frc robot mechanical principles
DESCRIPTION
FRC Robot Mechanical Principles. Review understanding from last week Robot agility and maneuverability? Chassis types & options Speed and Torque? Torque vs. Speed Gear ratios Breakaway torque limit 2 speed 3 CIM vs. 2 CIM 3 CIM + 2 Speed – vs. 3 CIM single speed Wheels: Friction. - PowerPoint PPT PresentationTRANSCRIPT
FRC Robot Mechanical Principles
• Review understanding from last week– Robot agility and maneuverability?– Chassis types & options– Speed and Torque?
• Torque vs. Speed – Gear ratios – Breakaway torque limit– 2 speed– 3 CIM vs. 2 CIM– 3 CIM + 2 Speed – vs. 3 CIM single speed
• Wheels: Friction
Continuing Subjects:
FRC Engineering/Design Review:
• Every year our Strategic Design has called for:– “Fast, Stable, Maneuverable With Good, Pushing
Power”– How do you get maneuverable – agile – quick turning?– How do you get stable?– How do you get both?
– How do you get Fast?– How do you get good pushing power?– How do you get both?
• Chassis & Drive train layout defined by middle of week 1?
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
1.46875
2.3523.5wide x 10 length
23.5 wide x 16" (6wd) =
23.5" wide x 10" long
2.35:1
An example of an 8WD agile & stable tank drive layout
FrictionClassical Friction Theory
• Torque at wheel imparts a “Drive force” at wheel carpet contact point
• This is reacted by a “Friction Force” of up to the “Friction coefficient” times the weight on the wheel– The friction coefficient is a characteristic of the materials involved– If the Drive force is greater than the Friction force, the wheels will slip
• The maximum Torque that can be transmitted by the drivetrain is the “Breakaway Torque” that creates a Drive force equal the Friction coefficient x Weight on wheel = m * m * g
Weight = mass*gravity =
m*g
Drive Force = Torque/radius=m*m*g
Torque
Friction reaction force
Drive Motors, Transmissions, Sprockets and Wheel Diameter
• How to translate speed of motor to speed of robot?– Motor speed inputs into transmission with a gear ratio
• Motor load results in speed loss
– Transmission output to sprockets connected by chain• Ratio of sprocket teeth decreases speed
• Overall Ratio includes motors, transmissions, sprockets/belts, wheel diameter
Whee
l
Motor
Sprocket
Transm
ission
Drive Motors, Transmissions, Sprockets and Wheel Diameter
• Simple Transmission Gearbox (as in the CIMple Gear box)– 2 CIM motor input
65 teeth
14 teeth
14 teeth
5300 RPMCIM Motor Free Speed
5300 RPMCIM Motor Free Speed
Output Speed = 5300 * 14/65 = 1150 RPM
Basic Relationships - Review
Wheel / Transmission Mechanics• Torque = Radius x Force = T (in-lbs)• Rotational speed = w (rpm)• Velocity = v = (w*2*P*r)/(60 *12) (ft/sec)• Frictional Coefficient = m “empirical” – test wheel grip to carpet, with
weight• Maximum Traction Force = FT = m x W (weight of the robot = mg)• Maximum Torque at wheel that can be transferred by friction
– Tm= m * W * radius• Max torque delivered by motor is at stall• Torque decreases with speed
w
Fw
Ft
T
r
Wv
Drive Motors, Transmissions, Sprockets and Wheel Diameter
Whee
l
Moto
r
SprocketTra
nsm
ission
Drive Motor RPM - no Load 5300 RPM CIM motorTransmission gear ratio 4.65 :1 CIMple gearboxTranmission output speed 1139.8 RPMSprocket 1 number teeth 12 s1Sprocket 2 number teeth 24 s2 Sprocket ratio 2.00 :1 s2 / s1Wheel Speed (no load) 569.9 RPMWheel Diameter 4 inchesLinear speed (no load) 9.95 feet per secondsMotor Load speed loss coef 0.81 acquired by measurement of loaded robotLinear speed (loaded) 8.06 feet per seconds
w (RPM)Velocity = v = (w*2*P*r)/(60 *12) (ft/sec)
COTS Drive Transmission Options
Drive Transmissions - CIM motor inputsName Vendor Gear Ratio # CIMsCIMple AM 4.65 2Toughbox AM 12.75 2Toughbox mini AM 10.71 2Supershifter AM 6 & 24 2 other ratios availableBall Shifter VexPro 3.66 & 8.33 2 other ratios availableSingle Speed Vexpro 7 or 6 or 5.33 3Dual Speed WCP 15 & 5.6 2 other ratios availableSingle Speed WCP various 2
Drive Motors, Transmissions, Sprockets and Wheel Diameter
• Spreadsheet simulations allow quick iterations to explore different combinations of gearboxes, sprockets and wheel diameters.
