My project for this weekend was to build a little decorative bauble, a piece of electronic jewelry. Seven high-brightness UV leds driven by a ring oscillator circuit arranged around a uranium-glass marble. You can see a few pictures of it here.
Some of you may recognize this as being similar to the power indicator on my walking robot. The difference with this piece, other than being stand-alone, is that the LEDs are made to flash sequentially rather than being on all the time. It makes it a lot more eye-catching.
Several of my friends are encouraging me to make and sell these as steampunk jewelry. That might end up being the only way I can afford new servos for the robot.
Saturday, October 31, 2009
Friday, October 30, 2009
Looking for servos...
The 2009 version of my walking robot was in part a test of the feasibility of using the cheap MG995 servo in a walking robot. I expected that even if these cheap servos didn't last long their cheapness meant replacing them wouldn't be unreasonably expensive. These servos lasted long enough in practice (although when they failed the failure mode what not what I expected) but their terrible position-holding ability made them unacceptable for what I'm trying to do. So the MG995 servos have to go.
I'm currently rebuilding the robot with some HS-5645 servos I have on hand, but I don't expect those to last long. One already has a stripped gear set, and the others don't look they have much life left in them.
I need new servos. Preferably metal-geared servos with motors rated for 7.4V lithium battery use. Unfortunately I'm also on a very tight budget for robots at the moment. My wife will only approve buying expensive servos if I can guarentee they will never fail, and who can guarentee that?
There's the HSR-8498HB. The low-end 7.4V robot servo from HiTec. Not cheap at $60, and a little less torque than the HS-5645MG. It has Karbonite gears, whose durability I would be very concerned about. The HSR-5498SG has steel gears, better torque, and is only slightly more expensive at $70. Both these servos are available with a rear axle point built into the case, which would remove the need for me to modify the case that way. On the downside these do not use the standard servo mounting points, which means they would not be useable with the current waterjet frame pieces. Although since all the strucural failures I had were at the standard mounting points, redesigning the servo-to-frame attachment method might be a good idea.
There's the $60 HDS-2288. Metal gears, rated for 7.4V, nice torque, uses standard servo mounting points. I'm not sure if they're actually available with any reliability, they seem to be sold out or on backorder everywhere.
The $20 Associated SHV1504MG has metal gears and is rated for 7.4V, and uses a standard servo case, but is terribly underpowered for this requirement.
Then Dynamixel servos. These look really interesting, having some amazing features way beyond normal servos in how they can be configured and what kind of feedback data you can get back from them. These don't use the standard servo control pulses at all, but instead have an addressed serial bus scheme, which I should be able to connect straight to the serial ports on the Xbee radio. I could use this to feed back voltage and temperature indications to the controller, maybe even translate joint torque into force-feedback somehow. Very interesting possibilities. And all these servos will run at 7.4V. They're actually designed for 9.6V, so 7.4V is a little on the low side but still within the rated voltage range.
On the down side most of the Dynamixel servos are completely outside my price range. The $45 AX-12 servo I could probably afford, but those have plastic gears which I'd prefer to avoid. The Dynamixel servos also would require a complete strucural redesign to use. That might be a plus after all, the AX-12 servos are far better suited to robotics from a strucural point of view than are standard hobby servos.
The last option would be to just make some frankenstein hack from my current servos, homemade die-cast cases, openservo boards, and rewound motors. Considering that as a backup plan.
I'm currently rebuilding the robot with some HS-5645 servos I have on hand, but I don't expect those to last long. One already has a stripped gear set, and the others don't look they have much life left in them.
I need new servos. Preferably metal-geared servos with motors rated for 7.4V lithium battery use. Unfortunately I'm also on a very tight budget for robots at the moment. My wife will only approve buying expensive servos if I can guarentee they will never fail, and who can guarentee that?
There's the HSR-8498HB. The low-end 7.4V robot servo from HiTec. Not cheap at $60, and a little less torque than the HS-5645MG. It has Karbonite gears, whose durability I would be very concerned about. The HSR-5498SG has steel gears, better torque, and is only slightly more expensive at $70. Both these servos are available with a rear axle point built into the case, which would remove the need for me to modify the case that way. On the downside these do not use the standard servo mounting points, which means they would not be useable with the current waterjet frame pieces. Although since all the strucural failures I had were at the standard mounting points, redesigning the servo-to-frame attachment method might be a good idea.
