Vex Servos

neoteric

I am a loyal Parallax user, and a strong hobby customer of Parallax's.· However, I like the VEX servos that Radio Shack sells.· They have a great shaft in them.· I picked a set up to try with my Stamp.· However, they do not seem to respond the same way.· In fact, sometimes they would work at different frequencies, and sometimes not.· Anybody know anything about these, who makes them, how I would program them with the stamp?· Appreciate the info.

Again, I state that I am a loyal Parallax customer.· Just like this Servo if I could get it to work.

Ken Gracey

Hello Neoteric,

I've seen a fair amount of discussion on the Vex servos on their discussion forum. It would be worth taking a look. They must have some people who know their servo signal and design better than us at Parallax. I think the link is www.vexrobotics.com.

Good luck -

Ken Gracey
Parallax, Inc.

Paul Baker

You have stumbled on one of the most appropriately named systems in existance. Vex robotics is indeed quite vexing (none of thier 34 MB+ of documentation explains anything of value beyond construction and very generalized explainations of components, even thier programming guide is a single splash page only describing what it's function is), but a brief search of thier discussion forum turned up this post by foobert in this thread http://www.vexrobotics.com/index.php/posts?thread_id=129·and he seems to indicate standard servo operations. (As a side note those animated Avatars everyone there likes to use are quite distracting). I fear your only hope of help is from some experienced user on their forums, the contact us only provides an email addy for the webmaster and links to non-existant pages on the Radio Shack website.

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Post Edited (Paul Baker) : 1/26/2006 6:26:44 PM GMT

neoteric

Yes, I absolutely tried handling them as normal PWM servos. They were erratic. Sometimes they would spin at 300, sometimes 1200, never at 650, 850. (actually, a few times they spun very slowly at 650) I was unable to discern any pattern at all. I tested my circuit with the Parralax servos, and they worked perfectly. Same code.

Maybe I was doing something wrong. The link you sent me certainly suggests they are standard 20ms pwm servos.

As for the VEX system, I rejected it out of hand and started with the BOEBOT. My interest was in learning electronics, not building a robot kit. The VEX kit is enclosed, and lots of stuff happens without any explanation of how the actual electronics work. I wanted to learn about NPNs and electrons... And I really have, thanks to the books, the NV articles, and the kind folks in this forum. I am building stuff I never dreamed I could, all of it from scratch at this point.

I just happen to like the heavy duty shaft on the servo. They are the only ones I have found like that. If there were some others, I would consider them, because the VEX add-ons are really priced at a premium. I

neoteric

PS, I'm actually helping others in the forum on simple questions. Not bad for a guy that didn't know how a battery worked 5 weeks ago.

Ken Gracey

Neoteric,

I have a VEX kit, so I know something about using this hardware.

My initial VEX build used the R/C interface in their skid-steer system. In this mode, the VEX controller was a pass-through for servo signals. When controlling the robot, we found much difficulty in proportional control, but it was quite easy to have full-on forward or backward. The difficulty resolving the servo's center position demonstrated itself in my attempt to do a "slow right" or "slow left" without a total full reverse/full forward to spin on a dime. We never pulled the servos off of the chassis to test them with a BASIC Stamp.

But, the people on the VEX discussion forum indicate that the signal is a 1-2ms pulse like our servos. If this is true, you can use our Robotics text for example code to control their servos. But, I suspect some difficulty around the center position. One member on the VEX list provided a link to a PWM web site. Just want you to know that the link he provided may pertain to motor control, but not to servo signals.

Ken Gracey
Parallax, Inc.

Paul Baker

Just goes to show the two addages: "Newer is not always better", and "Expensive is not always better" are true, ergo "Newer and Expensive is not always better". Makes you wonder what they are doing with all that money they are making if they aren't using quality components (is there even a centering adjustment on the servo? the docs make no mention of one, of course they dont mention much of anything [noparse];)[/noparse] ) and they don't seem to be paying anyone for technical support (I didnt see any company presence on thier discussion boards, compare that to the CEO of Parallax directly discussing this topic with you ).

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Post Edited (Paul Baker) : 1/26/2006 7:18:14 PM GMT

allanlane5

If the VEX intro kit wasn't $300, and THEN you pay more to add a processor box to it, AND if it didn't use really cheesy looking erector-set components, I would consider using it.

