# FORCE - PowerPoint PPT Presentation

1 / 28
Title:

## FORCE

Description:

### ... machines, hence their other name, dynamometers. ... Dynamometer types ... Transmission dynamometers are passive elements inserted into the system - they ... – PowerPoint PPT presentation

Number of Views:116
Avg rating:3.0/5.0
Slides: 29
Provided by: szu4
Category:
Tags:
Transcript and Presenter's Notes

Title: FORCE

1
MECHANICAL MEASUREMENTS
Prof. Dr. Ing. Andrei Szuder Tel.
40.2.1.4112604 Fax. 40.2.1.4112687 www.labsmn.pub.
ro szuder_at_labsmn.pub.ro
2
FORCE TORQUE MEASUREMENT
3
Force vs. Mass measurements
• Balance systems compare weight of the known
etalon with the force applied to the measuring
arm

F
Fe
• If the measured force, F is the weight of a
stationary object, then we balance the mass of
the object with the mass of the etalon.

4
Force MeasurementBy An ElasticElement
• Elastic elements are often employed for force
measurements via stress/strain Hookes law

Where K is the deflection constant and y is the
deflection at some characteristic point. Table
13.1 in the textbook lists some characteristic
cases
5
(No Transcript)
6
Important note Do not confuse the deflection
constant K with the Bridge Constant, also
labeled by K!
7
Torque Measurements
• Most often, torque sensors are used in measuring
the power of rotary machines, hence their other
name, dynamometers. In this use, we measure
dissipated energy by applying brake load to the
shaft of a rotary engine, and measuring the
angular velocity and torque.

where T is torque, F is force at displacement R
from the axis of rotation and ? is an angular
8
Dynamometer types
• The main challenges in dynamomemeter design and
use are controlled energy dissipation and signal
transmission from the rotary member to the
stationary equipment (using some kind of
brushes). Today wireless transmission systems are
often used.
• Based on the medium which dissipates energy
(heat), dynamometers are classified as mecanical,
hydraulic, electrical and transmission.
• Transmission dynamometers are passive elements
inserted into the system - they do not dissipate
energy.

9
Torque Measurement By Different Strain
Gauge Configurations
10
Dynamic response Second Order Systems
• Given the spring stiffness k, the damping
coefficient c and the mass m, the spring/mass
system with the damping element will follow

c
k
y
m
Fapplied
11
2nd Ord. System Response To A Step Function
If the system is excitedby a step function
The general solution can be expressed as
where
is the undamped natural frequency (in radians)
is the damped natural frequency (in radians)
is the critical damping ratio,
is the critical damping coefficient,
is the static displacement as time goes to
infinity and
is the response phase lag
12
The displacement due to an impulse signal will
depend essentially on two parameters the
critical damping ratio, ?, and the undamped
natural frequency, ?n.
Underdamped systems will swing above the
equilibrium few times before they settle at the
final level, while overdamped systems approach
the steady state from one side only!
13
2nd Ord. System Response To A Harmonic Excitation
If the system is excitedby a harmonic function
it will pass through a transient stage and settle
into a stationary oscillation, with a certain
amplification ratio, yd /ys, and a phase lag, ? .
where
is the excitation cyclic frequency (rev/s) and
is the system undamped natural cyclic frequency.
14
Notice that undamped system excited at natural
frequency will reach infinite amplification
ratio, i.e. it will come into resonance with the
excitation force. In practice, we try to limit
the useful frequency range to 20-40 of the
natural frequency, and have the damping ratio of
around 70.
15
Torque measurement
• Torque is measured by either sensing the actual
shaft deflection caused by a twisting force, or
by detecting the effects of this deflection.

16
Torque measurement
• To measure torque, strain gage elements usually
are mounted in pairs on the shaft, one gauge
measuring the increase in length (in the
direction in which the surface is under tension),
the other measuring the decrease in length in the
other direction

17
Torque measurement
18
Torque measurement
19
Torque measurement
20
Torque measurement
21
Torque measurement
22
Torque measurement
• The excitation voltage for the strain gage is
inductively coupled, and the strain gage output
is converted to a modulated pulse frequency
• Maximum speed of such an arrangement is 15,000
pm.

23
24
• Load cells became are the method of choice for
industrial weighing applications

25
them into electrical signals. The gauges
themselves are bonded onto a beam or structural
applied. In most cases, four strain gages are
used to obtain maximum sensitivity and
temperature compensation. Two of the gauges are
usually in tension, and two in compression, and
• Piezoresistive Similar in operation to strain
gages, piezoresistive sensors generate a high
level output signal, making them ideal for simple
weighing systems because they can be connected

26
• Inductive and reluctance Both of these devices
respond to the weight-proportional displacement
of a ferromagnetic core. One changes the
inductance of a solenoid coil due to the movement
of its iron core the other changes the
reluctance of a very small air gap.
• Magnetostrictive The operation of this sensor is
based on the change in permeability of
ferromagnetic materials under applied stress. It
is built from a stack of laminations forming a
load-bearing column around a set of primary and
secondary transformer windings. When a load is
applied, the stresses cause distortions in the
flux pattern, generating an output signal
proportional to the applied load. This is a
rugged sensor and continues to be used for force
and weight measurement in rolling mills and strip
mills.

27
• New Sensor Developments
• Fiber optic load cells are gaining attention
because of their immunity to electromagnetic and