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Advanced Programming in the VEX Environment. Peter Johnson Northrop Grumman Space Technology Programming Mentor, Beach Cities Robotics (FRC/FTC/VRC Team 294) 1 Mar 2008. Agenda. Sensors Recap Advanced Operator Control Advanced Autonomous Parting Thoughts. Sensors Recap.

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Advanced Programming in the VEX Environment


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    1. Advanced Programming in the VEX Environment Peter Johnson Northrop Grumman Space Technology Programming Mentor, Beach Cities Robotics (FRC/FTC/VRC Team 294) 1 Mar 2008

    2. Agenda • Sensors Recap • Advanced Operator Control • Advanced Autonomous • Parting Thoughts

    3. Sensors Recap

    4. Sensors – A Programmer’s Best Friend • Limit Switch • Connects to 1 digital input • 0 when closed, 1 when open • Use to limit range of mechanical motion in both autonomous and operator control modes • Fragile - always have a mechanical hard stop too! • Bumper Switch • More robust than limit switch, but otherwise operates identically • Can itself act as a mechanical hard stop

    5. Sensors – A Programmer’s Best Friend • Optical Shaft Encoder • Connects to 1 or 2 interrupt ports • Interrupt count (90 ticks/revolution) • With 2 interrupt ports, can also tell direction • Most useful on drivetrain or anything that rotates (like a lifting arm) • Useful for distance, rotation, and driving straight in autonomous • Ultrasonic Range Finder • Connects to 1 interrupt port and 1 digital port • Senses distance to a object in inches (2 to 100) • Useful for determining distance in a particular direction to walls, robots, or other objects

    6. Programming Sensors • Limit Switches and Bumpers • input = GetDigitalInput(X) • Optical Encoder • StartEncoder(X) • PresetEncoder(X, 0) • ticks = GetEncoder(X) • Optical Quadrature Encoder • Same as encoder, except two inputs and functions named with “Quad” • Ultrasonic Sensor • StartUltrasonic(interrupt, output) • distance = GetUltrasonic(interrupt, output)

    7. Advanced Operator Control

    8. Advanced Operator Control • Limiting range of motion with sensors • Toggle buttons • Push once to open, push again to close • Sequenced commands

    9. Advanced Operator Control Limiting Range of Motion with Sensors

    10. Limiting Range of Motion With Sensors • Hard stops prevent physical motion past endpoint • But motors will keep trying to drive, unless you program them not to! • Your driver will keep that button pushed • Why is this bad? • Possible physical damage: • Burn out motors • Strip gears / clutches • Drain battery (motors can use 1 A current at stall) • Don’t know when to stop driving motors in autonomous • The solution: Limit Switches and/or Bumpers! • Locate such that switch is activated when physical motion reaches endpoint • Override motor control when switch active

    11. Limiting Range of Motion – Take 1 while(1==1) { OIToPWM(1,1,1,0); // drive Motor 1 from Rx 1 Chan 1 min_limit = GetDigitalInput(1); max_limit = GetDigitalInput(2); if (min_limit == 0 || max_limit == 0) { SetPWM(1, 127); } } • What are the problems with this approach? • How would you fix these problems?

    12. Limiting Range of Motion – Take 2 while(1==1) { rc_input = GetRxInput(1, 1); min_limit = GetDigitalInput(1); max_limit = GetDigitalInput(2); if (min_limit == 0 && rc_input < 127) { SetPWM(1, 127); } else if (max_limit == 0 && rc_input > 127) { SetPWM(1, 127); } else { SetPWM(1, rc_input); } }

    13. Limiting Range of Motion – Final Version while(1==1) { rc_input = GetRxInput(1, 1); min_limit = GetDigitalInput(1); max_limit = GetDigitalInput(2); if ((min_limit == 0 && rc_input < 64) || (max_limit == 0 && rc_input > 196)) { SetPWM(1, 127); } else { SetPWM(1, rc_input); } }

    14. Limiting Range of Motion – Looking Ahead • What would you need to change if the limits are backwards? (you did test it, right?) • What would you need to change if the driver wants the motor direction (controls) reversed? • How would you handle multiple limits? Example: a grabber that had three limit switches: • Fully closed • Fully open • Object grabbed • How would you handle multiple motors? Example: two motors driving an arm, mounted on opposite sides • All of these scenarios can and will come up during robot design and test!

    15. Advanced Operator Control “Toggle Button” Operation

    16. “Toggle Button” Operation • Your driver comes to you and says: • “It’d be really nice if you could make this button work like a toggle: I press the button once to open the grabber, and again to close it. I don’t like having to remember to press the top button to close and the bottom button to open (or was that the other way around?). Oh, and the next match is in 15 minutes.” • Your response… “No, I don’t have time to test it!” • But the lunch break follows the next match, so you forgo food and start working… • Fortunately the grabber is driven by a servo, so this shouldn’t be too hard, right?

    17. Toggle Button Operation – Take 1 int grabber_closed = 1; while(1==1) { rc_input = GetRxInput(1, 1); if (rc_input < 64 || rc_input > 196) // either button { if (grabber_closed) { SetPWM(1, 255); // open it } else { SetPWM(1, 50); // close it } grabber_closed = !grabber_closed; // update state } } • Great start.. but then it gets tested…

    18. Toggle Button Operation – Uh oh • The grabber “stutters” on button presses… it will partly open and then close again, or vice-versa • It seems to be random whether it ends up open or closed • What’s the problem? • The code is in a forever loop, and as long as the button is held down, the first if condition is true • The code is oscillating between open and closed states • How to fix it? • Add a change in progress variable

    19. Toggle Button Operation – Take 1 int grabber_closed = 1; int grabber_changing = 0; while(1==1) { rc_input = GetRxInput(1, 1); if (!grabber_changing && (rc_input < 64 || rc_input > 196)) // either button { if (grabber_closed) { SetPWM(1, 255); // open it } else { SetPWM(1, 50); // close it } grabber_closed = !grabber_closed; // update state grabber_changing = 1; } else if (rc_input > 64 && rc_input < 196) // neither button down { grabber_changing = 0; } } • Back to the testing floor…

    20. Toggle Button Operation – Uh oh #2 • It’s better, but… it still occasionally “stutters” on button presses! • What’s going on? Let’s take a closer look at what happens mechanically when a button on the controller is pressed • This is called contact bounce • What does the button value look like when you rapidly read it via GetRxInput()?

    21. How to fix it? • Most reliable way is a timeout • Add a “changing” variable • Conditionalize the action on that variable • When a change occurs, start a timer • Don’t accept another input change for X ms • You can make it even more robust by resetting the timer on each pulse (so it’s X ms from the last transition)

    22. Example Debouncer Code rc_input = GetRxInput(1, 1); if (ring_grabber_changing == 0 && (rc_input < 64 || rc_input > 196)) { if (grabber_closed) { SetPWM(1, 255); // open it } else { SetPWM(1, 50); // close it } grabber_closed = !grabber_closed; // update state ring_grabber_changing = 1; } else if (ring_grabber_changing == 1) { if (!(rc_input < 64 || rc_input > 196)) { StartTimer(1); timer1 = GetTimer(1); timer2 = timer1 + 100; ring_grabber_changing = 2; } }

    23. Example Debouncer Code (cont’d) else if (ring_grabber_changing == 2) { if (rc_input < 64 || rc_input > 196) { // oops, got another press within the timeout period ring_grabber_changing = 1; } else { timer1 = GetTimer(1); if (timer1 > timer2) { ring_grabber_changing = 0; StopTimer(1); } } }

    24. Advanced Operator Control Sequenced Commands

    25. Sequenced Commands • Essentially a combination of operator control and a touch of autonomous • Instead of doing one action when the driver presses a button, do several • Example: • Raise arm • Open grabber • Lower arm • Simpler example: • Drive motor until it hits limit (without needing to hold button down) • Be careful • Make sure the driver really always wants the whole sequence to happen • Make sure your limit switches are correct and robust! • How to code it? • Combine push-button operation code (previous slides) with autonomous sequencing (coming up)

    26. Advanced Autonomous

    27. Advanced Autonomous • Multiple Autonomous Modes • Driving X Inches • Driving Straight • Making Turns (Turning Y degrees) • Sequencing Commands (Basic) • Sequencing Command (Advanced)

    28. Advanced Autonomous Multiple Autonomous Modes

    29. Multiple Autonomous Modes • Why is it desirable to have multiple autonomous modes? • Adjust strategy based on opponent’s strategy • Turn direction depends on side of field • Testing and calibration (more on these later) • Drive 10 feet • Rotate 360 degrees

    30. Mode Selection • So this sounds like a pretty good idea. • Only… how do we do the selection? • Easy: use VEX jumpers in digital input ports! • Jumper inserted: input=0 • Jumper not inserted: input=1

    31. Binary Encoding • Ideally we would like to use as few inputs as possible • One jumper per mode sounds straightforward, until you want 8 autonomous modes! • The solution: binary encoding of jumpers • Hint: Put a table like this one on your robot!

    32. Code Structure • Read the jumper inputs in Autonomous() • Create functions auto1(), auto2(), etc. and call them based on the jumper inputs • This helps better organize your code void Autonomous(unsigned long ulTime) { jumper0 = GetDigitalInput(1); jumper1 = GetDigitalInput(2); jumper2 = GetDigitalInput(3); auto_config = 1+(jumper0==0)+2*(jumper1==0)+4*(jumper2==0); if (auto_config == 1) { auto1(); } else if (auto_config == 2) { auto2(); } … }

    33. Advanced Autonomous Driving X Inches

    34. Encoders – Theory of Operation • Optical Encoders count the number of rotations of a shaft • They do this by shining a light through a slotted wheel onto a light sensor • This results in a pulse train • By hooking up the encoder output to an interrupt port and counting the number of interrupts, we know how many slots have passed by the sensor • Since the slots are equally spaced, we know the angle the wheel (and thus the shaft) has traveled • Quadrature encoders are a bit more clever and can determine the direction of rotation • I won’t cover quadrature encoders here

    35. Going The Distance • Encoders count 90 ticks per revolution • Knowing the gear ratio between the encoder shaft and the wheels tells you how many times the wheels rotate for each rotation of the encoder shaft • Knowing the diameter of the wheel tells you how far the wheel has traveled (its circumference) for each rotation of the wheel shaft

    36. Solving for Ticks • We want to find the number of ticks needed to go a certain distance, so let’s solve for ticks: • Now we just need two constants (wheel diameter and gear ratio), and we’ll be able to determine how many ticks it takes to go a certain distance! • Note the units of wheel diameter is the same as the units of distance (gear ratio is dimensionless) • Let’s write a function to do this calculation…

    37. Calculate Required Ticks int calc_required_ticks(float dist) // dist is in inches { float wheel_diameter = 2.75; // wheel diameter in inches float gear_ratio = 1; // assume 1:1 gear ratio return (90*dist) / (wheel_diameter*3.14*gear_ratio); } Or possibly better, since floating point math is SLOW: int calc_required_ticks(int dist) // dist is in inches { int wheel_diameter = 275; // wheel diameter in hundreths of an inch int gear_ratio = 1; // assume 1:1 gear ratio // scale dist to hundreths of an inch and do integer math return (90*dist*100) / (wheel_diameter*3*gear_ratio); }

    38. Finishing Up… • Now we just need a loop that drives until we hit the number of ticks we calculated: • Test drive 10 feet required_ticks = calc_required_ticks(10*12); ticks = 0; // reset encoders // start PWMs and encoders while (ticks < required_ticks) { ticks = GetEncoder(1); } // stop PWMs and encoders

    39. Left and Right Encoders • For reasons to be revealed shortly, you probably want to have two encoders: one on the right drive, one on the left drive. • Easy to modify loop to do this: • Note this stops when either encoder reaches its target required_ticks = calc_required_ticks(10*12); left_ticks = 0; right_ticks = 0; // reset encoders // start PWMs and encoders while (left_ticks < required_ticks && right_ticks < required_ticks) { left_ticks = GetEncoder(1); right_ticks = GetEncoder(1); } // stop PWMs and encoders

    40. Advanced Autonomous Driving Straight

    41. Driving Straight • Great! Now you’re driving exactly 20 inches! • But there’s a problem… for some reason your robot seems to be curving to the left (or right) • Why isn’t it driving straight!? • Motors aren’t perfect • Gears aren’t perfect • Wheels aren’t perfect • In short, there are many uncontrolled factors that can make one set of wheels rotate slower than the other set with the same motor PWM setting • For the purposes of this discussion, we’ll assume the wheel circumferences are identical – it’s relatively easy to scale the encoder counts for that

    42. The Goal • Ensure each side of the robot moves the same distance in the same time • Fortunately we already have the sensors we need to do this: encoders! • We can’t just do this at the end (wait for both encoder counts to reach the desired number), as that doesn’t prevent us from curving in the meantime, so… • Dynamically adjust each side’s drive motor power such that the encoder counts match • We can’t increase the motor power beyond 255 to “catch up” the slow side, so we need to decrease the motor power of the faster side

    43. Driving Straight – Attempt #1 left_power = 255; right_power = 255; // start drive PWMs and encoders while(1==1) { // Get encoder tick counts if (tick_count_right > tick_count_left) { // right’s ahead of left, decrease right power right_power = right_power - 1; } else if (tick_count_left > tick_count_right) { // left’s ahead of right, decrease left power left_power = left_power – 1; } // Update drive PWMs // Check for distance and stop } • What’s wrong with the above?

    44. Driving Straight – Attempt #2 left_power = 255; right_power = 255; // start drive PWMs and encoders while(1==1) { // Get encoder tick counts if (tick_count_right > tick_count_left) { // right’s ahead of left, decrease right power right_power = right_power - 1; left_power = 255; } else if (tick_count_left > tick_count_right) { // left’s ahead of right, decrease left power left_power = left_power – 1; right_power = 255; } // Update drive PWMs // Check for distance and stop } • Better, but we seem to be drifting faster than we correct! • We could just up the -1, but there’s a better way… use the error!

    45. Driving Straight – Almost Done! left_power = 255; right_power = 255; factor = 16; // or 8, or... // start drive PWMs and encoders while(1==1) { // Get encoder tick counts if (tick_count_right > tick_count_left) { // right’s ahead of left, decrease right power right_power = right_power – (tick_count_left-tick_count_right)/factor; left_power = 255; } else if (tick_count_left > tick_count_right) { // left’s ahead of right, decrease left power left_power = left_power – (tick_count_right-tick_count_left)/factor; right_power = 255; } // Update drive PWMs // Check for distance and stop } • This loop corrects harder the further away you are from matching tick counts • Only minor cleanups are needed to ensure you don’t saturate

    46. Using the Error • Using the error to proportionally determine how much to adjust the drive strength by is one of the simplest versions of a generic loop feedback controller known as “PID” • The PID factors: • “P” – proportional (to the error) • “I” – integral (total accumulation of error over time) • “D” – derivative (how fast the error is changing) • In the control loop just presented, • P=1/factor • I=0, D=0 • In small robots like VEX, it’s rarely necessary to use the other two PID factors • There’s not enough inertia to overwhelm the motors • I won’t delve into PID details (see Wikipedia instead)

    47. Advanced Autonomous Making Turns (Turning Y Degrees)

    48. Making Turns • Just turning is easy… drive left forward and right back, or vice-versa; but how to measure the angle? • One of the more reliable ways is to use a gyro sensor • Gyro measures rotational rate • Code integrates over time to determine angle • However, in FTC we can’t use one • Gyro isn’t a standard VEX sensor • Simply measuring time to turn is unreliable (just like measuring distance by time) • Next best thing: optical encoders on the drivetrain • Even better, we already have them there for driving a specified distance!

    49. Turning Ticks • So how far do your wheels drive to turn a full circle (360 degrees)? • Unfortunately, this can be quite hard to calculate • Wheel slip – depends on the surface • Non-centered axis of rotation • It’s easier to measure it empirically • Create an autonomous mode that tries to spin exactly 360 degrees based on a tick count • Adjust the tick count until the robot rotates 360 degrees

    50. Turning Ticks • Your rotation routine then just needs to scale that 360 degree rotation tick count by the number of degrees to determine how many ticks to rotate for: • You will probably have to recalibrate if the surface or robot weight changes