145 lines
6.9 KiB
Markdown
145 lines
6.9 KiB
Markdown
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title = "Stepper Motor"
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author = ["Dehaeze Thomas"]
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draft = false
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category = "equipment"
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## Types of Stepper motors {#types-of-stepper-motors}
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<https://blog.orientalmotor.com/stepper-motor-basics-pm-vs-vr-vs-hybrid>
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- Permanent Magnet
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- Variable Reluctance
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- Hybrid
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<a id="figure--fig:stepper-two-phase-hybrid-stepper"></a>
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{{< figure src="/ox-hugo/stepper_two_phase_hybrid_stepper.png" caption="<span class=\"figure-number\">Figure 1: </span>Interior of a two phase hybrid stepper motor. This motor has eight windings and 50 roto teeth" >}}
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<a id="figure--fig:stepper-hybrid-schematic"></a>
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{{< figure src="/ox-hugo/stepper_hybrid_schematic.png" caption="<span class=\"figure-number\">Figure 2: </span>Schematic of a two phase hybrid stepper motor. This motor has four windings and 15 pole pairs" >}}
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## Micro Stepping {#micro-stepping}
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From (<a href="#citeproc_bib_item_2">Ronquist and Winroth 2016</a>):
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> By varying the magnitude and direction of the winding currents, the rotor is continuously attracted in the desired direction.
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> A "step" occurs whenever a rotor tooth moves slightly to align itself to an electromagnet tooth.
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>
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> It is possible to decrease the step size of the hybrid stepper motor by using a control logic called **microstepping**.
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> As opposed to fully exciting each phase in turn, as described previously, microstepping involves transitioning between each phase shift.
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> That is, the current references are defined by sinusoidal signals displaced 90 electrical degrees from each other.
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> For most time instances, then, both phases are excited to a certain degree.
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> The result is that the electric position vector can be placed between two teeth.
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> The resolution of the motor has therefore been increased.
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From (<a href="#citeproc_bib_item_1">Condit 2004</a>):
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> There are several factors that affect the linearity of microstepping in real motors.
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> The first limitation is static friction in the system.
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>
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> [...]
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>
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> Another limitation is the fact that the torque versus position curve is not perfectly sinusoidal.
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> The toothed shape of the motor and other physical characteristics of the motor contribute to this.
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> Figure [3](#figure--fig:stepper-real-pos-vs-actual-pos) shows a plot of actual position vs expected position for a typical motor.
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<a id="figure--fig:stepper-real-pos-vs-actual-pos"></a>
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{{< figure src="/ox-hugo/stepper_real_pos_vs_actual_pos.png" caption="<span class=\"figure-number\">Figure 3: </span>Real vs actuator rotor position" >}}
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## Open Loop errors {#open-loop-errors}
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Nice references:
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- (<a href="#citeproc_bib_item_3">Vyas, Patel, and Shah 2015</a>)
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- (<a href="#citeproc_bib_item_2">Ronquist and Winroth 2016</a>)
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- <http://www.euclidres.com/apps/stepper_motor/stepper.html>
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<div class="seealso">
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References about these errors can be search for using "torque ripple", "Cogging torque" and "load dependent error" keywords.
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</div>
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### Error with period equal to one **turn** {#error-with-period-equal-to-one-turn}
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A stepper motor has a position error with a period equal to a full turn.
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An example is shown in Figure [4](#figure--fig:stepper-error-one-turn-period) (from (<a href="#citeproc_bib_item_2">Ronquist and Winroth 2016</a>)).
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The high frequency errors that can be observed have a period of one step (i.e. 200 periods each turn).
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<a id="figure--fig:stepper-error-one-turn-period"></a>
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{{< figure src="/ox-hugo/stepper_error_one_turn_period.png" caption="<span class=\"figure-number\">Figure 4: </span>Angle error of the stepper motor during a 100rpm (i.e. 0.6s per turn)" >}}
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### Error with period equal to one **step** {#error-with-period-equal-to-one-step}
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For a two phase stepper motor, there are (typically) **200 steps per revolution** (i.e. 1.8 degrees per step).
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Between each step, even when using some micro-stepping, there are some position errors that are due to non-perfect magnetic and electromagnetic fields.
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The period of this error is corresponding to 200 period/revolution.
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Then scanning, this can lead to **high frequency vibrations**.
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This is what is typically limiting the accuracy of the stepper motor (usually specified in between 3% and 5% of the step increment).
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This is approximately corresponding to **1mrad** and can be around 0.1mrad for best stepper motors.
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<div class="exampl">
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Consider a stepper motor with 200 steps by turn attached to a ball-screw with a pitch of 1mm per turn.
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A rotation of 1 turn per second will induce vibrations at 200Hz with an amplitude of \\(1\\,\mu m\\).
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</div>
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Note that this error is not a pure sine, it also has some harmonics.
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One way to reduce these errors is to use a ball-screw mechanism with a smaller pitch (or a reduction gearbox).
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The price to pay is smaller velocity (and even high vibration frequencies for the same velocity).
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### Load Dependent Error {#load-dependent-error}
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If the electromagnetic torque would be the only torque acting on the system, the electrical angle generated by the control system would correspond directly to the reference angle.
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The position error is to a large degree due to the so called load angle when the motor is positioned by an open-loop controller.
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The load angle results from applying an external torque to the stepper motor, **causing the magnetic rotor to be out of phase with the electrical field**.
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The most common way to limit these errors is to always operate the motor with its rated winding currents.
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This results in significant energy losses and heating of the motor which deprive the motor of its efficiency.
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Another option is to use a position sensor such as an encoder with a feedback controller.
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## Manufacturers {#manufacturers}
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| Manufacturers | Country |
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|--------------------------------------------------------------------------|----------------|
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| [AML](https://arunmicro.com/) | United Kingdom |
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| [Sanyo](https://www.sanyodenki.com/catalogs/servo/stepping_systems.html) | |
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## 2 phase VS 5 phase stepper motor {#2-phase-vs-5-phase-stepper-motor}
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<https://www.orientalmotor.com/stepper-motors/technology/2-phase-vs-5-phase-stepper-motors.html>
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## Bibliography {#bibliography}
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<style>.csl-entry{text-indent: -1.5em; margin-left: 1.5em;}</style><div class="csl-bib-body">
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<div class="csl-entry"><a id="citeproc_bib_item_1"></a>Condit, Reston. 2004. “Stepping Motors Fundamentals.” Microchip Technology.</div>
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<div class="csl-entry"><a id="citeproc_bib_item_2"></a>Ronquist, Anton, and Birger Winroth. 2016. “Estimation and Compensation of Load-Dependent Position Error in a Hybrid Stepper Motor.” Linköping University, Automatic Control; Linköping University, Automatic Control.</div>
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<div class="csl-entry"><a id="citeproc_bib_item_3"></a>Vyas, Darshit C, Jinesh G Patel, and Mrs Heli A Shah. 2015. “Microstepping of Stepper Motor and Sources of Errors in Microstepping System.” <i>Int. Journal of Engineering Research and General Science</i> 3 (2).</div>
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</div>
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