H-bridge or No H-bridge

ltmhall

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

Question 3# If I decide to use a h-bridge could I open up the servos
and solder two wires to the motor terminals and connect them to the
h-bridge.

Post Edited (ltmhall) : 11/4/2006 11:15:53 PM GMT

Martin Hebel

If you need to use a higher voltage/current/torque DC motor, an H-Bridge is most likely needed to drive it.

If you can get by using servos for motive force, great, you can always add some gearing to increase the torque though decrease speed. The servo is actually an analog device, there is a feedback network to a resistor, and an amplifier whose output is the error between the setpoint (your pulse width value) and the actual position. Based up on this error, the servo motor will drive in a certain direction and speed.

At least this is my understanding of a servo not really having had looked at the schematics on one.
-Martin

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Martin Hebel
StampPlot - Graphical Data Acquisition and Control
AppBee -·2.4GHz Wireless Adapters & transceivers·for the BASIC Stamp & Other controllers·

Russ Ferguson

Servos do have H-Bridges in them to drive the motor. The electronics inside the servo convert the pulse sent by the BS2 into the proper control signals for the H-Bridge.
NOTE: The controller board that is inside a continuous rotation servo can be removed and used to control other low current 6v motors (drawing less than 1 amp).

Answer to Question #1: The biggest advantage to using servo instead of using a separate H-Bridge and gear motor is that the servo comes with a built-in H-Bridge (w/speed control). It also has the motor and gears built-in that·provide reasonable torque and speed for a small mobile platform.

A secondary advantage is that almost all microcontrollers have a documented way to send control signals to servo, meaning that you could quickly swap out microcontrollers on your mobile platform. There is no special advantage to using the Basic Stamp II with servos.

Answer to Question #2: Adding bigger batteries and lots of sensor/controls is not normally a problem for the BOE-Bot platform. The H-Bridges in the servos are engineer pretty well - I have only heard of them burning up when the power is swapped to the red and black leads. They are not designed to·last 100,000 miles, but they are the absolute cheapest option available.

Answer to Question #3: Yes, you could replace the controller board in the servo with a simple H-Bridge. You would then have to come up with some way to add speed control to the H-Bridge. The controller board in the continuous rotation servo provides speed control to the H-Bridge that is on it.

Post Edited (Russ Ferguson) : 11/4/2006 11:59:08 PM GMT

PAR

ltmhall said...
.... 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.
.... 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

Question 3# If I decide to use a h-bridge could I open up the servos
and solder two wires to the motor terminals and connect them to the
h-bridge.

"... he didn't understand how it works without an h-bridge..."

The servo has its own internal electronics which take a signal from the Stamp and use it to determine how much to move the motor (which is powered independently from the signal).
You would want to look up on the web·how (modified continuous-rotation) servos, such as you are using, work, and provide this information to your instructor.

"He asked me if it had some kind of internal transistors on the outputs to drive it...."

Remember, the Stamp is not powering the servo (shouldn't be?). And, the signal from the Stamp which tells the (continuous rotation) servo which direction to rotate and how fast
is wired between a Stamp pin and the servo's electronics (pin), and doesn't need any resistor or other device between the two control·pins.

Q#1. If you need power not available from your servo, then·you might use·a simple dc·motor large enough and geared to deliver the power/speed you need; you'd use H-bridge(s) in order to control the direction and speed of the motor(s). You might also need to use other circuit components to deal with various power spikes and current draw conditions. The type of servo you are using now is ready-to-go, and cheap cost. But, it can not do everything you may need it to do.

Q#2. It depends on how much weight (load on the servo motor(s) and the servo's geartrain). At some point, adding load will exceed the capacity of the servos and something will break or burn.

Q#3. That would probably be a waste of the servo devices (and using the same motors would likely still not meet your power or geartrain requirements). Save the servos for other projects, and obtain adequately-sized motors and gearing.

PAR
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ltmhall

So to get the most of my servo motors 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.

Bruce Bates

ltmhall -

The best answer I can think of is as follows. Do you enjoy playing with fire?

If NOT, then stay within the manufacturer's recommendation for this and all other electrical or electronics parts. That way you know that they've been thoroughly tested within those ranges, and you can expect appropriate performance.

If SO, the "blood is on your hands"!

Regards,

Bruce Bates

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