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Modular CNC software for Arduino controller

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Motors

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Motors

Motors

Usually the motion of a CNC machine is performed by a specific type of DC motors named stepper motor. A stepper motor is one kind of electric DC motor used in the robotics industry.

Stepper motors are DC motors that move in discrete steps. They have multiple coils that are organized in groups called "phases". By energizing each phase in sequence, the motor will rotate, one step at a time.

With a computer controlled stepping you can achieve very precise positioning and/or speed control. For this reason, stepper motors are the motor of choice for many precision motion control applications.

Each motor has a stepper motor driver, it is a circuite that translates the commands of step forward/backward into the motor coils must be energized. As each step moves the motor a known distance it makes them handy devices for repeatable positioning. You can find more details here about the technology behind the stepper motors.

Pros & Cons

PORs
- Positioning: Since steppers move in precise repeatable steps, they excel in applications requiring precise positioning such as 3D printers, CNC, Camera platforms and X,Y Plotters. Some disk drives also use stepper motors to position the read/write head.
- Speed Control: Precise increments of movement also allow for excellent control of rotational speed for process automation and robotics.
- Low Speed Torque: Normal DC motors don't have very much torque at low speeds. A Stepper motor has maximum torque at low speeds, so they are a good choice for applications requiring low speed with high precision.

CONs
- Low Efficiency: Unlike DC motors, stepper motor current consumption is independent of load. They draw the most current when they are doing no work at all. Because of this, they tend to run hot.
- Limited High Speed Torque: In general, stepper motors have less torque at high speeds than at low speeds. Some steppers are optimized for better high-speed performance, but they need to be paired with an appropriate driver to achieve that performance.
- No Feedback: Unlike servo motors, most steppers do not have integral feedback for position. Although great precision can be achieved running ‘open loop’. Limit switches or ‘home’ detectors are typically required for safety and/or to establish a reference position.

Properties

When you bay the stepper motors for the DIY CNC machine, you must pay attention to some properties:

Step angle: stepper motors have a step angle. A full 360° circle divided by the step angle gives the number of steps per revolution. For example, 1.8° per full step is a common step size rating, equivalent to 200 steps per revolution. A smaller step angle makes the movements more precise but it also makes the movement slower.

Microstepping: A stepper motor always has a fixed number of steps. Microstepping is a way of increasing the number of steps by sending a sine/cosine waveform to the coils inside the stepper motor. In most cases, micro stepping allows stepper motors to run smoother and more accurately. Microstepping between pole-positions is made with lower torque than with full-stepping, but has much lower tendency for mechanical oscillation around the step-positions and you can drive with much higher frequencies because the number of steps per revolution increases.
If your motors are near to mechanical limitations and you have high friction or dynamics, microsteps don't give you much more accuracy over half-stepping. When your motors are 'overpowered' and/or you don't have much friction, then microstepping can give you much higher accuracy over half-stepping. You can transfer the higher positioning accuracy to moving accuracy too.

Bipolar/Unipolar: bipolar or unipolar refers to the internals of the motor, and each type has a different stepper driver circuit board to control them. For instance, A4988 is designed to control bipolar motors. Bipolar motors are the strongest type of stepper motor. You identify them by counting the leads - there should be four or eight. They have two coils inside, and stepping the motor round is achieved by energizing the coils and changing the direction of the current within those coils. Unipolar motors also have two coils, but each one has a centre tap. They are readily recognizable because they have 5, 6 or even 8 leads. It is possible to drive 6 or 8 lead unipolar motors as bipolar motors if you ignore the centre tap wires. A 5 lead motor has both centre taps connected, so re-wiring them to a 4 lead version requires at least opening the motor, if it can be done at all. The main feature of unipolar motors is that you can step them without having to reverse the direction of current in any coil, which makes the electronics simpler.

Holding torque: torque is the tendency of a force to rotate an object about an axis. Its unit is N·m. In the case of a motor, an easy way to see the torque is the force that a motor transfers to the end of an arm (with one meter length) connected to the motor shaft. Some time, for light motor, it is expressed as N·cm. The holding torque is the maximum force that the stepper motor can produce. Stepper motors do not offer as much torque or holding force as comparable DC servo motors or DC gear motors. Their advantage over these motors is one of positional control. Whereas DC motors require a closed loop feedback mechanism, as well as support circuitry to drive them, a stepper motor has positional control by its nature of rotation via fractional increments.

Size: The physical size of stepper motors are usually described via a US-based NEMA standard, which describes the bolt-up pattern and shaft diameter. In addition to the NEMA size rating, stepper motors are also rated by the depth of the motor in mm. Typically, the power of a motor is proportional to the physical size of the motor.

How a stepper motor works?

Stepper motors work on the principle of electromagnetism. There is a soft iron or magnetic rotor shaft surrounded by the electromagnetic stators. The rotor and stator have poles which may be teethed or not depending upon the type of stepper. When the stators are energized the rotor moves to align itself along with the stator (in case of a permanent magnet type stepper) or moves to have a minimum gap with the stator (in case of a variable reluctance stepper). This way the stators are energized in a sequence to rotate the stepper motor.

Architecture

The below animation shows how a stepper motor works.

NEMA

Refers to the frame size of the motor as standardized by the US National Electrical Manufacturers Association in its Publication ICS 16-2001. It specifies the 'face' size of the motor but not its length. For example a NEMA 23 stepper has a face of 2.3 x 2.3 inches with screw holes to match. Note: just because a motor is bigger does not mean it is more powerful in terms of torque. It is perfectly possible for a NEMA 14 to 'out pull' a NEMA 17 or a NEMA 23. Some characteristics of the NEMA motors are listed below.

Parameters

Model	Shaft Diameter (B)	Pilot Diameter (E)	Mounting Bolt Circle (C)	Spacing (D)
NEMA17	5 mm	22 mm	43.81 mm	31 mm
NEMA23	6.35 mm	38.1 mm	66.67 mm	47.14 mm
NEMA34	9.52 mm	73 mm	98.42 mm	69.6 mm
NEMA42	15.87 mm	55.52 mm	125.73 mm	88.9 mm

Heat
Most of the motors specs give the current for two coils that will give an 80 °C rise, i.e. they can run at 100 °C! Fortunately temperature rise is proportional to power, which is in turn proportional to the square of current (P=I²R), but torque is directly proportional so you can keep temperature under control without losing too much torque. For example, running a stepper at 70% of the rated current would result 70% of the torque and 49% (0.7²=0.49) of the power dissipation and thermal rise.

Power and current
All recent stepper controllers use a current-limiting design. Because of this, the resistance (ohms, Ω) of the coils does not matter, as long as it is low enough for the current to rise fast enough for the current-limiting design to come into play. If the resistance is too high (i.e. 24 V steppers) the current simply does not raise enough. For this reason, stepper motors rated for 3-5 V and 1-1.5 A are generally recommended, as these motors will perform near their peak torque with a current-limiting stepper controller (such as a Pololu A4988).

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