There's the $60 HDS-2288. Metal gears, rated for 7.4V, nice torque, uses standard servo mounting points. I'm not sure if they're actually available with any reliability, they seem to be sold out or on backorder everywhere.
The $20 Associated SHV1504MG has metal gears and is rated for 7.4V, and uses a standard servo case, but is terribly underpowered for this requirement.
Then Dynamixel servos. These look really interesting, having some amazing features way beyond normal servos in how they can be configured and what kind of feedback data you can get back from them. These don't use the standard servo control pulses at all, but instead have an addressed serial bus scheme, which I should be able to connect straight to the serial ports on the Xbee radio. I could use this to feed back voltage and temperature indications to the controller, maybe even translate joint torque into force-feedback somehow. Very interesting possibilities. And all these servos will run at 7.4V. They're actually designed for 9.6V, so 7.4V is a little on the low side but still within the rated voltage range.
On the down side most of the Dynamixel servos are completely outside my price range. The $45 AX-12 servo I could probably afford, but those have plastic gears which I'd prefer to avoid. The Dynamixel servos also would require a complete strucural redesign to use. That might be a plus after all, the AX-12 servos are far better suited to robotics from a strucural point of view than are standard hobby servos.
The last option would be to just make some frankenstein hack from my current servos, homemade die-cast cases, openservo boards, and rewound motors. Considering that as a backup plan.
Thursday, August 20, 2009
LiPolys and servos
Lithium-polymer batteries are inconvenient. They're physically fragile, awkwardly shaped (at least for fitting in cylindrical or spherical compartments), prone to turning into fireworks if mistreated, and have a nominal cell voltage of 3.7V. I can work around the shape and fragility, and installed a battery protection board to keep the cell from igniting, but the cell voltage is a real problem. Most cheaply and easily available servos are designed to operate at 4.8V or 6V, running off a 4 or 5 cell NiCad pack. One LiPoly cell is too low-voltage to run servos from, and two will deliver 7.4V nominal, and up to 8V when freshly charged.
I can't afford to use the high-end robot servos designed for 7.4V, but I also can't ignore the fact that LiPoly cells deliver three times the energy density of NiCads. So since 2007 I've been ignoring the manufacturer recommendations and running normal hobby servos off two-cell LiPOly packs. It's worked, mostly, and I think I'll keep doing it.
Looking at the components inside a typical digital servo does not reveal any obvious voltage limitation that would prevent operation over 6V. The components which limit the voltage at which the control circuit can operate are the motor drive FETs and the 3.3V voltage regulator for the microcontroller. The FETs in the MG-5645 servos have a breakdown voltage rating of 20V, which even assuming 2X overvoltage from EMF spikes should run safely off a 9.6V pack. The 3.3V regulator I can't find a datasheet for yet, but typically should have a maximum input voltage of at least 12V. Nothing on the PCB itself should fail to work at 7.4V.
The actual voltage limit seems to be in the servo motor. According to HiTec, the motor in their standard digital servos is limited to 6V. Now, back from my old robot combat days, I learned a lot about how much you could safely overvoltage motors, and just what caused them to fail when pushed to hard. Voltage alone does not destroy a motor. Armature windings can come apart from spinning the motor too fast, magnets demagnetized from too much current and heat, and too much load for too much time without enough cooling can cause the solder joints to simply melt, but voltage alone doesn't fry a motor. The actual point at which a motor will self-destruct is complicated and depends a lot on the gear ratio, actual load on the shaft, and thermal conditions.
It's probable that 6V is the point where these servos are rated for steady operation without failure. Pushing them past that increases the rate of failure, but the actual failure rate is also dependent on a lot of other factors including the quality of construction, load, and duty cycle. Running the MG-5645 servos at 7.4V, I had no failures of the motors or control boards, although I did break a lot of gear sets. The motors did get pretty hot after a few minutes of walking, but I didn't have any actually fail. With the HX12K servos I had two motors simply die, unsurprising considering their cheap construction. The MG996 servos will probably work no better at 7.4V. I may rebuild the robot with MG-5645 servos after all, since they seem to tolerate some degree of overvolting.
