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BS2 - SSC-32 Driving an H-Bridge... not working

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New ( offline )
Posts: 3
Posted: 2010-01-16 13:32 
Hello. I searched in vain but could not find a similar problem.

This is my issue:

The SSC-32 I have works great! It reliably controls servos with great precision.

The BS2 does NOT have a dedicated PWM out channel - which I need - so my hope was to use the SSC-32 as a middle man.

So, I am now trying to use the SSC-32 to provide a PWM signal to an H-Bridge (LMD1821T), and I cannot get the motors to move with any speed.

When I use the BS2's WITHOUT the SSC-32 to drive the H-Bridge using the PULSOUT command, such as PULSOUT 15, 2500 - the motors spin at full speed. But of course if the micro controller does anything else then the motor starts to perform erratically.

When using the SSC-32, I send the command SEROUT 15, 6 ["#15 P2500", 13] to the SSC-32, which is now driving the H-Bridge, and the motors spin, but very slowly. If I go to the other extreme and us P500 instead of P2500, the motors barley spin at all.

So it appears that the H-Bridge is getting a PWM signal... but for some reason I am not getting the results that I expected.

Can anyone tell me if it is possible to use the SSC-32 how I want to?

I have the SSC-32 with Atmel 168. I did perform a RESET DEFAULTS command to erase anything possible in the registers - but the SSC-32 is brand new so I didn't think that would help - and it didn't.

This setup is being used as part of my 4th Year Electrical Engineering Thesis - so any timely help would be greatly appreciated! My fallback plan is to use the HC12 micro controller...

Kelly


Guru ( offline )
Posts: 2251
Posted: 2010-01-16 13:49 
The pwm generated by the ssc-32 is not really suitable for directly driving an h-bridge (it is code to drive a chip inside a servo). There are off the shelf controllers (below) that are made to do this function. You don't give enough info on what you have to suggest a DIY solution

http://www.lynxmotion.com/Category.aspx?CategoryID=10

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Guru ( offline )
Posts: 4126
Posted: 2010-01-16 14:21 
I thought I just answered this post.

The SSC-32 drives R/C servos, which require a .5-2.5 mS pulse width in a "frame" of 20 mS. The LMD1800 series of H-bridges are designed to drive "DC servo motors", and require a 0-100% duty cycle at a 20 kHz clock rate.

The SSC-32 is only designed to drive the R/C servos, not "DC servo motors".

Alan KM6VV

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New ( offline )
Posts: 3
Posted: 2010-01-16 17:14 
Thanks for the quick responses.

Alan - I see that you did answer a similar issue - I guess I just missed it.

The Basic Stamp is nice because it allows for fast code writing - but it obviously has many limitations.

I think my best course of action will be to switch to the HC12 which has 8 PWM channels.

My next project (once I graduate) is to tackle a two legged robot - so the SSC-32 will NOT go unused! In fact, I purchased and received 20 BMS-620 servos a few months ago when I was on a spending spree!

Kelly


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Guru ( offline )
Posts: 4126
Posted: 2010-01-16 17:21 
Sounds like a good plan!

You probably know that the AtomBasic and AtomBasic Pro chips are upward compatible, and give much more power in computations.

The SSC-32 will serve you well in a legged robot!

Alan KM6VV

Scooter wrote:
Thanks for the quick responses.

Alan - I see that you did answer a similar issue - I guess I just missed it.

The Basic Stamp is nice because it allows for fast code writing - but it obviously has many limitations.

I think my best course of action will be to switch to the HC12 which has 8 PWM channels.

My next project (once I graduate) is to tackle a two legged robot - so the SSC-32 will NOT go unused! In fact, I purchased and received 20 BMS-620 servos a few months ago when I was on a spending spree!

Kelly

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New ( offline )
Posts: 3
Posted: 2010-01-16 18:00 
My University uses the HC12 - so this will be the cheapest option for me because I can simply sign out whatever I need. The main reason I initially choose the BS2 was because I'm a Mac user, and it's much easier to develop for the BS2 because the IDE is a native OSX app (plus I have 4 or 5 of them just lying around). When I switch to the HC12 I'm back working with Parallels (PC Emulator).


 

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