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I have project that needs motor. Motor should only turn one direction and should rotate 1 or 2 times at a time. Motor speed needs to be low and torque must high. So I made this with stepper motor and stepper motor driver. I want to get system better.

I think this stepper motor and motor driver overkill because these cost a lot. I want to try to use DC gear motor instead of stepper motor and I want to drive this DC gear motor with half H bridge.

This motor has to stop always same spot. If we take the area where the motor starts to rotate as 0, the motor should always stop at 0 degrees, not 3 or 5 (until 0.5 is okay I guess.)

enter image description here

So I think, I can do this with magnet. Lets say motor needs to turn like 2 second to reach start point. I will control the PWM with microcontroller and I will say turn 1.95 second. I will add the cover with the magnet( this will be magnet 2 not on the photo) above the starting point in this photo. This magnet2 will attract magnet1 . This 0.05 degree loss will never happen.

  • Is this the correct way to stop motor always the same spot ?

  • Since motor needs to turn only one direction, Is it good to use Half H bridge with Fet Driver IC ? Here is my schematic (actually data sheets typical connection.)

enter image description here

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    \$\begingroup\$ Might want to check out how wiper motors on cars do it - how do you think they get the blades to park in the same spot? \$\endgroup\$ Commented Feb 15, 2022 at 14:02
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    \$\begingroup\$ tlfong01 and Solar Mike thank you, I will check them out. \$\endgroup\$ Commented Feb 15, 2022 at 14:18
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    \$\begingroup\$ You're unlikely to get any pair of magnets that size to make the geared output shaft stop in the same position. They simply won't be strong enough to overcome the forces required to turn the gears. You should look instead at putting 1 or more sensors on the output shaft to determined when to stop driving the motor. You could add a hall sensor to detect the position of the magnet on the gear. \$\endgroup\$ Commented Feb 15, 2022 at 14:22
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    \$\begingroup\$ Getting a geared motor assembly like that to stop to within \$\pm 0.5 ^\circ\$, over any significant changes in torque, environmental conditions, and mechanism aging, is going to require pretty tight control on the output shaft position. You'll need to measure the output shaft position to at least a precision of a degree, with 1/10th a degree being much better. You'll either need to approach the set point very slowly, or you'll need to also monitor the input shaft (motor) speed. And, finally, if the gear train has a backlash of more than \$\pm0.5^\circ\$, it won't work with changing torque. \$\endgroup\$ Commented Feb 15, 2022 at 15:07
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    \$\begingroup\$ Thank you brhans and TimWescott , I will use your suggestions. \$\endgroup\$ Commented Feb 15, 2022 at 17:56

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You describe wanting to stop a uni-directional DC motor in the same spot. So that doesn't mean holding it in that spot, just stopping it rotating.

A DC motor is typically much cheaper than a stepper motor for a given output power. So it's a good move economically.

If you drive a DC motor using a half-bridge, grounding the other motor terminal, your half-bridge output will have the following effect:

Drive high DC motor is powered and turns Drive low DC motor is shorted and brakes High-impedance DC motor will coast if rotating

So your motor control system (MCU etc) can drive the H-bridge in the following ways for these DC motor activities:

  • To spin, PWM to H-bridge: PWM mark = drive motor high, PWM space = motor high-impedance
  • To soft-brake, PWM to H-bridge: PWM mark = drive motor low, PWM space = motor high-impedance
  • To hard-brake, control H-bridge to drive motor constantly low

You can then calculate/experiment motor movement algorithms that best suit the mechanical arrangement and loads that you have. This just gives you the elements of motor control.

For example, you might spin towards the stop point, soft-brake as it's near it then hard-brake.

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    \$\begingroup\$ TonyM, thank you for great explanation. I will try to implement them. \$\endgroup\$ Commented Feb 15, 2022 at 14:19
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Magnet 1 shown in the photo can be used to close a reed switch or activate a hall effect position sensor. Then a microcontroller can "read" the state of the reed switch or hall effect sensor to sense the stop position.

In the photo magnet 2 is not shown but a worm gear is shown at the far right which appears to be driving the gear holding magnet 1. Typically when the worm gear stops the gear holding magnet 1 will also stop and magnet 2 would have no further effect on the position. This assumes that when the motor stops there is no momentum causing the gear to continue to rotate further holding magnet 1.

If however the position gear holding magnet 1 would continue to move on momentum then magnet 2 might not be strong enough to cause a "cog" position when adjacent to magnet 1. In that case the best approach is to sense the position with as much resolution as required and to stop the DC motor via some algorithm based on the position sense data.

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  • \$\begingroup\$ Thank for your comment. I will use reed switch I guess. \$\endgroup\$ Commented Feb 17, 2022 at 4:29

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