Commonly asked questions. (New teams look here!)

Due to the well-founded frustration of people asking questions about constantly answered problems, I’d like to dedicate this thread for common answers to common questions. For example, I’d answer a common question like this:

Blah blah blah helpful guidance on DR4Bs blah blah blah…
post a couple of helpful links and whatnot

Make sure when you answer a question to put the related topic in bold, and keep the answer tidy.
Also, not chit chat or quoting people. This is meant to be a clean-looking thread where new teams could scroll down the page to find the answer that they need.

Some ones we need first:
Lift help (DR4Bs, Scissor lifts, and chain bars)
PID Loops
Basic code

Please keep this an active thread full of useful information!

There also exists a vex wiki:

its also mostly irrelevant nonsense

is there any way for a mod to “sticky” a thread like this to the top?

I think a stickied FAQ, or noob question thread could be potential very useful. Many new teams all ask the same questions.

@DRow can we do this?

Pid loops:
Pid loops are used whenever you need to accurately control the position of a mechanism. A PID controller continuously calculates an error value as the difference between a desired setpoint and a measured process variable and applies a correction based on proportional, integral, and derivative terms (denoted P, I, and D respectively) which give their name to the controller. Explaining every part of how a pid loop works is complex, but here’s a great resource that explains what they are, how to use them, and some code. PID Control (with code), Verification, and Scheduling - Robots For Roboticists

Double reverse four bars, while seeming to be a great option, have one significant downside, they are hard to build! Here’s some tips I have from building 5 of them now. Firstly, slop builds up quickly. Any place where slop can occur needs lots of attention to detail and careful constructing to minimise it. Second, don’t be afraid to rebuild. Must likely, after you build your first dr4b, you will encounter many problems. Find all the problems you have with your lift, and rebuild with those in mind. Third, use others experience. Watching videos from skyrise can show you how others have built dr4b in the past, and you can learn from how they constructed them.

1)You cannot score highest stack in skills
2) you can hold a mobile goal and cone at the same time
3) you can only hold 1 cone at a time, unless your stabilizing when you can hold any cone on a stack, and it wont count against your possession limit
4) check your brackets {} before asking on the forum for coding help
5) spread your motors across ports 2-9, there is a breaker on ports 1-5 and 6-10, if you use too much power you will pop your breaker and that 1/2 of your robot will be unsuable
6) dont worry about speakers and LCD’s, if you cant program them, you should probably work on your robot before you worry about thoes
7) the general ratios for a drive are (on 4") 1:1 4 motor High Speed, 6 motor 1:1 turbo

edit: please watch any youtube videos out there before you go asking about strategy, designs, or gear ratios

This post was ghost written by Anomaly.

Hi! I’m going to try to answer the most basic questions newer people may have with as much detail as possible to help reverse the trend that motivated me to leave the forums.

Question: Remote won’t connect.

Answer (borrowed from one of my posts):

  1. Make sure firmware is downloaded. I know you said the firmware is good, but make sure the joystick, cortex, and vexkeys have the latest firmware, and that the robotC version you are using is current. This is most likely the issue.
  2. Make sure the vex keys don’t have any obvious physical damage. If a vex key is cracked or coming apart, it can cause wireless failures.
  3. Make sure the orange download cable you’re using is functional. If the download cable is broken, pairing won’t work. To be 100% the download cable isn’t the point of failure, try using another download cable.
  4. Follow these steps in this order to pair the joystick and the cortex.
    a.) Power on the joystick and cortex. Make sure both have good batteries.
    b.) Link them together with the orange download cable until the lights on both are green.
    c.) Unplug the download cable and plug in the vex keys.
    d.) Only after the keys are in, power cycle both the remote and the cortex.
    If you are still experiencing issues, it is possible the joystick or cortex is broken.
    Question: What are pneumatics and do I need to use them?
    Answer: Pneumatics (not hydraulics!) are pressurized air tanks that operate pistons and are controlled with sensors called solenoids. You do not need pneumatics, in fact you can have 12 motors if you don’t use pneumatics and only 10 if you do, but pneumatics are beneficial for many subsystems. Pneumatics have a faster motion than motors and the pistons are lighter, but the tanks are heavier and you will run out of air with more than 40-50 actuations. To increase this upper bound, you can always use a pressure regulator to decrease the air flow into the piston and give you more actuations.

Question: How does skills work?

Skills is a 1 minute “match” where your robot is the only on the field. You can score for either alliance, and no highest stack bonus or autonomous bonus is given. Your driver skills score (1 minute of drive control) and programming skills score (1 minute of autonomous) are combined to make a “robot skills” score. By February, the teams with the top 50 scores in the world are automatically invited to worlds. Separately, if too few teams qualify for a state or regional championship, the teams with the highest regional skills scores will qualify.

Question: What kind of lift is best? How should I build it?

This depends on your goal. Scissor lifts and DR4Bs are the tallest linear lifts, 4 bars have the least friction, and 6 or 8 bars are the tall non linear lifts.

How to build a scissor lift (borrowed from one of my posts):

The biggest issue a scissor lift faces is instability caused because the 2 halves of the lift can move independently of one another. To solve this issue, cross bracing, a relatively narrow lift, and independent PID on each side of the lift can ensure the entire lift moves rather than just 1 side. A few other innovations are possible as well:

VEX 7682 Wingus & Dingus Innovate Award submission 2015

While there is no one most efficient way to construct a scissor lift, using screw joints, reducing friction in each joint, reducing the number of joints, and using rubber bands to cancel the weight of the lift all increase the efficiency of the lift. It’s also possible to increase the speed by adding motors or increasing the gear ratio.

How to build a DR4B lift:

The above video shows a well implemented DR4B from 134D in Starstruck. A double reverse four bar, or DR4B, is built by stacking 2 four bars on top of one another. The 4 bars should be the same or roughly the same length, and they should be mechanically linked. They are almost always geared together, but standoffs between the top and bottom stage or zip ties work as well when weight is a huge concern and strength is not. The distance between the top and bottom bars in each 4 bar will determine how much the DR4B can rack forwards and backwards. A distance of 5-10 holes is conventional, and I recommend 8 holes to prevent forwards-backwards racking. Many DR4Bs have the motors mounted on the lift towers, allowing the motors not to be lifted. However, because this uses extra space and requires extra gears at the bottom, plenty of teams including 134D, place their motors directly on the “middle section” of the DR4B. The main disadvantage of this is the motors must be lifted, increasing the inertia of the lift.
134D Robotics
Vex Starstruck Robot Reveal 134D

To rubber band the lift, most teams put bands on both the top and bottom stages. This requires the rubber bands be run at a diagonal across the lift bars, tensioning the lift. See the video of 134D for an example of good rubber banding.

The final point to make about DR4Bs is that they tend to be quite wobbly. To prevent this, it is strongly advised that you run an X made out of c channels or half c channels from the left to the right side of the lift on both the top and bottom of the c channels, and then standoff these Xs together. Here is an example of a DR4B that uses these techniques, built by team 62 in Skyrise.

Personally, I would very strongly advise against using a scissor lift OR DR4B as your first lift, but a DR4B is much easier to build correctly than a scissor lift because there are fewer racking issues. Teams as dominant as 400X have struggled in the past to successfully reinforce scissor lifts. Please take this opinion with a grain of salt because there are a large number of people that disagree with what I just said.
Cameron S
VEX Team 62 Reveal Worlds 2015

Question: What is a PID loop? How do I create one? (Answer partially borrowed from one of my posts.)

Answer: A PID loop is a complex feedback loop, and no PID loop for someone else’s robot will work on your robot. As such, you need to learn how to write a PID loop yourself. The basic idea of a PID loop is the robot figures out where it is, where it wants to be, and then assigns the subsystem a motor value based on the distance between the target and the actual.

P, I, and D stand for proportion, integral, and derivative. The proportion tells the motors to go at a speed based on the distance between the target and actual values, the integral sets the motors to a gradually increasing speed if it is very close to but not actually at the target, and the derivative slows down the subsystem when it gets very close to the target to prevent it from overshooting.
If you’ve never used PID before, this can be a bit overwhelming, so I’ll teach you here how to write a basic P loop. A P loop is not quite as precise as a PID loop, but the P is by far the most important part of the loop. Below is the code I use in my P loop.
task liftPID()
errorLift = targetLift - SensorValue(liftPot); //find error between target and actual
motor(leftLift) = motor(rightLift) = errorLift * kpLift; //and set motors accordingly

As you can tell, this code is extremely simple. To break down the logic, the robot knows know where the lift is thanks to the lift potentiometer, and it knows where the lift want to be because you tell the robot in the code. Thus, the robot needs to figure out the distance between the target and the actual, and set the motors to something accordingly. If the distance between the target and the actual is 1000 tics, the motors will probably need to be set to something very high to move quickly, but if the error is just 10 tics, they will be set to something very low. In the specific case of my code, I calculate the error by subtracting the target from the sensor value of the lift. So, if the target is 1000, and the sensor value is 500, the error becomes 500. Next, I set the motors to that error times the “kp,” or constant of proportionality. You will need to tune your own kp based on your specific lift. If the kp value is too low, the lift will move too slowly, and if the kp value is too high, the lift will oscillate or spiral out of control once it hits the target. Fortunately, tuning the kp only takes a few minutes. If the kp is 0.1, the motors will be set to 500 x 0.1 = speed 50. This will adjust depending on the error every 25 milliseconds.
If you want the lift to go to position 2500 on the potentiometer, you would simply say (in a different task) targetLift = 2500;
If you are interested in creating a PID loop with the I and D values as well, I would strongly recommend checking out MartinMaVexForever’s exceptional youtube series on this topic. I’ve linked the first video below, and this is a great tool for learning PID control.

Good luck with your PID control!

part 1/2

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part 2/2

Question: How do I copy team whoever’s robot?

Answer: You don’t. That team put their reveal on youtube to share their ideas, not to allow other people to copy them screw for screw. However, if you see a subsystem you would like to replicate, you can definitely draw inspiration from it. Just try to copy that subsystem at the beginning, and then make modifications to it based on problems that arise. (If the subsystem is out of the size limit, make it smaller, etc.)
Let’s Make a PID Loop Episode 1 – Basic Setup

Question: What are the standard gear ratios for usual subsystems?

Answer: This will largely depend on your number of motors, friction, and battery voltage. However, below is my estimate for the fastest reasonable amount you can gear up each standard subsytem for this year’s game.

2 Motors:
Drive: 1:1 torque with 4” wheels.
Lift: 1:5 speed (varies for elevator lifts and very small, light lifts)
Chain bar: 1:5 turbo
Claw: 1:1
Swing arm: 1:3 turbo
Side rollers: 1:1 turbo with 12 tooth sprocket

4 Motors:
Drive: 1:1 speed with 4” wheels or 1:1 turbo with 3.25” wheels
Lift: 1:5 turbo (varies for elevator lifts and very small, light lifts)
Chain bar: 1:3 turbo
Claw: you shouldn’t be putting more than 1 or 2 motors on your claw :slight_smile:
Swing arm: 1:1 torque
Side rollers: you should only use 2 motors on side rollers

6 Motors:
Drive: 1:1 Turbo with 4” wheels or 3:1 torque with 3.25” wheels

Question: Why are my motors jittering?
Answer: Your motors may be stalling or causing the cortex to stall. The motors have a heat sensor to prevent safety hazards, so any activity that causes the motors to noticeably heat up will cause them to stall. To prevent stalling, add motors, decrease the gear ratio, or decrease the friction. Similarly, the cortex has a sensor on ports 1-5 and another on ports 6-10 to prevent too much power from being pulled. If your 4 drive motors are coming from ports 1-4 and your drive is stalling, try spreading out the ports to 1-2 and 9-10 to prevent this type of stalling.
Question: What is a good parts list for a beginning team?
Answer: The following list is one I created to help out my school’s teams. This is NOT the only parts list or correct parts list, but I think it is a good one. It will give you enough parts to build a basic robot with a small cushion of extra parts in case you run out of something, and it costs around $2000.

VEX Robotics Parts Wish List for 2017-2018 Season for 1 New Team

Part Name Number Cost per Part

1 VEX Bumper Switch (2-pack) 276-2159 $12.99
1 VEX Potentiometer (2-pack) 276-2216 $12.99
1 VEX Yaw Rate Gyroscope Sensor 276-2333 $39.99
1 VEX LCD Display 276-2273 $49.99
3 VEX 3-Wire Extension Cable 6" (4-pack) 276-1427 $4.99
12 VEX 2-Wire Motor 393 with Motor Controller 29 276-1668 $24.98
3 VEX Shaft Collar (16-pack) 276-2010 $7.99
1 VEX High Strength 84-Tooth Gear (4-Pack) 276-3438 $12.99
2 VEX 4" Omni-Directional Wheel (2-pack) 276-2185 $24.99
1 VEX Star Drive Coupler 8-32 x 0.500" (25-pack) 276-4989 $1.99
1 VEX Aluminum C-Channel 1x3x1x35 (2-pack) 276-4359 $15.99
5 VEX Aluminum Bar 1x25 (16-pack) 276-2307 $29.99
1 VEX Aluminum C-Channel 1x5x1x25 (6-pack) 276-2290 $39.99
6 VEX Aluminum C-Channel 1x2x1x35 (6-pack) 276-2289 $34.99
1 VEX Hinge (2-pack) 275-1272 $9.99
6 VEX 7.2V Robot Battery NiMH 3000mAh 276-1491 $29.99
1 VEX Battery Clip (4-pack) 276-4042 $4.99
4 VEX Smart Charger v2 with Power Cord 276-2519 $18.98
1 VEXnet Backup Battery Holder 276-2243 $9.99
3 VEX High Strength Gear Kit 276-2250 $18.99
1 VEX Gear Kit 276-2169 $12.99
1 VEX Washer, Steel (200-pack) 275-1024 $4.95
2 VEX Washer, Teflon (25-pack) 275-1025 $4.95
2 VEX Drive Shaft Bar Lock (8-pack) 275-1065 $6.45
7 VEX Bearing Flat (10-pack) 276-1209 $4.99
3 VEX Additional High Strength Chain 276-2172 $24.99
1 Star Drive Screw 8-32 x 2.000" (50-pack) 276-5004 $4.99
1 Star Drive Screw 8-32 x 1.750" (50-pack) 276-4999 $5.99
1 Star Drive Screw 8-32 x 1.500" (50-pack) 276-4998 $4.99
1 Star Drive Screw 8-32 x 1.250" (50-pack) 276-4997 $4.99
1 Star Drive Screw 8-32 x 1.000" (100-pack) 276-4996 $6.99
1 Star Drive Screw 8-32 x 0.875" (100-pack) 276-4995 $6.99
1 Star Drive Screw 8-32 x 0.750" (100-pack) 276-4994 $5.99
1 Star Drive Screw 8-32 x 0.625" (100-pack) 276-4993 $5.99
1 Star Drive Screw 8-32 x 0.500" (100-pack) 276-4992 $4.99
1 Star Drive Screw 8-32 x 0.375" (100-pack) 276-4991 $4.99
2 Star Drive Screw 8-32 x 0.2
2 VEX High Strength Sprocket 18 Tooth (4-Pack) 276-3878 $12.99
2 VEX High Strength Sprocket 24 Tooth (4-Pack) 276-3879 $12.99
5 VEX Pillow Block Bearing & Lock Bar Pack 276-2016 $7.99
1 VEX Battery Extension Cable 276-3442 $4.99
15 VEX Nylon Spacer Variety Pack 275-1066 $4.95
**Note: each price is the cost of 1 set. I VEX Nylon Spacer Variety Pack, not 15, costs $4.95.
Question: I need design ideas.
Answer: Youtube has a huge array of matches and reveals posted. The best match I personally have seen (on youtube or in person) I’ve linked below, but better matches will occur as the season goes on, and it’s only a matter of time before these designs are obsolete.

Look for good concepts, clever ideas, and good strategies. If you can combine them successfully, you will have a very strong robot.

I hope this post will help to answer some of the basic questions newer members have. @Mira, if you could submit this as the answer to the original question, it would make this information more accessible and hopefully help some of you newer builders and programmers get started. Best of luck to everyone this season, and thank you for reading!
lol ok thats it sorry its so crazy long but pls post it i appreciate it

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Question: What type of drivetrain should I use?

Answer: This is totally up to you, but I will overview some of the common types of drivetrains that are used.

-Tank drive: One of the most common types of drivetrain, this drivetrain usually has 4-8 wheels, with half of them on each side, all facing forward. This drive does not usually mean using actual tank treads. Steering is done by moving one side of the drive faster than the other side. The advantage to this drive is it has lots of pushing strength, and it’s simple. Here is an example of a basic tank drive:

-Slide drive: Another drive type that is good for beginners. Slide drives are similar to tank drives, but all of the tank wheels must be omni wheels. In addition, it has one or more omni wheel mounted perpendicular to the other wheels. This allows it to strafe. Here is an example of what one could look like:

-Swerve drive: This drive system is more complicated and doesn’t usually have as much pushing force. However, it is very good at maneuvering, since the front wheels can turn to any direction. These are often complicated to program. Here is an example of what one could look like:

-Holonomic drive: This is one of the agilest drive systems. It requires 4 omni-wheels. Programming is often very complicated. It can move in any direction at any time and is about 1.4X faster than a tank drive. Here is what the most common type looks like (there are several variations):
-Mecanum drive: This is another type of holonomic drive, but the way it works is very different from the other types. Mecanum wheels are expensive and have lots of friction. However, they look pretty cool. Mecanum drives can also go in any direction and are pretty agile. Here is an example of one:

I realize these are not all the types of drivetrains, but this was just an overview of the more common ones. You can also look up any of these drive systems on YouTube to see how they work and get tips on programming them. Good Luck!

QUESTION: “My Pneumatic Cylinder moves slowly in one direction”

ANSWER: you are using an exhaust flow control valve on one end of the pneumatic cylinder. The flow control valve is the 90° fitting supplied in the pneumatics kits. There is an adjustment screw in the valve to adjust from fully stopped to fully open. Unless you actually have a need for speed control (rarely), buy additional straight cylinder fitting here: or here:

The attached document provides more information about pneumatics to supplement the stuff from VEX:
pneumatics information.pdf (313 KB)

how do you make a cone chain bar?

There is a helpful thread on Chain Bar Lifts
Chain Lift Tutorial

My computer sucks! How do I do cad?
It’s very simple, the version of the cad program you download corresponds with how much processing power it will take. If you can’t run Autodesk inventor 2017, try 2015 or 2012, it WILL run better

Can someone send me the invite link for the discord?

There’s a discord? Please send me the link as well thank you.

Hey I was wondering if anyone knows if we can use the 12 cone preloads during autonomous mode?

Yes you can. Reason being that they are said to be usable during a match. Then if you look at the definition of a match it is defined as including both the driver and autonomous periods :slight_smile:

This whole thread hasn’t really been turning out as planned…