mechanical actuation system lecture 6 (chapter 8)

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


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


x

y

z


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


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


How any dof?


How any dof?

Samsul Tongaji, PSM 2006/2007


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


8.2.2: Loading

• Mechanisms are structures and transmit and support loads

• Analysis – determine loads to be carried by individual elements


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


Rotary-to-rotary motion transmission mechanism

• Examples are:

1. Gears

2. Belt

3. Pulley


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


• 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


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


Gear types – bevel, spur, worm, helical


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


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


www.co-design.co.uk/dpg/bel/bel8.gif

6.7.1: Types of belts


www.co-design.co.uk/dpg/bel/bel8.gif


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


Sprocket and chain


Robot power chair http://www.fatnfast.com/robot/

The objective was/is to evaluate:

• The practicalities of 4x4 steering

End of Lecture 6

mechanical actuation system lecture 6 (chapter 8)

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