I used to think Parallax components were expensive -- until I looked at every other robotics-type kit out there. Lego Mindstorms come closest at $200 per kit -- but even there you're limited to ONLY three motors and three sensors. And any result looks like Lego, no offense. Nice processor in the brick, though.

I really appreciate the Parallax approach of documenting EVERYTHING, and then assisting with everything. And their processor is simple enough you can get on with interfacing it to hardware, instead of wrestling with assemblers, compilers, debuggers, o'scopes, logic analyzers, board designs, schematics, etc.

Oh, and those symptoms you're pointing out tend to be either grounding issues, or power-supply issues.· Or poor quality in the servos, which haven't been a problem with the Futaba stuff sold by Parallax.

Paul Baker

If you are looking for a high quality horn, here is a single sided·one, and here is a dual sided one·(actually looking at the specs for the second state they only work with a certain HiTec servo, it may be true or just a marketing ploy to buy that servo).·Lynxmotion also sells snap rivots, ball links, threaded rods, etc for use with servos if you are looking to modify the standard servo (Tower Hobbies, the first two links, does as well but they are more expensive). You may, in the end, have to chalk up the Vex servo as a learning experience.


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Post Edited (Paul Baker) : 1/26/2006 9:52:56 PM GMT

bulkhead

Paul, he may be referring to to the servo's output spline. If that's the case, you could get a metal geared servo (and an alum servo horn like Paul said).

VEX kits are quite pricey. Their starter kit includes a reciever unit that has a "programmable microcontroller" within, BUT you can't use it without the cable and software. The cable and software are sold as the "programming kit" and costs $100.

There's no doubt VEX components are more expensive and less reliable (and with less support) than parallax products, but they do introduce a lot of people into robotics. Without Lego or VEX, many people wouldn't be able to make the jump from basically nothing, to programmable stamps. They serve as entry level platforms for beginners to learn from, and without new people entering into this hobby, it would go extinct.

Paul Baker

I agree there is a definite niche for the "build it and run it" approach to robotics, I just dont believe you should have to pay more money for less functionality or expandability. I think Parallax made a great stab at the market with thier Scribbler robot. It seems that Vex was conceived as a pre-built approach to the First Robotics competition, then at sometime the marketing department said "Hey lets sell it as a general robot, and get Radio shack to push people into buying it" (From several people's RS experience, its clear the RS big-wigs have really pushed the store managers into getting thier employees to push customers towards Vex and away from Parallax with nothing to back it up except "It's newer"). It just seems from my perspective of sitting on the outside, that the company's priorities in order are: marketing, technology, support. The exact opposite of what thier priorities should be. Anyways Ive vented enough about Vex.

You are probably correct about the spline, I couldn't find a good picture or a·diagram of the servo, and I dont frequent RS's that frequently anymore.

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Post Edited (Paul Baker) : 1/26/2006 11:05:45 PM GMT

Kenny Gardner

I was one of the first to poo poo the Vex because of the price. I would make fun of it, calling it an Erector Set, and a really lame robotics set. But, I still wanted one. I fought myself back, saying it was too expensive.

Then came Christmas and Radio Shack offered a $50.00 rebate. I· bought one. Two weeks later, Radio Shack inadvertently cut the price by $50.00 and still offered the rebate - a hundred dollars off. I bought another one. In addition to the two kits, I'm a regular at VexLABS and stocked up on hardware. Not for use with just the Vex, but with all robots that I make, including additions to the BOE-Bot.

thier programming guide is a single splash page only describing what it's function is

My programming guide is 59 pages. The documentation for Easy C (Vex's programming language) is well over 100 pages (I purchased the full version).

they don't seem to be paying anyone for technical support (I didnt see any company presence on thier discussion boards

Where are you looking for the Support Forum? Have you tried here:

http://www.vexlabs.com/forum/

is there even a centering adjustment on the servo

Motor Control (centering) is done in software or by programming the Transmitter.

I suspect that Vex was designed as a low cost alternative to the First Robotics kits. If you think Vex is expensive, check out the First stuff:

http://www.ifirobotics.com/

The controller is powerful and worth most of the price of the kit. The Sonar Sensor is a Devantech SRF-04. The Wheels are awesome, as are the Tank Treads and·Sprocket and Chain Kit. I don't know who manufactures the Servos, but their quality is no worse than the Futaba's that Parallax uses.

