I figured I’d start off nice and simple and do an SES arm, getting some experience along the way. I based this on (read: ripped off completely) the example SES arm pictured on the main site, except I’ve swapped the C-bracket at the wrist for an ASB-16 offset bracket for wrist rotation. I’ve made 3 mistakes thus far. First, I tried designing it using a BS2 and no SSC-32. Second, I did not take into account the lengths of the servo cables (an order for extenders is soon coming Jim!), and third, I am using HS-422s. Has anyone gotten an arm to support its own weight using 422s? This arm has a total of 7 servos and feels fairly heavy.
Using the SSC-32, I am definately missing something. Issuing pulse commands seems to only send one pulse, and it takes several repeats of the command to get the servo to the position I want. How do I get the servos to move fully to a position and hold it?
I am also not sure about this ABS-16. It seems one of the “wings” may be bent in too far, but I had trouble getting the wris rotation servo seated, and it is definately pinching the cable. The strain relief is bent almost 90 degrees! The wing bend is also causing trouble with attaching to the universal bracket above it.
I’ve not been able to test it with all servos runing, only up to the elbow flex, but it doesnt seem this arm can support its own weight. If I need to upgrade to stronger servos, its gonna sit on my shelf for another month while my budget recycles. Overall though, definately been a learning experience, and I am one happy hobbyist. I WILL get it working!
The HS-422 is an analogue servo. It needs to be told continuously its position. A basic stamp (BS2) simply isn’t fast enough to do this for many (really over two) servos. The SCC-32 should work fine with analogue servos. If you’re having issues first make sure everything has enough power. Are you using VS to power everything or have separate VS and VL? If you are only using one power supply, it’s at least 6V and can supply enough current for all your servos? Also, check your servo connections. Loose or otherwise bad connections will prevent the signal from the SCC-32 from reaching the servo (which since you have analogue servos, it needs to do). Since you say a strain relief is being pinched, I believe this might be part of your problem. Finally, check baud rate, TLL/ RS-232 and your serial connection to the BS2. You might have it set improperly.
As far as weight, are you using the balancing springs? I believe the pics on the SES section show the arm made with springs since it is rather heavy. Here is the pic of what I think you are trying to make: lynxmotion.com/images/jpg/aarm05.jpg.
Also, ABS-16…Do you mean ASB-16? That isn’t in the SES arm I linked above.
Needless to say, if you can afford digital or stronger servos, they do make life a lot easier.
Yup, that photo is fairly close to whats sitting on my table right now. No, the ASB-16 isnt in the photo, I took some creative liscense. If you look at the wrist design of the arm in the photo, you will see it cant rotate, only flex. So I replaced the C-bracket the little gripper is mounetd to with the ASB-16 and antoher servo.
I expect its power issues I’m having with the servos needing to be constantly updated. I broke the wiring harness I was using to givethe thing 6V from the wall, so I started using a 9V battery. Yes its trying to power everything. I expect many of my problems will be solved once I get the bot back on wall power.
On the positioning though, I’ve been trying to learn the SSC-32 as well. I’ve been using the servo move commands like #3 P1500 and such. Am I correct in assuming this sends a single pulse only? That would explain the behaviour I’m seeing as well.
Lol. Didn’t realize I was replying to both your threads. Yea, you need a lot more power. See my post on the other thread for more on that.
Also, since to the stock design needs a spring because its streching what the servos can hold, adding additional degrees of freedom (and hence more weight) might not be the best idea. I’d try the arm with a purely stock configuration first.
Yea, the command you’re sending there is a single move. Look at the SCC-32 manual under group move for how to move an ensemble of servos. However, you need to address your power issues before you can think about being able to move multiple servos.
OK, I believe I’ve solved the power problem, sort of. I’ve got a walwart power supply but its only rated for 1A. I can only power 2 servos before the bloody thing craps out on me. The 422s are little better. Clearly I need a stronger walwart and stronger servos for this arm, which is going to leave me with a few unused 422s once I replace em. I’m thinking a 422 based BRAT is going to be my next project. Ah well, done with the drawing board, back to the shopping cart!
The SSC-32 is resetting. Power the VL input with a 9vdc battery and power the servos from a separate 6vdc supply. For an arm our 2amp regulated wall pack should be sufficient, but more current certainly wouldn’t hurt. The wire on the servo installed in an ASB-16 does have to bend, but shouldn’t be a problem. It’s not been an issue with my testing, it can’t be helped anyway. Robot arms should use stronger servos starting at the base and you can use less powerful servos as you go up. Hope this helps.
