Author Topic: H Bridge  (Read 1530 times)

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Offline gkiverm123Topic starter

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H Bridge
« on: November 19, 2014, 09:35:41 AM »
I'm trying to build an H Bridge to control the direction of a parallax continuous rotation servo (data sheet attached). The maximum voltage of this servo is specified to be 6 V and I can only apply 5 volts to the H bridge from my arduino (also need to use PWM). Thus, to maximize power input to the motor, I've been told to minimize the voltage drops across the transistors I use (which makes sense). Most people have recommended to use MOSFETs over BJTs. My plan is to use a simple configuration shown here (http://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridges-the-basics/) or this H bridge http://www.precisionmicrodrives.com/application-notes-technical-guides/application-bulletins/ab-002-discrete-h-bridge-circuit-for-enhanced-vibration-motor-control-haptic-feedback

Can anyone recommend MOSFETS that I could use? When I search on digi key I get tons of results and can't figure out which one to choose. Ideally I wouldn't spend more than $10 - 12 total on all mosfets for two h bridges.

Offline jkerns

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Re: H Bridge
« Reply #1 on: November 19, 2014, 11:34:32 AM »
You do not want an H bridge to control the servo. An H bridge will work for a DC motor where you are directly controlling the power to the motor, but not for a servo like this where the servo assembly has it's own internal H bridge and motor control.

The servo is controlled via a pulse modulated signal that tells the servo the speed and direction. Low pulse durations turn the servo one way, high pulse durations will turn it the other way. Read the specs for the exact pulse durations. Note that the key to this is that it is not a "duty cycle" but a "pulse duration" system - the time between pulses is not very critical.

Power for the servo can be separate from the control input as long as they share a ground - so your Arduino can supply 5 volts while the actual power is 6 volts
« Last Edit: November 19, 2014, 11:37:45 AM by jkerns »
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

Offline gkiverm123Topic starter

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Re: H Bridge
« Reply #2 on: November 19, 2014, 08:57:35 PM »
Sorry, I realized my original post was wrong. Since this is a school project, we can't use the I/O pin of the servo. In fact, I think the servos are modified so there isn't a wire leading out of the I/O pin from the motor anyways. I didn't realize until this now, so the data sheet isn't all that useful. So basically the servo more like a typical DC motor with a V_DD and ground. I found some H Bridges online http://www.precisionmicrodrives.com/application-notes-technical-guides/application-bulletins/ab-002-discrete-h-bridge-circuit-for-enhanced-vibration-motor-control-haptic-feedback.

Not sure how good they will work. Since my arduino will be used to power a bunch of other sensors, I plan on powering the motor, V_DD with 6 V. The MOSFET voltages will be controlled with the arduino. Does this sound like a reasonable plan?

Offline jkerns

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Re: H Bridge
« Reply #3 on: November 20, 2014, 07:57:58 AM »
The link you provided doesn't work, but in general: Yes, the idea of building a MOSFET H bridge using 6V for the motor power supply and the Arduino for the control is sound.
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

 


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