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Vorpal The Hexapod Troubleshooting Guide

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Revision as of 21:44, 7 January 2018 by Vorpalwiki (talk | contribs) (No lights at all)

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Hexapod works at first, but loses height while walking, and/or hip servos lose home position

This problem is called the "drifting servo" problem and it has recently been identified as a type of defective servo that our prior testing procedures did not detect. We recently received a batch of servos where a percentage of them have this problem. If you have at least two of these bad servos you will have this issue.

The symptoms are:

  • In either demo mode or RC mode, the robot starts up and seems to be working at first.
  • Within 30 seconds to 1 minute, the robot starts "losing height". The knees drift so it gets lower and lower until it is "crawling" on the ground or can't move at all.
  • Another related issue is that hip servos may lose home position such that they do not return to neutral, and this gets worse and worse until legs start crashing into each other.

The only fix is to replace the bad servos. You will need to determine how many servos are bad, then (if you bought the kit or servos from Vorpal Robotics) send email to support@vorpalrobotics.com and report how many replacement servos you need. Also give us enough information to find your account on Kickstarter, Indiegogo, or our Vorpal Robotics Store depending on where you purchased the kit. (If you did not purchase the servos or kit from us, you need to get new servos from whatever vendor sold them to you.)

Here is a procedure for determining how many servos have this issue:

  • Leave the hexapod off for at least 10 minutes. The drifting servo problem occurs as the servos heat up during use so we want them to be room temperature to start.
  • Turn the dial to STOP, then turn the robot on. The legs should now go to the normal standing position. Very slowly turn the dial clockwise until the legs go into ADJUST mode, where the knees come out straight from the robot body.
  • Observe the position of the knee servos. Confirm that they're all coming straight out from the robot body. Make a note of any that are slightly high or slightly low.
  • While still holding the robot in the air, turn the dial to DEMO mode. Let the robot run through two or three demo cycles (a cycle ends with a pause of several seconds).
  • Turn the dial all the way counterclockwise again to STOP mode, then slowly turn clockwise into ADJUST mode again.
  • Write down on a piece of paper any servos that are now obviously too high. For knee servos, make sure you write down the LARGER of the two numbers on the leg.
  • Keep observing the robot for a minute or so. You should see the ones that are too high slowly move back down into neutral position as they cool. This would confirm that those servos do have this issue.

Repeat the above procedure to find any hip servos (the ones attached to the main body) that drift. In the case of hip servos, you do not need to use ADJUST mode since they are always at 90 degrees in STAND mode. Observe how many of them do not return to 90 degrees after running DEMO mode. Again, remember you need to start from a cool robot so turn it off for a few minutes before testing.

Count up how many servos are bad and send that number to support@vorpalrobotics.com (assuming you bought them from us). Make the subject SERVO DRIFT and we will send the new servos out right away.

Servos Can't Lift Hexapod or are Sluggish


  • Vorpal can't stand up at all, servos seem to struggle.

Possible causes include:

  • Batteries are low or dead. Recharge the batteries. It is also possible that you have batteries that are just no good. Cheap no-name brand batteries often vastly overstate their capabilities.
  • Improper surface: Vorpal is designed to work on reasonably smooth surfaces such as tile floor, wood floor, smooth table top, desk top, etc. It will not work on most carpets. There are "carpet feet" being developed by some Makers which may solve this in the future.
  • Too much friction on servo bearing. Make sure your 3D printed models are cleaned up, especially the little hemisphere on each servo mount on the chassis, and also the little hemispherical hole in the leg hinges. This hemisphere is the bearing opposite the servo shaft, and if it has drips or other defects there may be too much friction for the servos to move smoothly. Sandpaper any areas that look misshapen or rough, or put a drop of silicone grease on the hemisphere. (Other types of oil will also work, or even petroleum jelly, but be careful and apply with a Q-tip or paper towel very carefully, because some kinds of oil may eat through electronics).
  • Legs out of adjustment. If the legs are adjusted such that Vorpal "rides too low" then the amount of torque required to walk increases and the legs may not be able to keep up with the requirements of the walking gait selected. Put the robot in ADJ mode (turn the dial fully counterclockwise to STOP, then slightly clockwise until the knees go out straight). Now, it's not always possible to make the servo horns come out totally straight from the body due to the way servo horns are manufactured. But, if you can't make it come out 90 degrees straight, it's better for the horn to be a little too clockwise than to be a little too counterclockwise. Too clockwise will make the leg lift the robot body a tiny bit higher, which is fine, but too much counterclockwise will make the robot ride "low" and stress the servos more.
  • Bad servo. If one or more servos are bad, the other servos may struggle to lift the robot's weight fast enough to keep up while walking. If a servo is obviously bad (chattering excessively, twitching, not moving at all, or clearly not pulling its fair share of the load) then that's the probable culprit.
  • Bad BEC. It is possible for the BEC (which is on the battery/switch assembly and provides 5V regulated power to the robot) to go bad if it gets overheated too many times, and in this case it may not be able to output enough amps to keep the legs moving properly. This is the least likely cause, please eliminate other causes before requesting new hardware.

