mechanical actuation system lecture 6 (chapter 8)
TRANSCRIPT
Mechanical Actuation System
Lecture 6
(Chapter 8)
SME 3252: Mechatronics Lecture 6
Introduction• Mechanisms are devices which can be considered
to be motion converters • Transform motion from one form to another• 3 common motion transmission mechanism:
1. Rotary-to-rotary motion transmission mechanism – gears, belt and pulley
2. Rotary-to-translational motion transmission mechanism – lead-screw, rack-pinion, belt-pulley
3. Cyclic motion transmission mechanism – linkages and cams
SME 3252: Mechatronics Lecture 6
8.2: Types of motion
• Motion of rigid bodies – combination of translational and rotational
• Kinematics of rigid bodies (Dynamics SME1213)
• Translational motion – one or more of 3 axes
• Rotational motion – one or more of 3 axes
SME 3252: Mechatronics Lecture 6
x
y
z
SME 3252: Mechatronics Lecture 6
Exercise• Analyse the motion of the following
mechanisms and state whether they are pure translation, pure rotation or a mixed of translation and rotation:
1. The keys on a computer keyboard
2. The pen in an XY plotter
3. The hour hand of a clock
4. The pointer on a moving coil ammeter
5. An automatic screwdriver
SME 3252: Mechatronics Lecture 6
6.2.1: Freedom and constraints
• No. of degree of freedom – no. of components to motion that are required to generate motion
• Problem in design – reduce d.o.f requires orientation constrains
SME 3252: Mechatronics Lecture 6
How any dof?
SME 3252: Mechatronics Lecture 6
How any dof?
Samsul Tongaji, PSM 2006/2007
SME 3252: Mechatronics Lecture 6
Exercises
• By examining the following mechanisms, state the number of degree of freedom each has:
1. A car hood hinge mechanism
2. A windscreen wiper mechanism
3. Your knee
4. Your ankle
SME 3252: Mechatronics Lecture 6
8.2.2: Loading
• Mechanisms are structures and transmit and support loads
• Analysis – determine loads to be carried by individual elements
SME 3252: Mechatronics Lecture 6
0.125kg
Torque
Required force, Fv
Friction force, fv
0.08m From total weight, M;M = Body weight + battery weight + servo motor weight + leg weight + othersM = 1 kg + 0.5 kg + 0.055 kg (8 motors) + 0.1 kg (4 legs) + 0.5 kg = 2.84 kgm = M/4 = 0.947kg
Thus, F = 0.3 x 0.947kg x 9.81 m/s2 = 2.786NThe minimum required torque, T = n(Fv .r)Where;n = Safety factor, 1.5r = Turning radius, 0.08Thus, T = 1.5 × 2.786N × 0.08m = 0.3343 NmFrom the calculation, the minimum required torque to perform the vertical leg movement is 0.3343Nm.
From static friction theory,Fv = fv and Fv = μNFv = μ(mg)Where;Fv = Force min for actuation, Nμ = Static friction coefficient, 0.3m = Robot weight imposed on each leg, kg g = Gravity, 9.81m/s2
By:
Lim Kim Fung, PSM 2006/2007
SME 3252: Mechatronics Lecture 6
Rotary-to-rotary motion transmission mechanism
• Examples are:
1. Gears
2. Belt
3. Pulley
SME 3252: Mechatronics Lecture 6
SME 3252: Mechatronics Lecture 6
Gear System
• Observed variables: torque , acceleration , velocity and displacement for input and output gear
• System parameters: number of teeth or radius which gives gear ratio:
•
b
a
b
a
N
N
r
rn
SME 3252: Mechatronics Lecture 6
• No slipping occurs: raa = rbb
• Differentiate with respect to time:
• Or ra a = rb b
• In gear ratio:
• Force acting on both teeth:
• Therefore:
dt
dr
dt
dr bbaa
nN
N
r
r
b
a
b
a
a
b
a
b
b
b
a
a
rrF
ab n 1
SME 3252: Mechatronics Lecture 6
8.5: Gear trains
• Gear trains – a series of intermeshed gear wheels
• A compound gear train where the two central gears revolve together at the same speed on the same shaft
SME 3252: Mechatronics Lecture 6
Gear types – bevel, spur, worm, helical
SME 3252: Mechatronics Lecture 6
6.6: Ratchet and paw
• The ratchet is the toothed wheel, the pawl is the arm that locks it in position
• The purpose - to limit rotation to one direction only
• It is often used as a safety device to prevent reverse running of a winch or as part of the wrench in a socket set
SME 3252: Mechatronics Lecture 6
SME 3252: Mechatronics Lecture 6
6.7: Belt and chain drives• A typical Vee belt drive system is shown below• In this eg. the driver pulley is larger, the driven
pulley will run much faster.• Driver / Driven = Speed of Driven / Speed of
Driver
SME 3252: Mechatronics Lecture 6
www.co-design.co.uk/dpg/bel/bel8.gif
6.7.1: Types of belts
SME 3252: Mechatronics Lecture 6
www.co-design.co.uk/dpg/bel/bel8.gif
SME 3252: Mechatronics Lecture 6
6.7.2: Chains• Use of chain – to prevent slip, lock into teeth on
rotating cylinders• Component of chain drives. • Eg of chain drive - bicycle
SME 3252: Mechatronics Lecture 6
Sprocket and chain
SME 3252: Mechatronics Lecture 6
Robot power chair http://www.fatnfast.com/robot/
The objective was/is to evaluate:
• The practicalities of 4x4 steering
SME 3252: Mechatronics Lecture 6
End of Lecture 6