This robot can mount these parts with high accuracy-
Also at any position within its workspace this robot can hold its workpiece with full force even at standstill.
So what kind of motor is used here? Indeed this small robot uses stepper motors.
Stepper motors can be classified as DC and synchronous motors.
These motors move in discrete angle steps - but how do so?
Here you get an overview about our topics in this video:
To interrupt the path of electric eddy currents, the stator, the fixed part of the motor, is made up of stacked laminations
Here you see the stator of a very simple stepper motor with four stator teeths and two motor coils.
When the motor coils are connected to DC voltage, a magnetic field with a north and south pole is excited at his teeths.
The magnetic poles can be interchanged by reverse the current flow. By switching the current of the motor coils in a certain sequence, a
rotating magnetic stator field can be created.
So, let's place a permanent magnet within the stator.
It is obvious that this magnet aligns with the poles of the stator. That's how a permanent stepper works.
This type of stepper has high torque, but also a big step angle.
You can detect a step angle of 90 degrees by a simple equation. You can calculate this angle:
Now consider a stepper motor with a rotor of two pole pairs.
The stator has four phases now. You can get a much smaller step angle as you can proof by this given calculation.
With the variable reluctance stepper, you can get a low-cost stepper motor here. The rotor is made of soft iron.
Notice that this stepper here has more stator poles than the rotor. The coils are energized in a certain sequence.
So only one pair of rotor teeth are alined to the stator at a time.
So let's start to energize the stator coils and watch the steps.
The rotors soft iron salient poles offer a lower reluctance to the magnetic lines.
Because these magnetic lines intend to go the shortest way, torque is created.
The big disadvantage hereby is the lack of a permanent magnet. This has a negative effect on the torque that is significantly decreased.
The most commonly used stepper nowadays is the so-called hybrid stepper, which combines aspects of both permanent and
Variable reluctance stepper. With this motor, you can achieve very small step angle.
To understand this motor. Let's have a look at its rotor structure.
The rotor consists of a permanent magnet and two cups rings of soft iron.
The surface of these cups have precisely ground teeths. In this example 50 teeths.
When these cups enclose the magnets, one ring becomes the North Pole the other the South Pole.
To give these cups and offset to each other, one cup is rotated. Here by 3.6 degrees.
To get a better understanding,
let's switch to a two-dimensional view.
The stator field in full step mode switches 45 degrees within one step. The rotor
however, rotates only for 1.8 degree. But how so?
Note that the rotor has 50 teeths and the stator 48.
So you can distinguish sections where different magnetized poles are aligned to each other, and other
sections where the opposite poles are only half aligned to each other - as you see.
Opposite poles attract each other, on the other hand between evenly magnetized poles there is repulsion.
The motor is now in a stable position.
Now let's switch this motor to the next step. The rotor rotates by 1.8 degrees to reach a new stable position.
There are three commonly used excitation modes for stepper motors: Full step, half step and micro stepping.
By these modes you determine the performance and torque of the stepper.
The full step drive mode provides high torque, because always all coils are energized at any time.
The rotor needs four steps to complete a
360 degree rotation. However, you have full and constant torque all these steps.
However, this mode does not improve the resolution.
In Half mode step sequence you get a step angle half to the angle in full mode.
So the resolution already increased to the double. On the other hand you get no constant torque.
However microstepping is the most common method of controlling stepper motors nowadays.
Here the coils are provided with a voltage similar to the form of sine wave.
As a result, you get smooth motion of the rotor, less stress to the parts and the accuracy of the stepper motor increases significantly.
Unfortunately in microstepping the torque is decreased about 30 percent.
For the user the torque speed curve is a decisive criterion for selecting a stepper. Let's have a closer look.
The Pull-in torque shows the maximum value of torque at given speeds
that the motor can start stop or reverse without losing steps.
Once the stepper started, it can pass to the slew range by a ramp, which means increasing the frequency steadily
The maximum start frequency refers to an operation with no load.
If the motor is run outside of the pullout torque, it will stall.
In summary the stepper motor can be characterized as a DC synchronous motor.
Its main applications are positioning as it has full torque even at standstill without heating up.
For simple positioning you don't even need a measuring system, because the motor rotates with fixed step angles.
The disadvantage of a stepper motor is the limited speed or rpm you can achieve.
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