One Servo Doesn't Work

Most likely causes include:

  • The servo has gotten unplugged from the servo controller. Open up the hexapod by removing the screws holding down the electronics caddy and check the wiring.
  • The servo has a frozen gear. Occasionally when a servo goes to its maximum position it may get stuck there. You will know this is true if the servo won't move easily even with power off. You can usually fix this by turning power OFF, then gently applying pressure to the servo in the direction you would expect it to move until it "unsticks". If the same servo keeps getting stuck, you should replace it.
  • The servo has gone bad (either gears are broken or wiring is damaged). There are several possible causes for this, but in any case the only fix is to replace the servo.

One Servo Jitters Constantly

It is normal for there to be some humming sounds while the robot stands as the servos activate and deactivate to maintain the standing posture. But very loud chattering or large twitching motions are not normal.

  • This typically means the servo has gone bad and its electronics are not responding properly. First check to make sure the servo's wire is completely inserted in the servo controller, but if that looks ok the only fix is to swap the servo.

Gamepad Won't Control Hexapod

  • Make sure the battery is charged in the gamepad and in the robot.
  • Make sure the hexapod dial has been turned to RC (remote control mode).
  • Make sure the gamepad is turned on and you see lights through the cover.

If you see lights through the holes in the center of the cover, observe the blinking pattern. Here's what the blinks mean:

  • NOT CONNECTED: When the Bluetooth modules are not connected, there will be a fast, steady blink.
  • IN THE PROCESS OF CONNECTING: When they first start to connect, they will blink slowly at first, then will go dark for a couple of seconds. This process will only take about 5 seconds.
  • CONNECTED: When they are properly connected, then there will be a 2 second pause followed by two quick blinks, and this will repeat over and over as long as there is a connection.

The next few sections explain what to do in each of these situations.

Lights indicate connection is good, but robot not responding

This could be because you've not hooked up the wires properly on the Bluetooth module, either the one on the robot or the one on the gamepad. Carefully check connections.

There could be a general issue with the robot. See if it works in DEMO mode. If DEMO mode doesn't work, you probably have an electrical issue and need to check all connections. It is possible you have a bad component (Arduino or Servo controller) and need to contact support@vorpalrobotics.com.

Lights never show a connection

If the Bluetooth modules are not paired after about 10 seconds (two quick blinks followed by 2 seconds dark means they're connected properly), then the possible issues are:

  • Make sure both the robot and the gamepad bluetooth modules have lights.
  • Make sure the GND wire on the nano in the gamepad is properly connected. For example, if you accidentally connected it to something like RST then the nano will have lights and appear to be working, and the Bluetooth modules can even connect to each other, but in reality the Nano is constantly resetting itself and never running the program.
  • If you have several hexapods, make sure you're using the correct gamepad that's matched with your hexapod. If need be, open up the gamepad and the hexapod and check the labels on the Bluetooth modules, make sure the labels match. If the labels are different, the two modules won't auto-pair and connect. You may have mixed up modules when building several robots.
  • To get further information, flash the gamepad with the DEBUG version of the gamepad firmware and see if the messages are correct. See the article Debugging Gamepad Issues for more information.

No lights at all

If you do not see any lights at all in the center of the gamepad, then:

  • The gamepad battery may be low or dead.
  • Your Bluetooth module may not be getting power, check wiring, especially the VCC and GND connections. Make sure jumper wires do not fall loosely off the pins, if they do you should swap to a new jumper wire or press on the little plastic tab on the connector with the hex key to tighten up the wire.

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