I can't afford to use the high-end robot servos designed for 7.4V, but I also can't ignore the fact that LiPoly cells deliver three times the energy density of NiCads. So since 2007 I've been ignoring the manufacturer recommendations and running normal hobby servos off two-cell LiPOly packs. It's worked, mostly, and I think I'll keep doing it.
Looking at the components inside a typical digital servo does not reveal any obvious voltage limitation that would prevent operation over 6V. The components which limit the voltage at which the control circuit can operate are the motor drive FETs and the 3.3V voltage regulator for the microcontroller. The FETs in the MG-5645 servos have a breakdown voltage rating of 20V, which even assuming 2X overvoltage from EMF spikes should run safely off a 9.6V pack. The 3.3V regulator I can't find a datasheet for yet, but typically should have a maximum input voltage of at least 12V. Nothing on the PCB itself should fail to work at 7.4V.
The actual voltage limit seems to be in the servo motor. According to HiTec, the motor in their standard digital servos is limited to 6V. Now, back from my old robot combat days, I learned a lot about how much you could safely overvoltage motors, and just what caused them to fail when pushed to hard. Voltage alone does not destroy a motor. Armature windings can come apart from spinning the motor too fast, magnets demagnetized from too much current and heat, and too much load for too much time without enough cooling can cause the solder joints to simply melt, but voltage alone doesn't fry a motor. The actual point at which a motor will self-destruct is complicated and depends a lot on the gear ratio, actual load on the shaft, and thermal conditions.
It's probable that 6V is the point where these servos are rated for steady operation without failure. Pushing them past that increases the rate of failure, but the actual failure rate is also dependent on a lot of other factors including the quality of construction, load, and duty cycle. Running the MG-5645 servos at 7.4V, I had no failures of the motors or control boards, although I did break a lot of gear sets. The motors did get pretty hot after a few minutes of walking, but I didn't have any actually fail. With the HX12K servos I had two motors simply die, unsurprising considering their cheap construction. The MG996 servos will probably work no better at 7.4V. I may rebuild the robot with MG-5645 servos after all, since they seem to tolerate some degree of overvolting.
Wednesday, August 19, 2009
Post GenCon report!
The robot ran at GenCon, getting in about half a dozen full charge cycles in over about three days before it was past the point where I could keep it going. Ultimately while it did succeed at the goal of entertaining others and myself, the performance and reliability weren't quite what I had hoped.
As I'd expected, all the reliability problems were with the servos. Unexpectedly, I didn't have a single gear failure. No stripped or broken teeth on any of the gears, and even the issue with the press-fit third gear coming apart that I'd seen early on one servo during testing didn't come up. I did have failure of the mounting tabs on three of the servos. The plastic web where the servo mounting tabs meet the cases is very thin on these servos. These mounting tabs can take the loads created by the servo torque, but the bending and twisting forces induced by the way all the forces from each leg are transmitted through the servo mounts appear to be more than they can take. I had servo mounting tabs break off on three servos, and so far have found cracks in the case of one more servo.
I also had motor failures in two servos. In both cases while walking the robot suddenly lost all power, apparently due to the overcurrent trip on the battery protector board. In both cases once power was cycled one of the servos no longer functioned, not even in the passive motor-braking mode the servos default to on powerup. I had thought it due to controller board failures, but on disassembly and testing found both motors had failed open. I haven't taken apart the motors to find out while they failed yet, but I suspect heat-induced failure of the connections to the brushes, or the brush holders or springs.
I had expected these servos to have a high failure rate, and designed the robot for easy access to the servos, so even with the high failure rate of these servos I was able to keep the robot working until I ran out of unbroken servo cases. I wasn't happy about having to repair the robot as often as I was having to recharge it's batteries, but I wasn't completely surprised.
