Well, this is my first post, and I’m really stumped as to why my hexapod will not do what I want it to do. I am using the old black H2 “stealth,” and I tabled it for a year because this problem frustrated me so much! Now, I have decided to fix it because I had the rest of the project functional (Infrared and touch sensors, wireless PS2 control, with basic functionality programed into it).
Being the poor student I was, I bought a Basic Atom pro and Parallax Servo Controller (because I couldn’t afford a lynxmotion one as well), and I built and programed the robot to work just fine. But the problem is, the servos do not have nearly enough torque as they should–the robot cannot lift itself up to take a step! Since it drained batteries very, very quickly (3-5 minutes for a set of 4 AAs), I figured it was a short somewhere. So, I removed every motor and reconnected them one by one, waiting to see if any motor caused a larger drop in the battery voltage (all the motors were unloaded because I had suspended it to make working on the inside while it was on easier). No motor that I could tell caused a noticable drop in voltage (or at least, no more noticable than the last). Next, I tested the robot on a smooth floor so that I could see which joints in particular were not moving the range of motion they should. The front pair of legs was doing the complete range of motion (and sliding across the ground), but the center and back pairs of legs were not lifting off of the ground. What could the problem be, and how should I go about pinpointing and fixing it?
P.S. The problem doesn’t lie in software or the servo controller, because when suspended the robot’s gait works exactly as programmed. The motors just don’t produce enough power to move the damn thing.
I’m using the Mini Atom bot board, and I have an IRPD and PS2 controller jack (without any controller connected, and from Lynxmotion’s site) connected to the board as well as the PSC.
4.8V should be fine for the servos; they are rated for 4.8V-6V, and besides, I know that the servos work at 4.8V because I previously ran the robot successfully with everything the same, less the servo controller and basic atom and plus an OOPic-R. So its not just the voltage of the batteries.
Are your logic and servo power supplies seperate? Could it be that under load the servos draw too much for the voltage regulators on the logic side or simply noise causing resets? Are these batteries connected directly to the servos or is there some sort of regulator?
The 4.8V pack is connected directly to the servos, the 9V battery is regulated via the 5V regulator on the Atom Bot Board (which works, I tested the voltages), and they only share a common ground, so it isn’t a brownout issue. In fact, as I said, the microcontroller is fully functional, because I tested, using the Serial Port to communicate, both PS2 Controller functionality and all of the sensors’ functionality.
To narrow it down try a wall wart supply. Something around 6 volts and several amps. Servos can run at 4.8 but at significantly reduced strength, though it would surprize me if 4.8 reduced it to a point where it couldn’t stand.
If that solves it you can focus on the batteries, connections and the like. If it doesn’t I’d look at the refresh rate on the pulse lines (should be around 50hz).
I’m using the servos with the kits. Nothing has got hot, but the batteries become warm-lukewarm. They are still cool enough that if they were food I was microwaving, I’d microwave them for 30+secs more with a 1kw+ microwave. So that’s not too hot (I’m a terrible judge of temperature). I’m going to try a wall wart right now, and I’ll report on the results.
Well, just a question, you mention that you are using NiMH batteries… What is their capacity (mAh) ?
Moreover, you say that you have already had your setup working properly but without the servo controller, so how did you control the servos ?
The problem with the EH2 and only 4.8V NiMH is that you will have at least 6 servos working at the same time, thus eating a lot of power. If your batteries are of poor quality, the voltage drop induced by the battery internal resistance will kill the whole thing…
I would really recommend you to power your servos directly from an RC car battery pack (7.2V, 2400mAh), this is the setup I am using with my EH2, and I can run it for ~45min without problems…
Your problem is voltage. I have always recommended 7.2vdc for 12 servo hexapods. With the old H2 Stealth, you have a double whammy, because the legs do not have the mechanical advantage like the current EH2 has. Well, it has a slight mechanical advantage, but, it’s not as effective. If you use a 7.2vdc, 1600mAh pack your servos will have almost twice the power as they do at 4.8vdc.
Well, I got a new battery pack, 7.2V from a brand new cordless drill (so it should be able to provide enough amperage) but it did not do the trick. when I connected it all, the voltage dropped from 7 to 3 volts, and the legs barely moved. Clearly, this doesn’t make sense, because I’ve now tried two power sources that should have provided sufficient current, and they have not. I guess the problem must be somewhere in the wiring (a short?), but I don’t know any good ways to track the lost energy down except by checking for heat with my finger, which has not been successful yet. If anyone has any other ideas, please tell me. I used the continuity tester on my DMM, and it didn’t detect a short between the positive and negative terminals of the motor driver board.
Also, I ugraded all the wiring for the power to 14GA wires, so that they would be eliminated as a potential power issue. Could the problem be from the switch on the hexapod (its the one the kit came with)?
You’re not telling the whole story. Where are you measuring 3vdc at? At the battery?, at the servo controller posts? at the servo connections? If you’re measuring 3vdc at the battery, then charge it, or pitch it, cause it’s bad. If you’re measuring it at the servo connections, and there is still 7.2vdc at the battery, then where is the extra voltage going?
Check the switch with an ohm meter. Anythings possible. I suspect your problem is with wiring or interconnections.
Well, I measured the vastly dropped voltage at the battery terminals, but I have determined that the battery and the servos are not the problem. As Andy suggested, I’m no longer a poor student and so I hooked up one of the couple of spare servo controllers I have lying around using the old batteries (because I didn’t want to go to the trouble of pulling my new battery system out of the robot. The servos certainly ran at full torque, because while they were simply flailing about, a single leg could lift the whole robot, and it was rotating and translating, even though single servos were responsible for the motion. Therefore, the problem must be with the switch or the servo controller. I am going to connect the new servo controller to the same rails the that the internal robot parts are supplied by, and hopefully that will solve this painful (for me) mystery and tell me what I need to buy to fix it.
Along similar lines, I have just received my Hex 3-R. I am not yet finished assembling it. However, once it’s completed, what sort of supply should I need to give it? I’m assuming something around 6 volts, but how many amps? I’d guess 1.5 or so, but that’s just a wild guess. How much current will the servos draw at peak, say when initially standing up? How about when just holding position? Any of you other H3-R owners have any current numbers?
Andy I followed you link and noticed in your post that you use an 11v battery that is regulated 5v and 6v respectfully. Might I ask what type of regulator you’re implementing? I too have decided to design a regulated power distributing circuit for battery implementation. I initially implemented an lm7805 type of voltage regulator however I came across some problems. I am now using a brown out IC (cant recall the type at the moment) and when shopping I was not able to find one that can handle more than 9 volts. For testing purposes I designed a rectified power supply using a transformer, rectifier, and lm317 (caps and pots, etc.) this is what I am considering to use to charge my batteries. If you can; would you elaborate on the trickle charge current and voltage values before a battery failure [boom]? I made the power supply adjustable (lm317) so I was thinking it would double for a charger. Is this going to be safe?
