BS2, USb servo controller, and batteries. Help.

tmshaw

Hello.
My project: I bought the USB 16 Servo Controller from Parallax, a BS2 Homework Board, and some servos. I am very newbish to electronics (voltage, circuits, etc.) but I have completed the "What's a Microcontroller?" book. I am building a Robot project. I bought the servo controller thinking it would take a load off the BS2, and let BS2 concentrate on other things. Here are my questions:

#1: How do I provide power to the USB Servo controller when it is not hooked up to USB on my laptop and is being controlled by the BS2? There is no info in the product docs that I could find. The board has a power connector built on, but I thought that was for attaching power to run the servos. Does it also provide power for the board when not connected to USB?

#2: If I provide power to the servo board through the built on connector, then how do I power the Servos? I understand that the board and servos should have separate power supplies -- but the servos have just the one 3-pin cable which has to plug onto the servo controller, meaning I have to use the connector on the board for the servo power, right?

#3. About using BS2 to control the servo board: I had read that the point of the servo board was to control the servos and let the BS2 do other things. Yet, the Servo board doesn't have any eeprom for storing info (as far as I could tell in the product docs) so the BS2 STILL has to control the servos right? Looking at the sample code for controlling the servo board with BS2, the BS2 still has to do all the commanding of the servos. So is the USB servo board just providing I/O pins for more servos and not really helping the BS2 out at all? I had assumed it would be sort of like this situation:

BS2: "Hey whats that over there?" (it used a sensor to "see" something)
BS2: "Hey, Servo board: walk forward!" (BS2 doesn't need to send pulsecodes for the servos, just a command to the servo board)
Servo Board: "Yes sir! Right away!" (Servo board runs "Walk Forward" routine that was created with the PC software.)
BS2: "Sweet! now while I'm walking, I can use my sensors to look for objects in my path."

Since it doesn't do that, I'm not sure of the utility of the Servo Board for anything other than tethered-to-the-PC, direct servo control.

Sorry, I'm just confused and trying to wrap my head around things.

Thanks for any infos and help.

Post Edited (tmshaw) : 8/13/2008 8:37:24 PM GMT

Franklin

the servo board will move a servo to a position and keep it there until the stamp tells it to do otherwise. It has no programming space to download movement sequences, those need to be done on the stamp.

BS2: "Hey whats that over there?" (it used a sensor to "see" something)
BS2: "Hey, Servo board: move forward!" (turn on servo 13 and 8)(BS2 doesn't need to send pulsecodes for the servos, just a command to the servo board)
Servo Board: "Yes sir! Right away!" (Servo board "moves Forward" .)
BS2: "Sweet! now while I'm walking, I can use my sensors to look for objects in my path."
BS2: Oops somthing in the way "Servo board stop servos 13 and 8."

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
- Stephen

SRLM

Lots of questions...

The documentation:

http://www.parallax.com/Portals/0/Downloads/docs/prod/motors/28823-PSCusbRevB-v1.2.pdf

The manual/documentation describes some of the things that you want to know. The servos are powered through the two big screws which can be connected to your choice of power supply. Note that the voltage range recomended it 4 to 7.5. You'd probably do best with 5. The onboard electronics:

"1. The power source for the servos must be a separate power source from that of the 5 VDC used
to supply the on-board logic.
2. Connect the three pin cable from the PSC-USB to: VDD (Red), VSS (Black), and the white wire to
your choice of I/O pin. VDD must be 5 VDC. Note: the idle state of the serial line is 5 VDC."


Its very easy to have a BS2 control the servos via the servo board. If you look on page ten, it gives the command on how to set the servo. The BS2 sends the command once and the servo board takes care of the ramping and updating of the servo. It does not appear to be able to do one of precharted setups that the USB connection uses. To control the robot in your situation, you would send the serial command to set the servo forward, then it can forget about the board.

If you are using a rolling robot, this is fine. However, if your robot walks, chances are that there is a very specific pattern of servo positions and movements needed. For this you'll probably need to get a slave BS2 or figure out how to hijack the USB.

SRLM

Hmm. I guess we gave an answer at the same time... (or, rather, I was really slow in researching it all and Franklin beat me to it)

tmshaw

oh wait.... so, regarding power supplies, the PSC is receiving power for itself from the Basic Stamp 2 board? Is that what this drawing from the manual is showing?

I have a board of education Homework Board however. It doesn't have the switch that needs to be set to "position 2". Plus it runs off a built in 9 Volt battery. Is a nine volt going to be able to run both PSC and Boe board?

Acutally, now that I think about it, that is how it works. I was just seeing the cable from the BOE board as sending the serial info, forgetting the power part.

Post Edited (tmshaw) : 8/13/2008 9:45:22 PM GMT

Chris Savage

It depends on the version of your board. If your BOE is a Rev A or Rev B serial board then you cannot connect the PSC to the servo headers as they would have whatever the V_IN voltage was.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Chris Savage
Parallax Tech Support

SRLM

You want to make sure that the USC microcontroller is given five volts. You can use the regulated 5 volts from the board, or you can provide your own 5 volt regulator. Keep in mind that regulators need a couple of volts in difference between input and output.

The three position switch on the BOE is

1: Off
2: Power to everything minus the 4 3 wire cable headers
3: Power to everything

Just make sure that the cable on your Homework board recieves power when you want, and you're all set.

tmshaw

Thanks for all the info, guys. It has been very helpful. I'm gonna try this all out- will probably have more questions in a bit. [noparse]:)[/noparse]