As to the original question, I assume that when you say Servo, you are talking about the Vex Motor, which is a continuous rotation servo. Whether you are or not, I can use my Vex Servos, continuous and regular, with Parallax servo code. ServoTest.bs2 from What's a Microcontroller works just fine with my Vex regular Servos. The Boe-Bot code works just fine with my Vex motors.

Kenny




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--

Kenny Gardner
GAP Development Company
http://www.gapdev.com/
·

Chris Savage

One thing I have not seen mentioned is whether the pulses rest HIGH or LOW.· Try running some sample code using a LOW and then a HIGH in the initialization.

' {$STAMP BS2}
' {$PBASIC 2.5}
 
index  VAR  Byte
 
LOW 0
 
FOR index = 500 TO 1000
  PULSOUT 0, index
  PAUSE 20
NEXT

' {$STAMP BS2}
' {$PBASIC 2.5}
 
index  VAR  Byte
 
HIGH 0
 
FOR index = 500 TO 1000
  PULSOUT 0, index
  PAUSE 20
NEXT

You might also try changing the PAUSE value from 0 through 100 to see if maybe the refresh rate of pulses is different.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Paul Baker

My programming guide is 59 pages. The documentation for Easy C (Vex's programming language) is well over 100 pages (I purchased the full version).
Having not purchased the kit, I do not have access to any printed materials. The documentation availible online, which looks as though its the "official" documentation, has a single page for the programming.
Where are you looking for the Support Forum? Have you tried here:
·
http://www.vexlabs.com/forum/
No I didnt see the forums, the only link I could find on the website http://www.vexrobotics.com·was a link to thier message board, which only has a hundred or so posts and none which seem to be from company employees, I am curious why they would maintain a message board and a forums with no apparent connection between the two.
Motor Control (centering) is done in software or by programming the Transmitter.
Nifty, but that means you have to buy the entire system to just center a servo, a very "Mac'ish" thing to do. (Not a knock on Macs themselves, just Apple's historical business practices).

I suspect that Vex was designed as a low cost alternative to the First Robotics kits. If you think Vex is expensive, check out the First stuff

That may be the case, but it doesn't mean it a good value. I could come out with a clone of a Toyota Matrix for $50K and say "but its much cheaper that a Farrari, so its a deal".

Look Im not trying to knock your choice, Im sure in many respects its a great system. But, being a Parallax customer, Ive come to expect a lot of value for a reasonable price and customer support that will bend over backwards to help, I just dont see Vex meeting that high level of expectation.


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Paul Baker

Ok I recind my comments about thier tech support, I have had a chance to look at thier forums abit and the moderators are good at answering questions. It is the most bizarre setup I have ever seen though, where users pose a question in one forum and an answer is provided in another catagorized forum which only moderators seem to be able to do. I guess I can understand why they set it up that way, but it doesn't seem to foster any community discussions, perhaps thats what the message board is for, I dunno Im still scratching my head. Anyways, this thread has been hijacked enough, the best bet is to ask the question on the forums, I saw a post where the moderator spoke of the variations in the center pulse value.

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Post Edited (Paul Baker) : 1/27/2006 5:56:17 AM GMT

Wreno

I have played with the vex servos and Motors, but not with direct microprocessor control. Basically they both take a standard 1-2 ms pulse for location (servo) or speed/direction information (motor). Basically, the "motor" is a servo box and motor with a real ESC (not a hacked servo board).

On standard RC equipement, they have worked with all RX outputs I have hooked them to, and with servo testers. They may or may not have a slightly higher minimum operating voltage than some servos (something to look for, especially if you are running a 3.3v system). It likes, as I recall, a minimum of 4.8v, and is 9v (or greater) tolerant.

The square shaft and clutch are interesting, and mounting is a tad different than other servos. The servos and motors, at $19.95 each, are not particularly expensive relative to servos of similar specifications, and are sold separately with a spare set of gears.

There is a sidebar in the December, 2005, issue of Servo Magazine with more info (olook under Radio Contro model Warship Combat.

Hope this proves helpful.

Wreno