[FTC]: Common failures with TETRIX

Since, this is the first time the NXT/TETRIX kit has been used, I think it shows great potential, but it’s still got a long way to go before I’d call it a great system.

The only way to improve, is to identify and correct problems, so I thought I’d compile a list of common problems we had/saw during this FTC season. I will pass these on to NI/LEGO Education/PITSCO/Hitechnic to see if improvements can be made in future designs.

See if you agree with these, or have any additional suggestions.

  1. Wobly Brass Bushings.
    These seem reasonably snug when first installed, but after several hours of driving, most of our gears and wheels seem to wobble quite a bit. We used the propper wheel mounts, and two points of support, but they are still getting looser. Not sure that they are reusable for another season.

  2. Bending Axles.
    To aggravate the problem above, any time an axle has any length that’s unsupported, they seem to bend when they take a hit. Perhaps they could be made with a better grade of Aluminum.

  3. Encoder cables.
    The thin wire used to connect the encoders to the Motor controller are incredibly brittle, especially where they are crimped into the encoder connector. These need a higher strand count, and a different grade of plastic to be able to withstand even a single season.

  4. Motor burn out.
    With the problems associated with glitches in the Motor controllers, it’s common for a motor to drive for several seconds when it’s not meant to. If the motor cannot rotate, this often leads to burned up motors. There is meant to be some overload protection, but it’s clearly not enough to prevent letting the magic smoke out. These motors are TOO expensive to let a software glitch destroy them.

  5. Motor drive shaft bearings coming loose or free.
    All of my encoder housings now spin freely on the motor shaft, despite the set screw being fully tighened. It appears that the brass bushing that they are attached to eventually comes loose. In one instance, the bushing has actually broken free of the gearbox. This motor was driving a 40 tooth gear which was driving an 80 tooth gear. The setup had no latteral load (except due to normal gear-gear interaction). This sort of catastrophic failure needs to be prevented.

  6. The comunications between the NXT and Motor Controller needs to be made MUCH more robust. Many of the supposed field/bluetooth issues are really related to this interface, since they can be “corrected” simply by cycling the 12V motor power. I will tollerate many things in a robot, but I won’t be at the mercy of random software glitches (and neither will my S/W team). As far as I’m concerned, getting a good Auto mode is the real goal for any of these games, so the closed loop control needs to be bullet-proof. This interface glitch MUST be fixed WELL before the next season.


We experienced all of these things (anyone want to buy a bag of “slightly used” 12v motors?).

There is another thread we need to start on, “what would you add to the kit next year?”

Oddly enough, we never ran into any of these problems at all, despite really beating on our robot sometimes. (IE falling off the ramp, high speed collisions, motors getting stuck and still trying to move.)

So either we got really lucky, or you got really unlucky. I have heard lots of stories about damaged parts, but that could be a case of a vocal minority.

  1. I would like to be able to choose whether the motor controller goes in the sensor port or the motor port, we wanted to use 4 sensors, but we only used 2 NXT motors. If thats not possible with the hardware, OK, there is nothing that can be done.

  2. A part that is like the flat bracket but shorter would be very useful.

  3. I think new kits should include 2 motor controllers by default. Having only 1 controller severly limits your options.

  4. Cheapers gear. Even if they have to be plastic.

Would it be so much to ask for a continuous rotation servo or two :wink:

  1. Wobbly Brass Bushings. Our team is seeing this.

  2. Bending Axles. We haven’t notice this. I believe the axles are steel though.

  3. Encoder cables. We’ve definitely experienced this.

  4. Motor burn out. We’ve been fortunate enough to not experience this. Probably because the clamps always give out first.

  5. Motor drive shaft bearings coming loose or free. As far as I know we haven’t experienced this. We will have to check the robots over to make sure.

  6. The comunications between the NXT and Motor Controller needs to be made MUCH more robust. This is one of our biggest issues. Our Autonomous programs are only 80% at best because of this. I hope this gets fixed.

  7. The split clamps will not take anywhere near the torque needed. We have these monster motors but transfering the torque takes modifications to the clamps.

