Stepper Motor Introduction

Kinco Stepper Introduction

A stepper motor (also referred to as step or stepping motor) is defined as an electromechanical device achieving mechanical movements through conversion of electrical pulses. Stepper motors are driven by digital pulses rather than by a continuous applied voltage. Unlike conventional electric motors which rotate continuously, stepper motors rotate or step in fixed angular increments. A stepper motor is most commonly used for position control. With a stepper motor system design, it is assumed the stepper motor will follow digital instructions. One important aspect of stepper motors is the lack of feedback required to maintain control of position, which classifies stepper motors as open-loop systems. Even so, many customers add encoders to stepper motors.

The main components of a stepper motor are the shaft, rotor and stator laminations, magnets, bearings, copper wires and lead wires, washers, and front and end covers. The shaft of a stepper motor is typically made of stainless steel metal, while the stator and the rotor laminations are comprised of silicon steel. The silicon steel allows for higher electrical resistivity which lowers core loss. The various magnets available in stepper motors allow for multiple construction considerations. These magnets are ferrite plastic, ferrite sintered and Nd-Fe-B bonded magnets. The bearings of a stepper motor vary with size of the motor. The housing materials are composed of various other metals like aluminum, which allow for high resistance to heat.

The main use of stepper motors is to control motion, whether it is linear or rotational. In the case of rotational motion, receiving digital pulses in a correct sequence allows the shaft of a stepper motor to rotate in discrete step increments. A pulse (also referred to as a clock or step signal) used in a stepper motor system can be produced by microprocessors, timing logic, a toggle switch or relay closure. A train of digital pulses translates into shaft revolutions. Each revolution requires a given number of pulses and each pulse equals one rotary increment or step, which is only a portion of one complete rotation. There are numerous relationships between the motor’s shaft rotation and input pulses. One such relationship is the direction of rotation and the sequence of applied pulses. With proper sequential pulses being delivered to the device, the rotation of the shaft motor will undergo a clockwise or counterclockwise rotation. Another relation between the motor’s rotation and input pulses is the relationship between frequency and speed. Increasing the frequency of the input pulses allows for the speed of the motor shaft rotation to increase.

A stepper motor varies per application in construction and functionality. The three most common stepper motor types are Variable Reluctance, Permanent Magnet, and Hybrid Stepper Motors. Kinco Stepper Motors are Hybrid Stepper, also known as High-Torque motors. Hybrid stepper motors incorporate the qualities of both the VR and PM stepper motor designs. With the Hybrid stepper motor’s multi-toothed rotor resemblance of the VR, and an axially magnetized concentric magnet around its shaft, the Hybrid stepper motor provides an increase in detent, holding, and dynamic torque. In comparison to the PM stepper motor, the Hybrid stepper motor provides performance enhancement with respect to step resolution, torque, and speed. In addition, the hybrid stepper motor is capable of operating at high stepping speeds. Typical hybrid stepper motors are designed with step angles of 0.9°, 1.8°, 3.6° and 4.5°; 1.8° being the most commonly used step angle. Kinco’s hybrid stepper motors are all 1.8 degree. These motors are ideally suited for applications having stable loads with speeds under 1,000 rpm. There are key components which are influential with respect to the running torque of a hybrid stepper motor: laminations, teeth and magnetic materials. The amount of laminations on the rotor, the precision and sharpness of the rotor and stator teeth, and the strength of magnetic material are all factors in designing for optimal torque output for hybrid stepper motors. High quality Kinco stepper motors are priced competitively, without sacrificing performance.

Brief Summary of Hybrid Stepper Motors:

  • Smaller step angles in comparison to VR and PM stepper motors
  • 1.8° is the most common step angle
  • Rotor is made of a permanent magnet with fine teeth
  • Increase in detent, holding, and dynamic torque

How to Select a Stepper Motor

There are several important criteria involved in selecting the proper stepper motor:

  1. Desired Mechanical Motion
  2. Speed Required
  3. Load
  4. Stepper Mode
  5. Wiring Configuration

Generally, stepper motors are operated without feedback in an open-loop fashion, and sometimes match the performance of more expensive DC Servo Systems. As mentioned earlier, the only inaccuracy associated with a stepper motor is a noncumulative positioning error which is measured in % of step angle. Kinco stepper motors are manufactured within a 1.5% step accuracy.

Motion requirements, load characteristics, coupling techniques, and electrical requirements need to be understood before the system designer can select the best stepper motor/driver/controller combination for a specific application. While not a difficult task, several key factors need to be considered when determining an optimal stepper motor/driver solution. The system designer should adjust the characteristics of the elements under his/her control, to meet the application requirements. Kinco offers many options in its broad line of stepper motor products, at several price points, allowing for the maximum amount of design flexibility. Although it may appear overwhelming to choose, the result of having a large number of options is a high-performance system that is cost-effective. Elements needed to be considered include the stepper motor, driver, and power supply selections, controllers, PLCs and HMIs. One must also take into consideration the mechanical transmission, such as gearing or load weight reduction through the use of alternative materials. Some of these relationships and system parameters are described in this overview.

Inertial Loads
Inertia is a measure of an object’s resistance to a change in velocity. The greater an object’s inertia, the greater the torque is required to accelerate or decelerate it. Inertia is a function of an object’s mass and shape. A system designer may wish to select an alternative shape or low density material for optimal performance. If a limited amount of torque is available in a selected system, then the acceleration and deceleration times must increase. To optimize efficiency in stepper motor systems, the coupling ratio (gear ratio) should be selected so the reflected inertia of the load is equal to, or greater than, the rotor inertia of the stepper motor. It is recommended that this ratio not be less than 10 times the rotor inertia. The system design may require the inertia to be added or subtracted by selecting different materials or shapes of the loads.

NOTE: The reflected inertia is reduced by a square of the gear ratio. Speed is increased by a multiple of the gear ratio.

Frictional Loads
All mechanical systems exhibit some frictional force. The designer of a stepper motor system must be able to predict elements causing friction within the system. These elements may be in the form of bearing drag, sliding friction, system wear, or the viscosity of an oil filled gear box (temperature dependent). One must select a stepper motor that can overcome any system friction and still provide the necessary torque to accelerate the inertial load.

NOTE: Some friction is desired, since it can reduce settling time and improve performance.

Positioning Resolution
The positioning resolution required by the application may have an effect on the type of transmission used, and/or the selection of the stepper motor driver. For example: A lead screw with 5 threads per inch on a full-step drive provides 0.001 inch/step; half-step provides 0.0005 inch/step; a microstep resolution of 25,400 steps/rev provides 0.0000015 inch/step.

Microstepping
In the stepper motor microstepping mode, a stepper motor’s natural step angle can be partitioned into smaller angles. For example: a conventional 1.8 degree motor has 200 steps per revolution. If the motor is microstepped with a ‘divide-by-10,’ then each microstep moves the motor 0.18 degrees, which becomes 2,000 steps per revolution. The microsteps are produced by proportioning the current in the two windings according to sine and cosine functions. This mode is widely used in applications requiring smoother motion or higher resolution. Typical microstep modes range from ‘divide-by-1 0’ to ‘divide-by-256’ (51, 200 steps per revolution for a 1.8 degree motor). Some microstep drivers have a fixed divisor, while the more expensive microstep drivers provide for selectable divisors. All of Kinco’s stepper motor drivers are microsteppers, and are offered in many different divisors.

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