The bigger problem was the poor control I had over the robot's motion. The lifelike movements people like about the robot, and the ability to make fins and subtle movements to interact with the crowd, depends on the servos closely matching the movement of the joysticks. On that this version of the robot failed. First of all, the precision of the HX12K servos is utter crap. There's a huge amount of play in the geartrain, which combined with a badly tuned control loop in the servo control board means that the position signal you're sending to the servo is going to be treated as at most a rough suggestion as to the position you'd like the servo to hold. There's a huge amount of overshoot and ringing, the robot's legs wandering and shaking randomly even when sent a steady position signal. Standing still with all four legs on the ground the robot would be stable, but it would shake uncontrollably when holding a leg in the air, and walking was jerky and unsteady. It was a goods thing thing that the ability to invert the legs and walk either side up was always a crowd favorite, because the robot would frequently fall over while walking.
The other problem was that the radio link, which worked fine at home, was unreliable in a convention hall full of cell phones and other electronics. Control was unpredictable - sometimes it would work fine at up to 30 feet, sometimes it would intermittently lock up at less than 5 feet.
I'm also not happy with the Wii Nunchuk joysticks. I just don't get the same fine-control ability as I had with the RC airplane radio set I used to use.
The radio range issue can be easily corrected. At the moment I'm using the 1mW version of the XBee radios. The Pro version of the XBee radio has a transmitter power of 60mW. More power might be the brute force solution of the problem, but that's fine with me. I also have some ideas for rebuilding the controllers with better joysticks stripped out of an old RC airplane set.
The control issues with the servos I can't do anything about without replacing them. Poor position-holding seems to be an unsolvable issue with the MG995/HX12K servo. I suspect that even if I transplanted the control boards from the HS-5645MG servos in my junk box the insane amount of play in the geartrain would prevent them from being accurate enough for my needs.
Some of the more astute of you might wonder, if I've got nice high-quality Hs-5645MG servos in my junk box, why am I wasting my time with cheap import crap like the HX12K? It's because the HS-5645MG gearboxes kept failing. The HX12K servos, for all their faults, at least have bulletproof gears. I may try rebuilding the robot with my old HS-5645MG servos anyway - the spring shock absorbers I built into this version of the robot's legs may be enough to save the gears from failing. There is also a new servo out, the TowerPro MG996, which is an upgraded version of the MG995/HX12K servo with better accuracy and reliability, which I might buy and try out eventually. For the time being my budget is shot and my wife's patience with robots that break after a minute of walking is expended, so buying any new servos will have to wait.
As I'd expected, all the reliability problems were with the servos. Unexpectedly, I didn't have a single gear failure. No stripped or broken teeth on any of the gears, and even the issue with the press-fit third gear coming apart that I'd seen early on one servo during testing didn't come up. I did have failure of the mounting tabs on three of the servos. The plastic web where the servo mounting tabs meet the cases is very thin on these servos. These mounting tabs can take the loads created by the servo torque, but the bending and twisting forces induced by the way all the forces from each leg are transmitted through the servo mounts appear to be more than they can take. I had servo mounting tabs break off on three servos, and so far have found cracks in the case of one more servo.
I also had motor failures in two servos. In both cases while walking the robot suddenly lost all power, apparently due to the overcurrent trip on the battery protector board. In both cases once power was cycled one of the servos no longer functioned, not even in the passive motor-braking mode the servos default to on powerup. I had thought it due to controller board failures, but on disassembly and testing found both motors had failed open. I haven't taken apart the motors to find out while they failed yet, but I suspect heat-induced failure of the connections to the brushes, or the brush holders or springs.
I had expected these servos to have a high failure rate, and designed the robot for easy access to the servos, so even with the high failure rate of these servos I was able to keep the robot working until I ran out of unbroken servo cases. I wasn't happy about having to repair the robot as often as I was having to recharge it's batteries, but I wasn't completely surprised.