Just a reminder that you need to be a little careful with walworts since sometimes the voltage is unregulated. Your electronics have a regulator, so they’re not going to overvolt. However, the servos do not. Overvolting a little is okay, and actually will give them a little more torque, but overvolting by a lot might be causing some stability issues. If you have a meter, I’d hook it up and see what the base voltage is, and how it changes as you add servos.
My personal experience is that unregulated walworts are over-rated current wise, and generally have rather poor voltage/ current curves. I had a 12V 3A rated walwort that would put out around 17V on low (<100 mA) loads, and drop to less than 9V with a 2A load. I tried using that to power my NiMH charger, and realized that I needed something better. I got around that by splicing into a spare computer’s 12V (30A rated) rail. Unfortunately I don’t know any common household devices that put out in the 6.0-9.0V range at serious current. If you can handle just running at 5V (you’d need a seperate VL) you could tap into the 5V rail on a computer. You probably should look at a Lynx supply (2.5 Amp regulated 5V), a serious battery pack, or making some kind of other power supply. One do-it-yourself solution might be the Lynx 6.0V regulator with a 12V tap from a computer. However, this would require a bit of wiring, and I’m not 100% sure it could handle that situation without extra cooling. However, something in your next basket should be a fix for your power issues.
I agree with what you said, but I would go one step further and say, never use an unregulated supply for servos! This is because the voltage fluctuates depending on how much work the servo is doing. Not a good thing in any motor control circuit.
Meter? What meter?
Ya I know I’m courting disaster without one, I lost most of my electronic gear in a very rushed emergency move a while back and am slowly, oh so slowly, rebuilding my workbench.
Speaking of throwing money at the problem, I just placed an order for the 6v 2a wallwort and a few servo extender cables. With any luck this will be the final order for this arm, and I can mvoe on to bigger and badder price, er, projects.
Hey Jim, for the record, can you tell me what servos were used in that arm in the pic I based my design off of? I’ve got the additional weight of the ASB-16 and the extra 422 on the wrist but I dont think its making that big of a difference in the overall weight of things.
D’oh!
Well then this probably wont be the last order, though I’ll probably cheat some and just order a brat with a 6pack of 645s and toss my 422s on the brat frame instead. That should nicely finish one project and start another!
Hopefully by the end of next month, and before I have to go into the hospital, I will have phase two of my Octabot completed and be ready to start into a new project. I already pretty much know, between two possibilities, what I want to build next - either a tri-wheel rover or one of my Walk 'N Roll robots.
OK, got my latest order today (Thanks Annette), and have learned the following. I overestimated the length of one of my control lines, so another extension cable is needed. The lynx power pack did solve my power issues nicely. I really, really, REALLY need better than 422s on the shoulder and elbow flex, and its not a good idea to test out shoulder rotation with this design unless youve got the base FIRMLY anchored to something. A 422 is just fine here though, as all the weight is supported by the heavy duty bracket. Blasted thing swung itself right off my desk!
So, I’ll be ordering at least 3 645s and a servo extension cable, and the 475 for the wrist if I can work it into the budget. Christmas for me is likely to wait until january. But by God I WILL get this blasted thing working. Now its personal
Hmm… I’ve seen some mounting rails on some of the lexan sumo kits. Are these rails thin enough to maybe slip under the base as stabilizers?
Methinks there’s a misconception, above.
I believe you were asking if sending “#3 P1500” tells the SSC-32 to send a single pulse of 1.5ms to your servos.
If that’s the case, then the answer is no.
Upon power up, the SSC-32 will send no servo pulses, so analog servos will not be holding a position.
Once an initial position is sent to the SSC-32 for a give servo, it will (almost) imediately send the pulse out to the servo.
This “activates” the given servo, if you will.
Every 20 milliseconds (give or take), all “activated” servos get sent another pulse.
This continues until the SSC-32 is reset (which will occur with annoying frequency if you have power issues) or until the SSC-32 recieves a new instruction for that servo.
So, to answer your question, a single instruction to the SSC-32 will be repeatedly pulsed to your servos indefinitely, until you change something.
So, you’re right in thinking that the SSC-32 is only sending out a single pulse, but you’re right for the wrong reason.
It’s only sending out a single pulse because the servos that you tell to move are overdrawing the power supply and resetting the SSC-32.
D’oh!
I’ve seen some mounting rails on some of the lexan sumo kits. Are these rails thin enough to maybe slip under the base as stabilizers?
