What is a cnc? The stepper motor
Stepper Motors.
A stepper motor is a brushless, synchronous electric motor that converts digital pulses into mechanical shaft rotation. Every revolution of the stepper motor is divided into a discrete number of steps, one revolution equates to 360 degrees in many cases 200 steps equates to one revolution or 1.8 degree step angle. The Nema17 is a common motor and can be found in many open-source projects.
How to wire a stepper motor
A stepper motor will have anywhere from four wires all the way to eight wires, if your motor is a four wire two will be negative and two positive, if your motor has six wires three will be negative and three positive. Steppers need buffers and cannot be hooked up directly to the power source, unlike AC/DC motors, if you just provide power the motor will be damaged. In some wiring diagrams you will notice the connection of the steppers to the breakout-board is shown as "A+A-B+B-" this indicates a four wire stepper motor connection, where "A" is one coil and "B" is another. Some people will purchase driver chips like the "ULN2003" or "R6-754410" chip and insert it into some "breadboard" to control very small current motors.
A stepper motor must be sent a separate pulse for each step, so for one revolution 200 pulses will be sent to the breakout board, some allow for "Micro stepping" .
The stepper motor can only take one step at a time and each step is the same size. Since each pulse causes the motor to rotate a precise angle, typically 1.8°, the motor's position can be controlled without any feedback mechanism.
Although some have built in encoders to determine shaft location. As the digital pulses increase in frequency, the step movement changes into continuous rotation, with the speed of rotation directly proportional to the frequency of the pulses.
Step motors are used every day in both industrial and commercial applications because of their low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment. You see them in printers, DIY applications , and commercial machines.
Stepper Motor Advantages
1. Over engineered, reliable and accurate.
2. The motor has full torque at standstill (if the windings are energized) meaning it can hold an object in one spot if needed, it will hold its rated torque.
3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 to 5% of a step and this error is non-cumulative from one step to the next.
4. Excellent response to starting/stopping/reversing.
5. Very reliable since there are no contact brushes in the motor. Therefore the life of the step motor is simply dependant on the life of the bearing and windings.
6. The stepper motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control and they can closed loop motors if an encoder is used, reducing cost and increasing efficiency.
7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.
Types of Step Motors
There are three basic types of step motors:
(1) Variable reluctance,
(2) Permanent magnet,
(3) Hybrid.
The hybrid motors are most common today, the step motor combines the best characteristics of the variable reluctance and permanent magnet motors. They are constructed with multi-toothed stator poles and a permanent magnet rotor. Standard hybrid motors have 200 rotor teeth and rotate at 1.8º step angles. Because they exhibit high static and dynamic torque they run at very high step rates, hybrid step motors are used in a wide variety of commercial applications. Some of those applications are; computer disk drives, printers/plotters, CD players, and recently the Rep-Rap 3D prototyping machine. Some industrial and scientific applications of stepper motors include robotics, machine tools, pick and place machines, automated wire cutting and wire bonding machines, and even precise fluid control devices.
Step Modes
(1) Full Step.
(2) Half step
(3) Micro step.
The step mode is reliant on the driver, the driver board connects to the beak-out board. Most offer ½ step ability.
Standard hybrid stepping motors have 200 rotor teeth, or 200 full steps per revolution of the motor shaft. Dividing the 200 steps into the 360º of rotation equals a 1.8º full step angle. Normally, full step mode is achieved by energizing both windings while reversing the current alternately. Essentially one digital pulse from the driver is equivalent to one step.
Torque is lost when ½ or ¼ step is used.
Linear Motion Control
The rotary motion of a stepper motor can be converted to linear motion using a lead screw. If the lead is equal to one inch per revolution and it has a TPI of 32 and there are 200 full steps per revolution it will take 6400 steps to move one inch. Usually backlash is a concern; it can cause a part being machined to be cut out of tolerance so a special kind of thread is used called a ACME rod. The rod has a bearing on either end and both ends connect together to produce a lead-screw/ ball-screw.
Series vs. Parallel Connection
There are two ways to connect a stepper motor, in series or in parallel. A series connection provides a high inductance and therefore greater torque at low speeds. A parallel connection will lower the inductance which results in increased torque at faster speeds many driver boards have "Dip" switches to allow for a change in the way the motor is connected.
Stand-Alone Operation
In a "stand-alone" mode the indexer can operate independent to the host computer (PLC PC). Once downloaded to the non-volatile memory, motion programs can be initiated from various types of operator interfaces, such as a keypad or touch screen, or from a switch through the auxiliary I/O inputs. A stand-alone stepper motor control system is often packaged with a driver and power supply and optional encoder feedback for "closed loop" applications that require stall detection and exact motor position compensation.
Usefull Links:
http://en.wikipedia.org/wiki/Stepper_motor
http://www.reprap.org/wiki/Stepper_Motor
http://www.imagesco.com/articles/picstepper/02.html
http://www.ehow.com/about_4695704_what-stepper-motor.html
http://www.solarbotics.net/library/pdflib/pdf/motorbas.pdf
http://www.amci.com/resources/what-is-a-stepper-motor.asp
Google Stepper Motor Images
http://whatis.techtarget.com/definition/stepper-motor
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