1-Speed Drivetrain
Free Speed (RPM)
Stall Torque (N*m)
Stall Current (Amp)
Free Current (Amp)
Speed Loss Constant
Drivetrain Efficiency
Motor Stall Torque (in lbs)
Max (Stall) Acceleration
in/s2
CIM 5310 2.43 133 2.7 81% 90% 21.5055 1068.39 T = W*mu*R1104 W = mg = T/(mu*R)
# Gearboxes in Drivetrain
# Motors per Gearbox
Total Weight
(lbs)
Weight on Driven
Wheels Wheel Dia. (in) Wheel Coeff
Max wheel torque
(breakaway) in lbs.
Max acceleration (breakaway)
in / s2
2 3 140 100% 4 1.3 364 502.32 F = ma
F= T/R
Driving Gear
Driven Gear
Drivetrain Free-Speed
Drivetrain Adjusted
Speed
Pushing Match Current per
Motor
Wheel Max (Stall)
Torque (in-lbs)
Max continuous (40 Amp)
wheel torque
Max cont. acceleration
in / s2a = T/(R*m)
1 6 15.45 ft/s 12.51 ft/s 70.76 Amps 774.20 437.63 603.92 m=W/g g=386.4 in/s2
1 1.00 6.00 : 1 <-- Overall Gear Ratio m=T/(mu*R*g)1 1 1.55 Breakaway Amp a= mu*g1 1 33.27 1.202267179
Wheel Torque = Motor torque*Gear ratio
Gear Ratio Effects Gear Ratio Optimization Trades Off Speed and Torque
• Higher gear ratio– Lower max speed– More low end torque– May not be able to use all of Torque?
• Lower Gear Ratio– Higher max speed– Less max torque– May not ever get to top speed?
• Torque provides acceleration – T = F * r = m * a * r– increasing speed
• Torque decreases with speed
• Wheel friction limits amount of Torque that can be transmitted without spinning wheels
– Only get advantage of higher gear ratio if friction is high
– For Instance: m = 0.9 there is no advantage to a gear ratio above 7.3
• For typical m = 1.1 What is optimum gear ratio?
Torque=>
<= Speed
<= Distance
m = 1.3
m = 1.1
m = 0.9
Gear Ratio Max Speed
11.42 : 1
7.30 : 1
5.03 : 1 14.9 ft/s
10.3 ft/s
6.6 ft/s
2CIMS in each of 2 single speed gearboxes
Time (seconds)
Gear Ratio Effects
2 Speed Gearbox Allows Optimization of Speed and Torque
Torque=>
<= Speed
<= Distance
m = 1.3
m = 1.1
m = 0.9
Gear Ratio Max Speed
11.42 : 1
7.30 : 1
5.03 : 1 14.9 ft/s
10.3 ft/s
6.6 ft/s
2CIMS in each of 2 two speed gearboxes
Time (seconds)
• Desire to “shift” when acceleration (or Torque) crosses– Here shift from 11.43 ratio to 5.03 ratio
at about 25 in-lbs and 16 fps– Very slight advantage in distance /
time
• If m = 1.1 then get up to 320 in-lbs torque at low speed
• And up to 15 fps!
• Only is advantage if shifted at right times
• Driver shifting is difficult – Automation opportunity?– Read speed on encoder and shift
automatically?
2 CIM vs 3 CIM Drive3 CIM / Gearbox Drive Eliminates Need For 2 Speed Gearbox• 3 CIMs provide 50% more
torque at any gear ratio
• Minimal benefit for 2 speed gearbox – Friction becomes more
important than gear ratio
• Can have ~14 fps robot (very fast) and have max transmittable torque
• 3 CIMs provide quicker acceleration – getting more distance vs. time.– Equal to 2 CIM – 2 speed
Gear Ratio Max Speed
11.42 : 1
7.30 : 1
5.03 : 1 14.93 : 1
10.28 : 1
6.58 : 1
Torque=>
<= Speed
<= Distance
m = 1.3
m = 1.1
m = 0.9
3 CIMS in each of 2 single speed gearboxes
2 CIM vs 3 CIM DriveWhen May 3 CIM – 2 Speed Make Sense?
• Low gear ratio – high speed–High gear ratio set at level of
max useful torque benefit • and not trip breakers• Here for m = 1.2, Ratio~ 9:1
– Low gear maintains high acceleration
–Makes difference only if accelerating over 15 feet distance• At 20 feet may get up to
3-5 foot advantage• May not be controllable
Torque=>
<= Speed
<= Distance
m = 1.3
m = 1.1
m = 0.9
Gear Ratio Max Speed
9.50 : 1
5.33 : 1
3.44 : 1 21.8 ft/s
14.1 ft/s
7.9 ft/s
Drive SimulationAllows Convenient Evaluation Of Different Drive Train Configurations
• Useful to understand trends – But make sure to anchor to test data
• Includes considerations for:– Speed loss coefficient – how much slower motor is under load
• Free speed is 5300 RPM, loaded speed ~ 4300 RPM (81%)• May be dependent on gear ratio – further test data needed
– Torque accelerates speed, but torque reduces with speed– Speed desired called by voltage– Voltage drops when load is first applied, current spike
• Simulation– Iterative time step solution - excel– Test data can be taken to improve simulations– Spreadsheets from team 33 and 148 (JVN) used and here-bye credited
• Modified both in calculations and display.