The bigger problem was the poor control I had over the robot's motion. The lifelike movements people like about the robot, and the ability to make fins and subtle movements to interact with the crowd, depends on the servos closely matching the movement of the joysticks. On that this version of the robot failed. First of all, the precision of the HX12K servos is utter crap. There's a huge amount of play in the geartrain, which combined with a badly tuned control loop in the servo control board means that the position signal you're sending to the servo is going to be treated as at most a rough suggestion as to the position you'd like the servo to hold. There's a huge amount of overshoot and ringing, the robot's legs wandering and shaking randomly even when sent a steady position signal. Standing still with all four legs on the ground the robot would be stable, but it would shake uncontrollably when holding a leg in the air, and walking was jerky and unsteady. It was a goods thing thing that the ability to invert the legs and walk either side up was always a crowd favorite, because the robot would frequently fall over while walking.
The other problem was that the radio link, which worked fine at home, was unreliable in a convention hall full of cell phones and other electronics. Control was unpredictable - sometimes it would work fine at up to 30 feet, sometimes it would intermittently lock up at less than 5 feet.
I'm also not happy with the Wii Nunchuk joysticks. I just don't get the same fine-control ability as I had with the RC airplane radio set I used to use.
The radio range issue can be easily corrected. At the moment I'm using the 1mW version of the XBee radios. The Pro version of the XBee radio has a transmitter power of 60mW. More power might be the brute force solution of the problem, but that's fine with me. I also have some ideas for rebuilding the controllers with better joysticks stripped out of an old RC airplane set.
The control issues with the servos I can't do anything about without replacing them. Poor position-holding seems to be an unsolvable issue with the MG995/HX12K servo. I suspect that even if I transplanted the control boards from the HS-5645MG servos in my junk box the insane amount of play in the geartrain would prevent them from being accurate enough for my needs.
Some of the more astute of you might wonder, if I've got nice high-quality Hs-5645MG servos in my junk box, why am I wasting my time with cheap import crap like the HX12K? It's because the HS-5645MG gearboxes kept failing. The HX12K servos, for all their faults, at least have bulletproof gears. I may try rebuilding the robot with my old HS-5645MG servos anyway - the spring shock absorbers I built into this version of the robot's legs may be enough to save the gears from failing. There is also a new servo out, the TowerPro MG996, which is an upgraded version of the MG995/HX12K servo with better accuracy and reliability, which I might buy and try out eventually. For the time being my budget is shot and my wife's patience with robots that break after a minute of walking is expended, so buying any new servos will have to wait.
Thursday, August 13, 2009
Servo failures
I had another servo failure today, after a lot of walking around and showing the robot off. I'm seeing an interesting pattern. It's always the two servos which move the rear legs horizontally which fail. These seem to be under the most strain, being the servos responsible for most of the forward/upward movement of the robot with each step. Surprisingly enough, the gears are not failing at all. The cases are. The mounting tabs which connect the servos to the robot chassis have cracked on two servos now. I am also seeing failure of the case at the point where the gear shafts are held in place. Over time the shafts wear away the plastic until the shafts can wobble enough for the gears to jam or come unmeshed.
I have seem this type of failure before. Now that the problems with stripped gears appear to be solved it seems the plastic servo case is the next weak link. I have some ideas for reinforcing the servos but the machining involved may beyond the capabilities of my workshop.
I have seem this type of failure before. Now that the problems with stripped gears appear to be solved it seems the plastic servo case is the next weak link. I have some ideas for reinforcing the servos but the machining involved may beyond the capabilities of my workshop.
Wednesday, August 12, 2009
Update - day 1 of GenCon
Walked the robot around at the GenCon Stink for a while until one of the servos suddenly stopped working. Gears are intact. Seems to be a dead control board, although nothing is obviously burned out. Swapped the servo and the robot is walking again. Also one slightly cracked servo case and one loose servo horn screw, both easily fixed.
Radio range seems to be a lot shorter than at home. I blame the large number of cell phones. Must invest in higher power transmitter module after convention.
Radio range seems to be a lot shorter than at home. I blame the large number of cell phones. Must invest in higher power transmitter module after convention.
