Drive Motors EE 5351 – Lecture 6

Consider a mobile platform: Vehice Mass: 20 kg (5 kg supported by each wheel) Drive Motors: 2 Wheel Diameters: 6 cm

Performance Requirements: Top Speed: 2 m/sec Acceleration (0 to 2 m/sec): 3 sec Maximum Incline: 5% (1:20) Operating Surface: Indoor, hard surface (Crr=0.15)

Calculation of Forces Rolling Resistance : Frr = mgc rr = 20*9.81*(0.15) = 29.5 N Grade Resistance: Fgr = mg*sin(α), α=tan -1 (1/20) = 20*9.81*(.05) = 9.8 N Force for Acceleration up the grade: Facc = [ma] = [m(1 + c rr + sin(α))*Δv/Δt ] = [20( )*(2/3)] = 16 N Total Motive Force: Facc+Frr+Fgr = 55.3 newtons

Torque and RPM Torque on each Wheel: T = F x d / 2 wheels= Ftot * Radius / 2 = (55.3 * (.06/2))/2 = 0.83 N-m (7.35 in-lb) Wheel RPM at Top Speed: N = Vel* (1/πD) * 60 = (2* 60) / (π * 0.06) = 637 RPM Power Required when Accelerating to Top Speed: P = Tω = 0.83 * 637 * 2* π / 60 = 56 watts per motor

Sizing of the Motor: DC Motors tend to be High RPM, Low Torque Require a Gear-Head to translate to Low RPM, High Torque Gear-Head has Significant Losses – 30% Motor also has Losses – 20%-30% Pin = Pshaft/(η m *η gh ) = 56/(.7*.7) = 112 W

Motor Sizing… So, looking for a 100 Watt Motor that can produce about 1 N-m of Torque at 700 RPM Choice #1: Banebot RS-545 motor with 26:1 Gearhead: – 60 W motor, RPM, 0.3 in-lb Torque – With Gearhead: 540 RPM, 7.8 in-lb A Bit Small….

Motor Sizing … Choice # 2: Banebot RS-550 and 26:1 gearhead – 120 W motor at RPM, 4.4 in-lb motor – With gearhead: 650 RPM, in-lb (13 N-m) Overkill? Motor draws 10 amperes at full load, 1.2 A at no load. Can we supply enough battery capacity to provide a useful operating life for 2 of these motors?

Batteries