Report

DC Brushed Motors & Power Transmission 2014 FRC Ken Stafford Permanent Magnet Brushed? • Stator (Field) made of permanent magnets • Rotor (Armature) made with wire coils (electromagnets) • Commutator has brushes to pass current 2 The Basics… • Imperfect Transducers – Electrical Power to Mechanical Power – Electrical Power to Thermal Power! • Electrical Power (input) – Volts times Amps (Watts) – EG: CIM @ 40A has 480W input @12V • Mechanical Power (output) – Work divided by Time or – RPM times Torque (Watts or Hp) – EG: CIM (40A/12V) 3800rpm/6.15 inlbs=275W The more basic Basics… • Torque “twisting effort” – EG: shaft turning, force at the end of an arm, force at the circumference of wheel… “pushing/pulling strength” – Unlimited torque available through any motor with appropriate transmission • Power “rate of doing work” – EG: speed of lifting, torque times rpm, force times velocity… “robot/mechanism speed” – Maximum is set by motor design—only decreases through transmission Motor Parameters • Manufacturers provide varying data • Not too difficult to obtain experimentally with basic lab equipment • You need only four values to predict ideal performance – At full speed (no load) • Motor Speed (rpm) • Current (Amps) – At maximum torque (stall) • Torque (inlbs) • Current (Amps) Example: Taigene (Van Door) • Motor clamped in vise hooked to calibrated power supply • Free-running rpm by timed counting • Stall torque by linear force balance at end of measured arm • Current measured directly from power supply • Results: – Free running: 47.5 rpm @ 1.23 A – Stall: 360 in lbs @ 24.2 A Extrapolate Motor Performance Performance Map 60 50 40 Speed (rpm) Power (Watts) 30 Efficiency (%) Current (Amps) 20 Heating (Watts) 10 0 Note: This is at full rated voltage 0 50 100 150 200 250 Torque (in lbs) 300 350 So…what does this mean? • Max Torque occurs at zero rpm (stall) – Also produces zero Mech Power and Max Thermal Power – Lightweight, air-cooled motors will smoke in seconds More on Motors… • Max Power occurs at 50% Stall Torque, ~ 50% Stall Current, and 50% Free-running speed • Any sub-maximum power is available at 2 different operating conditions – High speed/low torque – Low speed/high torque • Max Efficiency occurs at ~25% Stall Torque or ~60% Max Power Typical FRC Motors • Sealed vs Fan-Cooled • Thermal Protection • Anti-backdrive vs backdrive resistant • Built in transmissions Motor Selection Criterion 1 Power Requirement 2 Weight of Motor & Transmission 3 Physical Size of Motor & Transmission 4 Backdrive Characteristics 5 Continuous vs Intermittent Operations 6 Efficiency 7 Availability Specific Recommendations • Sealed motors (eg CIM) – High torque, can handle intermittent high loads – Heavy • Application: – Driveline or other high power accessories – Must be kept LOW in chassis Recommendations Cont. • Fan-cooled motors (eg FP) – Very high power/low weight/ intolerant of high load • Applications: – Shooters/fans Recommendation Cont. • Worm Gear Motors (eg Van Door) – Thermal protection, backdrive resistant, low power, heavy • Applications: – Arm shoulder, turret – Low in chassis Design Example • Build a winch to lift a 100 lb robot 3 ft in 10 secs: – Mech Power = Work/Time * Conversion – Required Power = ((100 lb)(3 ft)/10 sec)(746 W/550 ft-lb/sec) = 40W – 12 of the 20 allowed 2014 FRC motors > 40W Taigene Performance Map 60 50 40 Speed (rpm) Power (Watts) 30 Efficiency (%) Current (Amps) 20 10 0 0 50 100 150 200 250 Torque (in lbs) 300 350 Design Example (cont) • From the Performance Map – It produces 40 W at either 100 or 275 in-lb • At 100 in-lbs it’s ~45% efficient; at 275, it’s ~18%! – Design your drum radius so it develops 50 lbs of force with 100 in-lbs of torque • Radius = 100in-lbs/100 lbs = 1 in 100 lbs 2 in 100 in-lbs Design Result Drum Radius Motor Condition Time to Lift <1.0 in Happy—cool >3.0 secs =1.0 in Working easy =3.0 secs >1.0; <2.75 in Getting warm… <3.0 secs =2.75 in Working hard =3.0 secs >2.75 in Unhappy—hot! >3.0 secs Design Details Cont. • If holding a lifter/arm in position is important do not rely upon motor torque (overheating) Design aexample: mechanical one-way clutch/ • Previous ~30W to hold @ 1.0 in retractable ratchet or @ balance drum; ~180W to hold 2.75 inmechanism drum! Transmission Essentials • Unless you are VERY lucky…you will need ‘em • Transmissions can: 1) Modify output speed/torque. Transmission Essentials • Unless you are VERY lucky…you will need ‘em • Transmissions can: 1) Modify output speed and torque 2) Change direction of rotation 3) Physically separate motor from device • They will ALWAYS 4) Reduce power through losses More Basics • For spur gears and chain/sprockets, it’s all about the number of teeth! • “Gear Train”, aka “Speed Ratio” e = Product of Drivers / Product of Driven = Speed out / Speed in = (Torque in / Torque out) * Sys efficiency Speed Example • Driving elements: – 16 T gear – 18 T sprocket • Driven elements: – 54 T gear – 28 T sprocket • e = (16 * 18)/(54 * 28) = .19 • With motor @ 5000 rpm Roller speed = 5000 *.19 = 950 rpm Torque Example • Same transmission: e =.19 • Losses occur at each “stage” – Typically 5% loss (η = .95) • You calculate you need 20 lbs force at the 2 inch dia roller Torque out = 20 lbs * 1 in = 20inlb Torque in = (.19 * 20 inlb) / (.95 * .95) = 4.2 inlb • Note: this would result in a 28 A load with the small CIM as shown “Roller” Chain and Sprockets • Parameters (ANSI) – Chain Number: • 1st digit = pitch in 1/8 inches • 2nd digit = “roller” (0), “lightweight” (1), or “bushed” (5) • EG: #25: ¼ inch pitch without rollers, #35: 3/8 inch pitch without rollers – Strength (breaking/working): • #25 = 875/140 lbs • #35 = 2100/480 lbs – “Working” implies industrial duty/life expectancy Chain Design Details • Sprocket sizes – Recommend 12T to 75T • Smaller causes vibration & excess wear • Larger leads to chain loss • Chain Wrap – 120 degree minimum for any drive or driven sprocket – Change routing, add idlers if necessary • Design in adjusters for long runs – Adjustment range should ideally be 2 pitch lengths (to avoid half-links) Gears, gears, gears! • Parameters (ANSI) – (Diametral) Pitch: P = Number of Teeth/ Pitch Diameter • Integer number, normally divisible by 4 • EG: 32, 28, 24, 20, etc • Larger the P, the smaller/weaker the teeth – Pressure Angle: the actual off-tangent angle force transmission • Typically 14.5 or 20 degrees • 14.5 degrees weaker, more efficient Spur Gear Design Details • Gear sizes – Recommend 12T to Infinity! (rack) • Smaller (“pinions”) are weak from undercutting—esp 14.5 degree PA – All gears with same P and PA will fit • Transmission axle separation – An exact, easily computed distance D = (Total number of teeth/P) / 2 – EG: 16T & 42T, 24 Pitch gears D = ((16 + 42) / 24) / 2 = 1.208 inches • Can be 95-98% efficient/stage 16T 42T Other Gear Stuff 50T • Worm Gears – Antibackdrive (Helix angle dependent, Window--yes; Van Door--no) – Woefully inefficient: η = .25-.75 – Very compact e = Number leads on worm / Teeth on Worm Gear = 4 / 50 = .08 – DO NOT USE FOR HI-POWER – Requires VERY secure bearing support 4 General Suggestions • Operate motors on left side of perf map • Air-cooled motors cannot operate near stall for more than a few seconds • Control top speed of operation by suitable gearing not by reduced voltage • Avoid powered anti-backdrive • Driveline ROT: no wheel-spin when blocked? = TOO HIGH GEARED! Overall Caveats • Real world motors in robots will not operate at the peak values predicted on the performance maps – Batteries will sag, voltage will be lost through conductors, etc • You need to consider mechanical transmission efficiency when calculating motor requirements • Be careful to note reference voltage in manufacture’s data—automotive use 10.5V commonly Questions?