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Stepper Motor Application


About Stepper Motor

Definition: A stepper motor or step motor or stepping motor is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any feedback sensor (an open-loop controller), as long as the motor is carefully sized to the application in respect to torque and speed.

What are stepper motors good for?

  • 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.

What are their limitations?

  • 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.
stepper Motor
 
Figure 1 - stepper Motor
  

Study

Stepper motor, with a cogged rotor (Figure 2) , is analyzed using EMS Transient magnetic analysis type.
The stator consists of four spokes, each one is surrounded by a copper coil. The current applied on each coil is a pulse signal with time delay. The electromagnetic force and torque exerted on the rotor by the coils are computed. The magnetic flux density path through the rotor and the stator for all time steps is also obtained. Moreover, the current density is given by EMS.                    

 
3D Model of Stepper Motor used in the simulation
Figure 2 - 3D Model of Stepper Motor used in the simulation
 

The Transient Magnetic module of EMS is used to compute and visualize magnetic fields that vary over time . These fields are typically caused by surges in currents or voltages. This type of analysis can be linear or non-linear. It also addresses eddy currents, power losses and magnetic forces. After creating a Transient Magnet study in EMS, four important steps shall always be followed: 1 - apply the proper material for all solid bodies, 2- apply the necessary boundary conditions, or the so called Loads/Restraints in EMS, 3 - mesh the entire model and 4- run the solver.

Materials

In the Transient Magnetic  analysis of EMS,  the whole properties of material are needed (Table 1).

Table1 - Table of materials
 
Components / Bodies Material Relative permeability Conductivity (S/m)
Rotor Mild Steel 2000 1.1e+006
Outer Air Air 1 0
Inner Air Air 1 0
Coil Copper 0.99991 57e+006
Stator Mild Steel 2000 1.1e+006

ElectroMagnetic Input

In this study, 4 coils (Table 3) are applied and the rotor  (Table 3) where we need to calculate the virtual work. 

Table 2 -  coils information

Name Number of turns
Wound Coil (1-4) 10

In Transient Magnetic we should specify the excitation waveform (voltage or current source) of coil as shown in the figure below.

Waveform of current excitation of the first coil
 
Figure 3 - Waveform of current excitation of the first coil
 
Table 3 -  Force and Torque information
 
Name Torque Center Components / Bodies
Virtual Work At origin Rotor and Permanent Magnets

Meshing

Meshing is a very crucial step in the design analysis. EMS estimates a global element size for the model taking into consideration its volume, surface area, and other geometric details. The size of the generated mesh (number of nodes and elements) depends on the geometry and dimensions of the model, element size, mesh tolerance, and mesh control. In the early stages of design analysis where approximate results may suffice, you can specify a larger element size for a faster solution. For a more accurate solution, a smaller element size may be required.

Mesh quality can be adjusted using Mesh Control (Table 4), which can be applied on solid bodies and faces. Below (Figure 4 ) is the meshed model after using Mesh Controls.

Table 4 -  Mesh control

Name Mesh size Components /Bodies
Mesh control 1 4.00 mm Coils / Rotor/ Stator
Mesh control 2 1.500 mm Inner Air
 
Meshed Model (Inner air showed in the left picture)
 
Figure 4 - Meshed Model (Inner air showed in the left picture)



Mesh details

 
Figure 5 - Mesh details

Results

After running the simulation of this example we can obtain many results. Magnetostatic Module generate the results of : Magnetic Flux Density (Figure 6), Applied Current Density(Figure 7), Force density, and a results table which contains the computed parameters of the model, the force and the torque(Figure 8) .

Magnetic Flux Density at 0.0015 s, the Flux is maximum near the excited coil
 
Figure 6 - Magnetic Flux Density at 0.0015 s, the Flux is maximum near the excited coil


Applied Current Density at 0.004 s, during every T/4 only one coil is excited



Figure 7 - Applied Current Density at 0.004 s, during every T/4 only one coil is excited


Torque generated by the Magnetic Flux in the rotor
Figure 8 - Torque generated by the Magnetic Flux in the rotor
 

Conclusion

By creating multiple studies, the user can change the materials, the number of turns, the current through each turn, and the geometry of each part.EMS allows the user to keep the same assembly file and associate each study with a design table position.  All these features are very helpful for designers and can be used to determine the Stepper motor parameters which must be changed in order to optimize the Motor performances. Hence, in addition of being fully integrated in SolidWorks and Inventor, EMS is also accurate and easy to use.