluctance motor, and contains an axially magnetized concentric magnet around its shaft . The teeth on the rotor provide a path which helps guide the magnetic flux to preferred locations in the air gap. The magnetic concentric magnet increases the detent, holding and dynamic torque characteristics of the motor when compared with both the variable reluctance and permanent magnet types.

FIG: HYBRID STEPPING MOTOR

CHOOSING A MOTOR

In making a choice between steppers and servos, a number of issues must be considered; which of these will matter depends on the application. For example, the repeatability of positioning done with a stepping motor depends on the geometry of the motor rotor, while the repeatability of positioning done with a servomotor generally depends on the stability of the potentiometer and other analog components in the feedback circuit.

Stepping motors can be used in simple open-loop control systems; these are generally adequate for systems that operate at low accelerations with static loads, but closed loop control may be essential for high accelerations, particularly if they involve variable loads. If a stepper in an open-loop control system is overtorqued, all knowledge of rotor position is lost and the system must be reinitialized; servomotors are not subject to this problem.

There are several factors to take into consideration when choosing a stepping motor for an application.

Some of these factors are what type of motor to use, the torque requirements of the system, the complexity of the controller, as well as the physical characteristics of the motor. The following paragraphs discuss these considerations.

Variable Reluctance Versus Permanent Magnet or Hybrid

Variable Reluctance Motors (VRM) benefit from the simplicity of their design. These motors do not require complex permanent magnet rotors, so are generally more robust than permanent magnet motors.

With all motors, torque falls with increased motor speed, but the drop in torque with speed is less pronounced with variable reluctance motors. With appropriate motor design, speeds in excess of 10,000 steps per second are feasible with variable reluctance motors, while few permanent magnet and hybrid motors offer useful torque at 5000 steps per second and most are confined to speeds below 1000 steps per second.

The low torque drop-off with speed of variable reluctance motors allows use of these motors, without gearboxes, in applications where other motors require gearing. For example, some newer washing machines use variable reluctance motors to drive the drum, thus allowing direct drive for both the slow oscillating wash cycle and the fast spin cycle.

Variable reluctance motors do have a drawback. With sinusoidal exciting currents, permanent magnet and hybrid motors are very quiet. In contrast, variable reluctance motors are generally noisy, no matter what drive waveform is used. As a result, permanent magnet or hybrid motors are generally preferred where noise or vibration are issues.

Unlike variable reluctance motors, permanent magnet and hybrid motors cog when they are turned by hand while not powered. This is because the permanent magnets in these motors attract the stator poles even when there is no power. This magnetic detent or residual holding torque is desirable in some applications, but if smooth coasting is required, it can be a source of problems.

With appropriate control systems, both permanent magnet and hybrid motors can be microstepped, allowing positioning to a fraction of a step, and allowing smooth, jerk-free moves from one step to the next. Microstepping is not generally applicable to variable reluctance motors. These motors are typically run in full-step increments. Complex current limiting control is required to achieve high speeds with variable reluctance motors.

Unipolar Versus Bipolar

Permanent magnet and hybrid stepping motors are available with either unipolar, bipolar or bifilar windings; the latter can be used in either unipolar or bipolar configurations. The choice between using a unipolar or bipolar drive system rests on issues of drive simplicity and power to weight ratio.

Bipolar motors have approximately 30% more torque than an equivalent unipolar motor of the same volume.

The reason for this is that only one half of a winding is energized at any given time in a unipolar motor. A bipolar motor utilizes the whole of a winding when energized.

The higher torque generated by a bipolar motor does not come without a price. Bipolar motors require more complex control circuitry than unipolar motors . This will have an impact on the cost of an application.

If in doubt, a unipolar motor or bifilar motor are good choices. These motors can be configured as a unipolar or bipolar motor and the application tested with the motors operating in either mode.

Hybrid Versus Permanent Magnet

In selecting between hybrid and permanent magnet motors, the two primary issues are cost and resolution.

The same drive electronics and wiring options generally apply to both motor types.

Permanent magnet motors are, without question, some of the least expensive motors made. They are sometimes described as can-stack motors because the stator is constructed as a stack of two windings enclosed in metal stampings that resemble tin cans and are almost as inexpensive to manufacture. In comparison, hybrid and variable reluctance motors are made using stacked laminations with motor windings that are significantly more difficult to wind.

Permanent magnet motors are generally made with step sizes from 30 degrees to 3.6 degrees. The challenge of magnetizing a permanent magnet rotor with more than 50 poles is such that smaller step sizes are rare! In contrast, it is easy to cut finely spaced teeth on the end caps of a permanent magnet motor rotor, so permanent magnet motors with step sizes of 1.8 degrees are very common, and smaller step sizes are widely available. It is noteworthy that, while most variable reluctance motors have fairly coarse step sizes, such motors can also be made with very small step sizes.

