Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: ltmhall on November 04, 2006, 05:10:43 PM

Title: H-bridge or no H-bridge
Post by: ltmhall on November 04, 2006, 05:10:43 PM
I'm really confused. I'm currently building a autonomous mobile robot for a class project. I'm using two servo motors and the Basic Stamp II. I already have it moving and it goes forward, reverse, right and left. I took it to class and my professor said it was okay, but he didn't
understand how it works without an h-bridge(he doesn't really know
a lot about the Basic Stamp). I explained how you send the 1ms or
2ms pulse, and the pulsout command. He asked me if it had some
kind of internal transistors on the outputs to drive it, and I told him I didn't know I would have to check the schematic. Originally when I started I thought I needed one, but after posting the question I found out I didn't. I have seen different designs online that use servo motors and the Basic Stamp II that do use an h-bridge and I'm not sure what the advantage is over not using one. I'm in the process of adding a deck to add more electronics. I'm not sure it this will cause a problems because my servos draw about half an amp.

Question 1# What is the advantage of using a h-bridge using servos  and the Basic Stamp II.

Question 2# If I add more weight and I'm using no h-bridge will this cause a problem

Title: Re: H-bridge or no H-bridge
Post by: dunk on November 04, 2006, 07:09:00 PM
hey ltmhall,
so one of the advantages of using servos is they have all the electronics you need to drive them built in.
when using a servo there is no need to use an H bridge. (there is also no real way you could use one.)

H bridges are needed when you want to drive ordinary DC motors directly from your microcontroller.
they would draw to much power to attach straight to your microcontroller. if you did that it might damage the microcontroller.
H bridges accept an input from the microcontroller and so allow your microcontroller to controll something that draws as much current as a DC motor.

don't know if you have seen this description of H-bridges yet:

good luck!

Title: Re: H-bridge or no H-bridge
Post by: Dosbomber on November 06, 2006, 02:33:34 AM
1: If you use servomotors you don't need to build an H-bridge (already been covered).  An H-bridge allows you to switch high currents through a DC motor in one direction or the other, allowing you to make the DC motor turn in one direction or the other (usually forward and reverse, in a robot) while controlling those switches (usually transistors or FETs) with a relatively low current controller, such as a microcontroller (Basic Stamp, in your case).

2: Weight on your robot has to do with motor torque and wheel traction.  If you were building a robot too large to maneuver with servomotors (they do build some big ones however), you could step up to DC motors (which would require an H-bridge), or stepper motors (which would require a wholly different type of controlling circuit).  These motors have more power than the standard hobby servo's you're probably using.
Title: Re: H-bridge or no H-bridge
Post by: ltmhall on November 06, 2006, 06:35:42 AM
So to get the most of my servo motors and make my robot faster should I apply the maximum rated voltage to them? And if so is it possible to go beyond that without causing damage to them. For instance they are rated at 4.8V to 6.0V could 6.5 V cause damage.
Title: Re: H-bridge or no H-bridge
Post by: dunk on November 06, 2006, 01:02:51 PM
i'd aim for 6Volts if i were you.
they'll probably operate OK at 6.5V for a while but at the expense of breaking sooner.
all devices ratings have some safety margin built in so going over the specification by half a volt probably won't cause immediate damage.
on the other hand, all battery packs have some change in the voltage they deliver. for example a 6.5Volt battery pack might actually deliver over 7Volts when fully charged. if you have already used up your safety margins you might start doing immediate damage.

Title: Re: H-bridge or no H-bridge
Post by: Admin on November 07, 2006, 09:11:26 AM
Remember that motors are designed to operate most efficient at the rated voltage. If you go below or above, batteries will not last as long per given output.

You can overrate at say 6.5 or 7V for short periods of time with no worry for damage, as damage in this range will only occur from overheating (so as to give it time to cool down).