  8. The cost of the gears and the poor attachment method.

  9. Lack of a longer axle.

  10. Inability to use more than 3 sensors.

  11. Lack of a decent way to mount the Lego motors.

Enough for now.

I forgot about the servo motors. We went thru them like butter at the beginning of the season.

Well , they aren’t magnetic, and they cut really easilly.

Just seem too soft for steel.

We’ve had issues with the wobbly bushings, encoder cables, and the communication between the NXT and the motor controllers (the last of which lost us the final round at Los Angeles, argh). There’s also been problems with the wires coming loose; crimping them onto the motors helped but there’s not much we can do about the ones that fall out of the motor controllers even when fully tightened.

We’ve also had issues with the power switch, which is fine in most respects but needs to be far stiffer, as it can be turned off by a well-placed puck, not to mention another robot. (We thought we had ours in a spot where nobody could get at it. We were wrong.) A couple teams lost matches at Los Angeles last weekend due to robots accidentally getting turned off.

On the flip side, we now have several funny stories to tell regarding interesting failures…

I actually just took apart a gearbox on the DC Gearmotors (it was stripped, or acted stripped, so why not) and found one of the pinion gears shattered. It is obviously a pressed powder/sinter type material. Anyone else experience this? Anyone know if there is a place to buy the gearboxes or gears for the gearboxes. It’s hard to know the final dimensions of this gear because it is in about 6-7 pieces. It appears to be a 9-11 tooth gear.

We’ve had a couple of gearheads fail, but haven’t taken them apart. The bushing snapping off the end is more common.

The gearbox broke once for us too…

after a younger member dropped it of the table, and it landed on the shaft. :open_mouth: :stuck_out_tongue:

All four of my motors burned out, this really irritated me and I’m making sure this never happens again. I recommend having touch sensor bumpers around your robot to make sure your motors aren’t stalling. I’d recommend not going past 60-80% motor power for a a few seconds.

Our axle heated up on the motor and is now force fitted onto my encoder and hub. Waste of a motor…

For a $25 dollar motor, and in a rough competition the motor tabs you hook up to, are way too thin and feeble. I lose a motor because of a lost tab. I am however happy.

I recommend checking over your motor and servo controller, the signals being sent to it, with a logic analyzer (I use the Saleae one, very good and affordable, http://www.saleae.com/logic/).

Queuing your controller takes way too long, I don’t think a regional has gone by without FMS issues, they really need to improve this. Teams need to be enforced about the bluetooth rules and how your NXT should be prepped.

The lack of sensor ports is extremely limiting. LEGO Motors can be difficult to attach, I usually have to do it by a zip tie and a flimsy beam. Although proved that with regular NXT models the size and form of the NXT motors can be taken advantage of, it’s just not right for Tetrix.

All of the wires should definitely be of a lower gauge. That being said some of them are of a low gauge and end up not fitting in the motor controller’s terminals without crimping or trimming.

My complaints to HiTechnic on the motor controllers, although a great thing to have for NXT fans, these are poorly made and priced. It is not a professional motor controller, and is definitely not as industry standardish as the rest of the kit. It’s a simple H-Bridge circuit with some power transistors! Maybe $10 dollars worth of parts, not $64 dollars!! And the lack of indicator LED’s is frustrating.

As for the Tetrix metal, although I love it’s amazing strength and extremely light weight, it’s overkill for FTC. The hole pattern and sizes of tubes and channels are suiting, I was satisfied with those aspects. The amount of metal that came with the kit is extremely pleasing, I can build an entire competition robot with it.

I don’t like the size of standoffs. 1" and 2" doesn’t exactly fit the hole patterns and widths of channels. They should be 1.25 and 2.25".

Having to buy multi nut pliers separately as some form of wrench that the original kit lacked was a disappointment. As with Vex, a kit that’s meant for a team of 3-10 only came with 1 set of hex wrenches. Luckily they were somewhat commonly available sizes.

We weren’t told on what setting to charge the battery at. For many teams there was a punishment to FIRST’s lack of documentation.

Although high resolution and lovely to program, the encoders are way overpriced.