Hybrid motors suffer some of the vibration problems of variable reluctance motors, but they are not as severe.

They generally can step at rates higher than permanent magnet motors, although very few of them offer useful torque above 5000 steps per second.

Functional Characteristics

Even when the type of motor is determined, there are still several decisions to be made before selecting one particular motor. Torque, operating environment, longevity, physical size, step size, maximum RPM– these are some of the factors that will influence which motor is chosen.

STEP SIZE

One of the most crucial decisions to make is the step size of the motor. This will be determined by the resolution necessary for a particular application. The most common step sizes for PM motors are 7.5 and 3.6 degrees. This corresponds to 48 and 100 steps per revolution respectively. Hybrid motors typically have step sizes ranging from 3.6 degrees (100 steps per revolution) to 0.9 degrees (400 steps per revolution).

Some stepping motors are sold with gear reductions which provide smaller step angles than are possible with even the finest stepping motors. Gear reductions also increase the available torque, but because torque falls with stepping rate, they decrease the maximum rotational speed.

For linear movement, many stepper motors are coupled to a lead screw by a nut (these motors are also known as linear actuators). Even coarse steps with this arrangement translate to very fine movements of the lead screw because of the gear reduction inherent to this mechanism.

TORQUE

Torque is a critical consideration when choosing a stepping motor. Stepper motors have different types of rated torque. These are:

Holding torque – The torque required to rotate the motor’s shaft while the windings are energized.
Pull-in torque – The torque against which a motor can accelerate from a standing start without missing any steps, when driven at a constant stepping rate.
Pull-out torque – The load a motor can move when at operating speed.
Detent torque – The torque required to rotate the motor’s shaft while the windings are not energized.

Stepping motor manufacturers will specify several or all of these torques in their data sheets for their motors.

The dynamic torques, pull-in and pull-out, are a function of step rate. These torques are important for determining whether or not a stepping motor will “slip” when operating in a particular application. A “slip” refers to the motor not moving when it should or moving when it should not (overrunning a stop). In either case, the result is the controller will no longer know the position of the motor. Open loop positioning fails in this case. The motor must be adequately sized to prevent this from happening or a closed loop feedback system employed.

The pull-in torque offered by a stepping motor depends strongly on the moment of inertia of any load rigidly attached to the motor. This makes this torque figure somewhat problematic because the moment of inertia of the rig used to measure this torque is rarely stated in manufacturers data sheets and is rarely equal to the moment of inertia of the load actually driven in the application. Most manufacturers provide torque curves in their data sheets.

LONGEVITY

Another factor to consider when choosing a motor is the longevity of the motor. Some of the questions asked should be:

• How long does the motor need to work properly?
• What environmental hazards will the motor be subjected to?
• What heat will the motor operate at?
• Is the motor’s operation continuous or intermittent?

Stepper motors by their very nature are more robust than other types of motors because they do not have brushes that will wear out over time. Typically, other components in a particular system will wear out long before the motor ever will. However, all stepper motors are not created equal and even the best motors will fail if the proper considerations are not made. The following are some design guidelines that influence motor longevity:

• Ball bearings vs bronze bushings – Ball bearings last longer than bronze bushings and do not generate as much heat, but they cost more.
• Motors that run near their rated torque will not last as long as those that do not. Motors should be chosen so that they will run at 40-60% of their torque rating.
• Protect the motor from harsh environments. Exposure, humidity, harsh chemicals, dirt and debris will all take their toll on a motor.
• Ensure adequate cooling. Motors generate heat and this must be dissipated. For motors that include an integral heat sink, ensure adequate circulation of cooling air. Other motors are designed to be cooled by conduction to the chassis on which the motor is mounted. Hybrid motors that sue rare-earth magnets are particularly heat sensitive.
• Finally, motors should be driven properly. This means special care should be taken to ensure the current rating of the windings are not exceeded.

 

ADDITIONAL INFORMATION

If the drive chip does not have internal clamp diodes, you need to supply them. The motor can produce >100V due to back EMF.

****************MAKE SURE ALL GRONDS ARE CONNECTED ****************

You reverse the motor rotation by reversing the sequence.

In the lab you will use the SAA1042 driver chip. This chip has a pin to control clock-wise (CW) and counter clock-wise (CCW) rotation and to select between full and half-step modes of operation.

Related information >> Stepper motors and drive methods.

 

 

 

 

 

 

 
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