Friday, August 7, 2009
Design goals for 2009 walker:
1. No structural joints made with glue or epoxy.
The first walking robot I made wasn't much more than servos, a battery pack and a radio receiver held together with silicone glue. That worked well enough as those robots were too feeble and lightweight to need much structural strength. Later, as I went to stronger servos, heavier batteries, and more rigid structure I experimented with various epoxies. To keep the weight down, I wanted to use the servo cases and batteries themselves as structural elements, which meant ignoring the intended mounting tabs and epoxying directly to the servos.
The glue and epoxy kept failing. In the end I had the best luck with cyanoacrylate superglues, but even those didn't hold reliably. And when I started showing the robot at conventions, gluing everything together had a second downside in making it much harder to easily replace broken parts. The final straw was when I switched from the hefty NiCad cells I had been using to LiPoly packs with minimal structural strength. When I needed to add a steel frame to take the place of the batteries that had previously made up the structural core, there wasn't much point in using glue rather than screws to attach to the servos.
When I set out to completely redesign the structure on the latest rebuild, I set a solid rule. No glued or epoxied structural joints. Everything structural would either be fastened with machine screws (if meant to be disassembled), or welded or soldered (if meant to be permanent).
I did end up having to use superglue to attach weld nuts (used to provide a second hinge point on each servo joint) to the inside of the servo cases. The glue is only needed to hold the nuts in place during assembly, as the screw which holds the hinge pin in place also holds the servo nut from coming loose. I also had to use gaffer's tape and zip-ties in a few spots to hold wires in place. Not an elegant solution, but acceptable for non-structural parts.
2. No purely decorative components
I've made the mistake in the past of loading decorative bodywork and cosmetic detailing on convention robots. On the 2006 version of the robot I built up complex anodized titanium bodywork around the servos and joints, with rubber tubing covering the servo and battery wires and thick aluminum tubes for the legs and central chassis. The result - excess weight, stiffness and interference in the joints, for bodywork and cable sheaths that the robot would work better without. That version walked about ten seconds before stripping a gearbox.
Faster battery drain and more frequent gear replacement made for less time walking the robot around and having fun. Having a robot that works reliably for a long time had to be my first priority. My second strict rule on this redesign was to have nothing on the robot which didn't in some way help the performance or reliability of the robot. This didn't mean no cosmetic details. It meant making those functional parts of the robot cosmetically interesting so I wouldn't have to add completely useless cosmetic parts. I was also willing to stretch my definition of what counted as a justifiable functional purpose. For example, the copper body shell technically serves the purpose of holding wiring in place out of the way of the legs, so that counts.
3. Easy to maintain
I've tried, and failed, to design the robot such that nothing breaks over the course of a busy weekend. Instead I've designed the current version for easy maintenance. Those parts which I carry spares for can be easily removed and replaced without having to break or cut anything. The servo gears which are the most frequently failing point can be replaced without even having to remove the servos from the robot.
Those parts which I don't have spares for can still be either removed for troubleshooting, or at least easily accessed by removing any bodywork and components around them. I don't at the moment have spares for the radio, the battery, the servo-control board, or many of the other internal components, but I can get to and troubleshoot all those parts in place easily. Having a thin waterjet-cut steel frame supporting an easily removable outer shell versus the closed-box structure on some previous designs makes maintenance a lot easier.
4. Safe for human contact
This robot is meant to be shown off at conventions, walking up to people, shaking hands, and even being picked up and handled. It needs to be able to do all this without injuring anyone. No sharp points or edges. I designed it with spherical and cylindrical body sections, and rounded feet rather than points. The antenna had to stick out somewhat, but I replaced the stiff wire antenna that came with the radio gear with a length of super-flexible wet-noodle wire.
It also means I had to try to avoid pinch points. The servos I'm using now have amazing torque, enough that a finger in the wrong place could be crushed if the robot moved while being held. Although the XBee radios are pretty good about rejecting bad data, I didn't want to rely on the control system to prevent any of the servos from moving at the wrong time. This robot is designed with large openings around the joints, with rounded edges on all the metal surfaces facing openings. This also helps with reliability - wide open joints with lots of clearance are a lot less likely to bind or jam, and make it easier to reach in and work on mechanical bits without taking the entire joint apart.
The first walking robot I made wasn't much more than servos, a battery pack and a radio receiver held together with silicone glue. That worked well enough as those robots were too feeble and lightweight to need much structural strength. Later, as I went to stronger servos, heavier batteries, and more rigid structure I experimented with various epoxies. To keep the weight down, I wanted to use the servo cases and batteries themselves as structural elements, which meant ignoring the intended mounting tabs and epoxying directly to the servos.
The glue and epoxy kept failing. In the end I had the best luck with cyanoacrylate superglues, but even those didn't hold reliably. And when I started showing the robot at conventions, gluing everything together had a second downside in making it much harder to easily replace broken parts. The final straw was when I switched from the hefty NiCad cells I had been using to LiPoly packs with minimal structural strength. When I needed to add a steel frame to take the place of the batteries that had previously made up the structural core, there wasn't much point in using glue rather than screws to attach to the servos.
When I set out to completely redesign the structure on the latest rebuild, I set a solid rule. No glued or epoxied structural joints. Everything structural would either be fastened with machine screws (if meant to be disassembled), or welded or soldered (if meant to be permanent).
I did end up having to use superglue to attach weld nuts (used to provide a second hinge point on each servo joint) to the inside of the servo cases. The glue is only needed to hold the nuts in place during assembly, as the screw which holds the hinge pin in place also holds the servo nut from coming loose. I also had to use gaffer's tape and zip-ties in a few spots to hold wires in place. Not an elegant solution, but acceptable for non-structural parts.
2. No purely decorative components
I've made the mistake in the past of loading decorative bodywork and cosmetic detailing on convention robots. On the 2006 version of the robot I built up complex anodized titanium bodywork around the servos and joints, with rubber tubing covering the servo and battery wires and thick aluminum tubes for the legs and central chassis. The result - excess weight, stiffness and interference in the joints, for bodywork and cable sheaths that the robot would work better without. That version walked about ten seconds before stripping a gearbox.
Faster battery drain and more frequent gear replacement made for less time walking the robot around and having fun. Having a robot that works reliably for a long time had to be my first priority. My second strict rule on this redesign was to have nothing on the robot which didn't in some way help the performance or reliability of the robot. This didn't mean no cosmetic details. It meant making those functional parts of the robot cosmetically interesting so I wouldn't have to add completely useless cosmetic parts. I was also willing to stretch my definition of what counted as a justifiable functional purpose. For example, the copper body shell technically serves the purpose of holding wiring in place out of the way of the legs, so that counts.
3. Easy to maintain
I've tried, and failed, to design the robot such that nothing breaks over the course of a busy weekend. Instead I've designed the current version for easy maintenance. Those parts which I carry spares for can be easily removed and replaced without having to break or cut anything. The servo gears which are the most frequently failing point can be replaced without even having to remove the servos from the robot.
Those parts which I don't have spares for can still be either removed for troubleshooting, or at least easily accessed by removing any bodywork and components around them. I don't at the moment have spares for the radio, the battery, the servo-control board, or many of the other internal components, but I can get to and troubleshoot all those parts in place easily. Having a thin waterjet-cut steel frame supporting an easily removable outer shell versus the closed-box structure on some previous designs makes maintenance a lot easier.
4. Safe for human contact
This robot is meant to be shown off at conventions, walking up to people, shaking hands, and even being picked up and handled. It needs to be able to do all this without injuring anyone. No sharp points or edges. I designed it with spherical and cylindrical body sections, and rounded feet rather than points. The antenna had to stick out somewhat, but I replaced the stiff wire antenna that came with the radio gear with a length of super-flexible wet-noodle wire.
It also means I had to try to avoid pinch points. The servos I'm using now have amazing torque, enough that a finger in the wrong place could be crushed if the robot moved while being held. Although the XBee radios are pretty good about rejecting bad data, I didn't want to rely on the control system to prevent any of the servos from moving at the wrong time. This robot is designed with large openings around the joints, with rounded edges on all the metal surfaces facing openings. This also helps with reliability - wide open joints with lots of clearance are a lot less likely to bind or jam, and make it easier to reach in and work on mechanical bits without taking the entire joint apart.
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