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 = mgcrr = 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 + crr + sin(α))*Δv/Δt ]
= [20(1 + 0.15 + 0.05)*(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/(ηm*ηgh)
= 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, 14000 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 17000 RPM, 4.4 in-lb motor
– With gearhead: 650 RPM, 114.4 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