Next large robot

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  • I didn't have to deal with that. There are a few sellers of these that have a warehouse in Oakland that they hold product in, and I live an hour from there in San Jose, so it was delivered with a lift-gate truck to my door as part of the $2500 price. I was expecting "75% completed kit" but it was pretty much plug & play. I've been happy with it.
  • I went and ordered the Boss Laser Cutter today. I ordered the LS1620 model but they upgraded me to the LS1630 model for the same price as the other unit. It seems they are phasing these out but haven't updated their website. They don't want to bring in any more of these so I agreed to take one. Now its a 2-4 week wait for delivery. In the meantime I have to figure out where to set this up in the basement and clear out a spot.

    Bob Sweeney
  • Unless you need to laser something bigger than 12" x 9", you can buy a 40W laser on ebay for $365, free shipping.

    A 100W laser can't cut any more materials that a 40W laser can. To laser all the plywood for my R/C airplane only takes an hour. I imagine then that a 100W laser would do it in 24 minutes. I did have to align the mirrors etc, but everything went smoothly.

    I hope for whatever you're spending, that at least it doesn't have stepper motors.
    I am the Master, and technology my slave.
  • The_Master wrote: »
    Unless you need to laser something bigger than 12" x 9", you can buy a 40W laser on ebay for $365, free shipping.

    A 100W laser can't cut any more materials that a 40W laser can. To laser all the plywood for my R/C airplane only takes an hour. I imagine then that a 100W laser would do it in 24 minutes. I did have to align the mirrors etc, but everything went smoothly.

    I hope for whatever you're spending, that at least it doesn't have stepper motors.
    This is a commercial laser and 100 watts may not cut/etch any more materials than a 40watt but it gives enough power to easily and quickly cut through fairly thick wood and plastics. I have several hobbies besides robotics that I plan on using the laser for making parts along with a few other side ventures I am exploring. It is probably over kill for what I will be using it for and expensive for hobby use but since you can't take it with you, might as well have fun along the way! Besides, think of all the new design robot bodies that can come out of this!

    I have been able to spend several hours working on coding the hexapod. I decided to experiment with the PID loop for the coxa by re-writing a new PID routine and making it even simpler. I got rid of the existing speed control code and only used the inherent ramping on the PID. Actually its a PI loop right now, I haven't added a derivative process yet. The proportional code was easy to integrate as it is simply the constant (kp) times the error (kp*error). I determined the kp value by simply incrementing it by 1 and then commanding a move. The larger the value the faster the leg moved until I got a value where the leg had significant overshoot. Then I backed the value back down to the previous value.

    The Integral portion is trickier but it improves the response rate as its output is added to the output of the Proportional code. The Integral value is based on the change in error over time, since the code time interval was essentially constant, I did not use any code to determine time. This simplifies the code to accumulating the error (errorsum = error + error sum) and then multiplying it by a constant (ki) (ki * errorsum). This value can grow pretty quickly so I put limits on its growth to prevent it going too far. Initial tuning of the Integral portion didn't go well as I was using integers for ki which cause the motor speed to accelerate much too rapidly and caused wide overshoots. That's when I figured out the ki value should be less than 1. Since I can't use floating point math (no cogs available to run the code), I set ki = 5 and then divided the integral result by 10. This produced a controllable leg movement that did not have any overshoot.

    A byproduct of this testing was that the encoder value at the end of one motor command now closely matched the last encoder value for the previous command. As I stated a few days ago I ran into issues where although the stop motor command was sent at the right time, the motor inertia caused enough additional movement that the leg would end up further away from the desired position.

    At this time I don't think I need to add the derivative portion of the control, I still have more testing to perform and I want to try a variety of integral constants to see what value gives the best response.
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