types of steppers
there are a wide variety of stepper types, some of which require very specialized drivers. for our purposes, we will focus on stepper motors that can be driven with commonly available drivers. these are: permanent magnet or hybrid steppers, either 2-phase bipolar, or 4-phase unipolar.
One of the first things to consider is
the work that the motor has to do. As you might expect, larger motors are
capable of delivering more power. Stepper motors come in sizes ranging from
smaller than a peanut to big NEMA 57 monsters.
Most motors have torque ratings. This is what you need to look at to decide if the motor has the strength to do what you want.
NEMA 17 is a common size used in 3D printers and smaller CNC mills. Smaller motors find applications in many robotic and animatronic applications. The larger NEMA frames are common in CNC machines and industrial applications.
亲朋游戏大厅下载官方下载 The NEMA numbers define standard faceplate dimensions for mounting the motor. They do not define the other characteristics of a motor. Two different NEMA 17 motors may have entirely different electrical or mechanical specifications and are not necessarily interchangeable.
The next thing to consider is the
positioning resolution you require. The number of steps per revolution ranges
from 4 to 400. Commonly available step counts are 24, 48 and 200.
Resolution is often expressed as degrees per step. A 1.8° motor is the same as a 200 step/revolution motor.
亲朋游戏大厅下载官方下载 The trade-off for high resolution is speed and torque. High step count motors top-out at lower RPMs than similar size. And the higher step-rates needed to turn these motors results in lower torque than a similar size low-step-count motor at similar speeds.
Another way to achieve high positioning
resolution is with gearing. A 32:1 gear-train applied to the output of an
8-steps/revolution motor will result in a 512 step motor.
亲朋游戏大厅下载官方下载 A gear train will also increase the torque of the motor. Some tiny geared steppers are capable of impressive torque. But the tradeoff of course is speed. Geared stepper motors are generally limited to low RPM applications.
Another thing to consider is how the
motor will interface with the rest of the drive system. Motors are available
with a number of shaft styles:
- Round or "D" Shaft: These are available in a variety of standard diameters and there are many pulleys, gears and shaft couplers designed to fit. "D" shafts have one flattened side to help prevent slippage. These are desirable when high torques are involved.
- Geared shaft: Some shafts have gear teeth milled right into them. These are typically designed to mate with modular gear trains.
- Lead-Screw Shaft: Motors with lead-screw shafts are used to build linear actuators. Miniature versions of these can be found as head positioners in many disk drives.
there are many variations in stepper motor wiring. for our purposes, we will focus on steppers that can be driven with commonly available drivers. these are permanent magnet or hybrid steppers wired as 2-phase bipolar, or 4-phase unipolar.
亲朋游戏大厅下载官方下载 a stepper motor may have any number of coils. but these are connected in groups called "phases". all the coils in a phase are energized together.
always energize the phases in the same way. One lead, the "common"
lead, will always be negative. The other lead will always be positive. Unipolar
drivers can be implemented with simple transistor circuitry. The disadvantage
is that there is less available torque because only half of the coils can be
energized at a time.
Bipolar drivers use H-bridge circuitry to actually reverse the current flow through the phases. By energizing the phases with alternating the polarity, all the coils can be put to work turning the motor.
A two phase bipolar motor has 2 groups of coils. A 4 phase unipolar motor has 4. A 2-phase bipolar motor will have 4 wires - 2 for each phase. Some motors come with flexible wiring that allows you to run the motor as either bipolar or unipolar.
This style is common in smaller unipolar motors. All of the common coil wires are tied together internally abd brought out as a 5th wire. This motor can only be driven as a unipolar motor.
This motor only joins the common wires of 2 paired
phases. These two wires can be joined to create a 5-wire unipolar
The 8-wire unipolar is the most versatile motor of all.
It can be driven in several ways: