Sensors 4 FRC Robots

1. Sensors 4 FRC Robots By Hugh Meyer Red Alert Robotics FRC Team 1741 Center Grove High School Greenwood Indiana October 5, 2013

2. Why Do We Need Sensors? • Protect robot from self destruction • Protect robot from driver destruction • Provide closed loop control • Sense position • Feedback for autonomous mode • Limit range of motion • Confirmation system is functioning properly • Gives electrical team something to do

3. Mechanical Limit Switches • Lever or roller actuated • Several sizes available • VR7 is popular, easy to work with, inexpensive, supplied in kit of parts • Simple to wire – connect red and black wires to NC and NO connections, connect white signal wire to the COM connection • Orient the lever so it wipes across the cam – Avoid having the switch become your mechanical limit

4. Mechanical Limit Switches • Signal is on/off or true/false or 1/0 • Connected to digital I/O on the Digital Side Car • Software reads a one or zero • Switch connections will be labeled

5. Mechanical Limit Switches • Common • Normally Closed • Normally Open • Common terminal is the output • NO and NC would be ground and + 5 volts

6. Magnetic Reed Switches • High reliability – Reed contacts are sealed in a small glass cylinder • Easy to wire – Connect one terminal to the black wire and the other to the white wire • Used on Pneumatic cylinders, but the cylinder must be ordered with the magnetic ring option • Magnet is the element in motion to be sensed by the reed switch

7. Fixed Pressure Switch • Used to control air compressor • Switch closes when pressure is below a predefined low limit value and opens when pressure is above a high limit value • Simple to wire – Black and white wire connect to the switch

8. Pressure Transmitter • Provides an analog or digital signal indicating the exact pressure • Analog wiring is simplest – Black and red wires supply power to sensor and the white connects to the analog input • Digital device will connect to a SPI, I2C, or serial port

9. Push Buttons • Momentary contact • Suggest connecting all three wires – Black and red to NC & NO contacts and white wire to the COM contact • Available in many different sizes, shapes, color, and styles • Driver station ultimate control

10. Toggle Switches • Generally not momentary action – switch stays at position when released • Available in many different contact arrangements • FRC usage can generally use the simple SPDT (single pole double throw) • Center off is available if appropriate

11. Quadrature Encoders • Measures speed, distance, and position • Signal can return to the cRIO or Jaguar, or both • Requires two digital inputs if you want directional information and highest resolution • Signals are 90 degrees out of phase

12. Quadrature Encoders • Different sizes available • Several styles available • Kit of parts encoders must be installed carefully • Encoder disk must be correct distance from optical element • Need oscilloscope to verify correct operation

13. Quadrature Encoders • Available in different resolutions • The line count specifies the resolution • Higher count gives the I and D components of PID faster information for better response • Lower count is more tolerant of alignment errors • Several functions in WPILib are available depending if you want speed or distance • Incremental device – If you want absolute position then you need to home the system, or always start from a known position

14. Potentiometer Absolute Encoder • Always know where you are • Standard pot supports up to 270 degrees of rotation • Multi-turn pots can support more than one rotation • Home switch and sequence not required • Analog voltage output is proportional to position • Black and red wires go to outer connections on pot • Signal comes from the wiper terminal

15. Absolute Magnetic Shaft Encoder • Similar to a single turn pot, but allows full 360 degree rotation with no stops • Available with an analog or a pulse width modulated digital output • Output signal is proportional to the absolute shaft position • Different bearing configurations are offered • Can be used as operator control as well as sensor on robot • Connected to analog bumper on analog I/O module

16. Rotary Magnetic Encoder • No mechanical contact • Accurate alignment is important • Data sheet is on KOP webpage • Several IO options

17. Linear Motion Magnetic Encoder • No mechanical contact • Accurate alignment is important • Data sheet is on KOP webpage • Several IO options • Quad outputs

18. Linear Motion Optical Encoder • High precision • Any length encoder strip • Quad output • US Digital • Resolution of sensor and strip must match • Includes index mark for home position

19. Gear Tooth Encoder • Alignment not critical • Works with steel chain sprockets or gears • Pulse width modulated output • WPI Library has a function to read these type of sensors

20. Ultrasonic Distance Sensor • Measures distance to a target • Two styles of sensors available – Dual element and single element • Available with analog or digital outputs • Digital output would be the preferred method • Good for localization for autonomous mode • Sensitively spec is used for scaling voltage data to feet or inches • Get that information from the data sheet

21. Gyro - Inertial Turn Rate Sensor • Measures the rate of turn • Can be used for heading data for autonomous drive mode • Can be used to create a field oriented style drive scheme • Can be used to create a target oriented style drive scheme • If oriented properly can be a tilt sensor, although the accelerometer can do that naturally with the Z axis data

22. Inertial Accelerometer Sensor • Measures rate of change in motion and the direction of gravity • Three axis sensor – X, Y, Z • Z axis measures direction of gravity and can be used as a tilt sensor • X axis, if oriented properly, will measure forward and backward motion • Y axis, if oriented properly, will measure sideways motion

23. Inertial Sensor • Should be mounted on a mass that is suspended in foam • This acts as a low pass filter and will significantly reduce high frequency noise • Accelerometer output is available in digital or analog outputs • Analog outputs require one analog input for each axis • Digital outputs require four digital I/O connections on the digital side car • Use the SPI interface software functions to get the data • Use the digital outputs – This prevents the bias from becoming an issue

24. Inertial Sensor • Rate gyro output is available as an analog output • Connect this output to the analog bumper input • Use shielded wire for a clean signal • System must measure the bias when the sensor is initialized • System must be absolutely stationary when the system is measuring the bias • If your gyro is drifting it is likely because the system was in motion during the initialization period

25. Inertial Sensor • If you have a onboard air compressor it should not run during the gyro bias measurement time • Sensitivity must be passed to the gyro class when it is initialized • Sensitively is found on the data sheet for the device • Accelerometer sensitivity will be on the order of 300 mV/g • Gyro sensitivity will be on the order of 7 mV/degrees/s • Read the data sheet provided by FIRST and the data sheet from the manufacturer • The FPGA in the cRIO does the integration for you to get an angle

26. Inertial Sensors More Information • Kevin Watson has some good information about these types of sensors • He has some well commented code if you really want to get into the details • http://kevin.org • The FPGA in the cRIO really does the hard work integrating the gyro rate data to get an angle

27. Optical Transmitted Beam • Consists of two elements – a transmitter and a receiver • A signal is output when the light beam is blocked • Alignment is important • Simple packaging is readily available off the shelf • Power must be supplied to both the TX and RX

28. Optical Transmitted Beam • Use a regulated power output. Most require 24 volts which you can get using the solenoid bumper wired to the 24 volt supply • Detected object must be large enough for the sensor to see

29. Optical Retro Reflective • Uses a retro reflective target • Only one device • Target can be a flag in motion • Target can be stationary and have an opaque flag block the light – The flag must not be reflective, or it will be detected

30. Optical Vane Transmitted Beam • One piece housing in a U shape – also called slot sensor • Available in several different sizes • Will detect presence of any opaque object that is inserted into the U • Very effective for home sensors when used with incremental encoders • No mechanical contact for high reliability • Simple to actuate • Does not require precise alignment or mounting

31. LED & Photo Transistor • Roll your own for a fraction of the cost • Flexible designing package since you make your own • LED (LTE-4208) and Photo transistor (LTR-3208E) must be a matched pair • IR light is best because that will reduce the effect of ambient light • Works over a wide distance range

32. LED & Photo Transistor • Requires resistors in the circuit to work properly • LED needs a current limiting resistor in series with it to drop the 5 volts down to 1.2~1.6 volts • Size this resistor to draw 15 ~ 30 milliamps • Check the spec sheet for the target current • A good range will be 180 ~ 221 ohms if using a 5 volt power supply, I used 221 ohms for this device • You can calculate the value or measure it if you have an ammeter

33. LED & Photo Transistor • Photo transistor needs a pull up or down resistor depending on the wiring scheme you select • A good range will be 2k ~ 4k ohm, I used 3.16k for this device • Resistor can be above the transistor or below it • Above you get a high when the light beam is blocked • Below you get a low when the light beam is blocked

34. LED & Photo Transistor • R = 221 ohms • RL = 3.16k ohms

35. Current Sensor • Measures current flowing to a motor • Indicates if the motor is running within load specifications • Helps prevent “magic smoke” from exiting the motor • Hal effect sensors can measure DC current • IC based sensors can be inserted into the circuit • Jaguar speed controllers have current measuring ability built right in – you just need to ask it

36. Test Rig • Always measure motor current when building proto types • 12” piece of 10 AWG wire makes a good shunt resistor • 1 milli-volt equals 1 amp current flow

37. Cameras • Two types provided over the years from the kit of parts • Axis M1011 • Axis 206 • Any others are legal if they are IP based • Library provided by FIRST is very advance • Several teams got vision to work for them • Can be connected directly to the cRIO or radio • Vision processing can be done on cRIO or driver station

38. Tools Needed • Oscilloscope is very helpful • Voltmeter • Ammeter • Soldering Equipment • Connectors • Shielded wire

39. Software You Need to Write • Code that will read the sensor • Code that will log the data from the sensor • First thing you should do is hook up sensors you plan to use and start reading data • Don’t wait • Connect sensors and start reading data as soon as possible!!

40. Log The Data • Log the data to a file on the cRIO • Send the data to the driver station and log to a file on the DS • Add external device that logs to a memory card • Logomatic by Spark Fun Electronics • https://www.sparkfun.com/products/10216

42. Sensor Guidelines • It always needs a sensor • If a mechanism has limited travel then it needs an end of travel (EOT) limit sensor on both ends! • Start reading data from sensors as soon as you can so you will learn what is required and the type of data • Design your sensors into the system. Add on after thought mounting schemes tend to be unreliable

43. Kit of Parts Sensors • Use the resources provided by FIRST at the kit of parts website • They have data sheets and instructions specific to our application • http://www.usfirst.org/roboticsprograms/frc/2012-kit-of-parts-sensors

44. Suppliers • AndyMark - http://www.andymark.com/Default.asp • US Digital - http://usdigital.com/ • Digikey - http://www.digikey.com/ • Mouser Electronics - http://www.mouser.com/ • Newark Electronics - http://www.newark.com/ • Allied Electronics - http://www.alliedelec.com/ • Rockwell Automation - http://www.ab.com/catalogs/ • Automation Direct - http://www.automationdirect.com/adc/Shopping/Catalog/Sensors_-z-_Encoders • Spark Fun Electronics - http://www.sparkfun.com/ • Parallax Inc. - http://www.parallax.com/tabid/768/ProductID/92/Default.aspx • MaxBotix - http://www.maxbotix.com/ • Suzo Happ - http://na.suzohapp.com/pushbuttons/pushbuttons.htm • Penny+Giles - http://www.pennyandgiles.com/Joystick-Controllers-pg-28,2,,.php • State Electronics - http://www.potentiometers.com/select_joystick.cfm

45. Questions • Hugh Meyer • hmeyer@redalert1741.org • 317 786-9214 • Webpage: http://rar.meyermat.net/workshops/sensors/index.html