Anybody else had their servo bushings (the axle thingy that attaches to the horn) ware out beyond repair? Although somewhat my fault (:yikes: ) this was a huge annoyance and the servo’s were surprisingly weak (doesn’t really match the whole “real world application robotics” theme).

There wasn’t really a reliable way to attach the battery or the on/off switch. Zip ties worked but not really to satisfaction. I believe the kit came with one velcro strap to hold the battery in place. One doesn’t exactly do the trick for the weight and size of the battery.

How often have you got a hub stuck on an axle and you have to yank it out with two heavy duty pliers? For strong metal gears the teeth sure ware out, luckily still working and pretty much no skipping yet I’m not sure how many months of competition they would last.

I think everybody has tried out the pivot bearings only to find out the screw is too small to fit through the required amount of metal!

Back to lego to metal connections. I hated how the hard point connectors was only wide enough to fit a lego beam in one orientation. They could’ve also had a different form of hard point connector where it fits between two beams (i.e. the bottom of a lego motor).

Final complaint, how many people got their design screwed over by the backorder of motor contollers???

Sorry for the overly long post. Enjoy?

We used a dual servo mount for our battery holder, plus 2 zip ties and it was very sturdy.

That reminds me of another issue: We originally had our battery resting on two bolts. Probably a bad idea, because it shorted and caused the battery to heat up melt the blue plastic insulation.

We used the pivot bearings in our robot at one time, are you using them as suggested in the instruction manual?

First off, great idea! (battery holder) and wise warning

Yes I’m using the pivot bearings as instructed, maybe an isolated incident?

Perhaps. Do you mean the screw went all the way through the center of the bearing? Or through the small holes on the servo mount? None of our bearings had that problem, and if all of yours did that would be a huge coincidence.

I actually have a few unopened packages of bearings i haven’t tried yet. So it could really well be an isolated incident. For clarification, the screw went through the bearing and metal but wasn’t able to make it through the servo bracket. I’ll post a picture perhaps (lol if I kept the “mismanufactured” bearing set)

The pivot bearings are useless for everything EXCEPT what they’re used for in the manual. Most metal is just way too thick for the length of the screw; it only really works with the really thin stuff. It’s sad, because there’s so much we could have done with it…

add these two to the list:

  1. motors (with encoders and PID turned on) don’t brake consistently (if at all). If a motor has pressure against it and you set the speed to zero it will brake and actually stop (depending on the settings in place at the time) (this is for raising an arm for instance). If, however, the motor has been acting to slow the item down (think of lowering an arm under control) then when you set the speed to zero, you just remove power - you don’t stop it (most of the time). I haven’t looked into it to see if this is a symptom of the connection to the NXT, but I don’t believe that it is.

  2. more servo options, or some way to not reset the encoder position in both lego and drive motors when the program starts running.

Yes, Both the Lego and DC motors use pulse counting encoders, so they don’t have any absolute way of detecting their starting position.

You either need to have some way to “home” them, or some external absolute sensing device.

One idea to Home a motor is to add a touch sensor, and run the motor at low power until it hits the sensor (or some other hard stop). The you can reset the encoder. Remeber… use low power !!! Or, you can reset the encoder every time the arm moves past a light senor.

The other thing to consider is that it IS legal to add the Hitechnic soldered prototyping board, with external sensors … like a potentiometer. In theory you could add a feedback pot to an arm to sense the actual position, and just use the encoders for fine speed control.

Servos are great, but unfortunately you can’t read their starting position, so your first action has to be to move them to some safe position. If you can always physically place your servo arm in a known position (eg: all the way up or down), then if you can figure out the servo values that correspond to this starting position, that can be your first servo command… eg: Go to where I know you should be starting.

I do like that you can send the servo to position “0” which means “stay where I last sent you”. why? If you don’t continually send servo commands, they will droop down after about 2 seconds… so, to prevent this, you can have a loop that runs in the background sending “0” comands, which will keep them wherever they were last commanded. Unfortunately you can’t start out by sending “0” commands because there is no “history” and the servo will go to the center.

For us, it was that we had 2 of the C-clamps actually crack on us.