Electronic Weighing Machine

How an Electronic Weighing Machine works?

An electronic weighing machine is used for measurement of weight. It works works by using the principle of transducers. The device is mounted on elements called as load cells. These load cells are comprises of strain gauges as a transducing devices called Transducers.

Transducer

A Transducer is defined as a device which converts one form of energy into another form. It is used to convert mechanical forces or physical changes into an electrical signal.

Thus, the input of a transducer is in the form of a physical change (force, load, pressure, light, temperature etc.) and output is in the form of an electrical signal (voltage, current, frequency etc.) or electrical pulse.

Construction of transducer

A transducer converts a physical change into a corresponding electrical signal. Thus, a transducer has following two components–

1) Sensing Element

Sensing Element is the element which responds to a physical change in nearby environment. It is also known as Detecting Element of a transducer. Since, it is the first element in the process of conversion of energy form into other, it is also known as primary transducer.

It responds to a physical phenomenon or change in physical phenomena. Its response is closely related to the physical phenomena to have accurate measurement.

2) Transduction Element

A transduction element is the second part of transducer which receives the output of sensing element and converts it into an electrical signal. Since, it is the second component used in the process of conversion, it is also known as Secondary Transducer.

Working Principle of Transducer

A transducer is basically a combination of sensing and transduction element. The sensing element responds to the physical change and generates its output. This output may be in the form of displacement, angular displacement, strain etc. The output of the sensing element is then linked to the transduction element. This element is so designed to act upon the response of sensing element and generate a proportional output in the form of electrical signal. Which is then processed in the form of required output on display.

The advantages of converting physical quantities into analog electrical quantities are as follows –

• Electrical amplification and attenuation can be done easily.
• The mass-inertia effect is minimized, because electrical signals are not affected by the inertia of electrons which have negligible mass.
• The effects of friction are also minimized.
• The processing and controlling of electrical signal require less power.
• The electrical output can easily be used, transmitted and processed for the purpose of measurement.

Application of Transducers

Transducers have versatile applications in industries. In fact, control and automation is only possible by use of transducer. Some of the major applications of transducers are as follows –

• Transducers are used in home appliances such as Air Conditioners etc. to sense the room temperature and control it.
• Microphone is a transducer which converts sound wave into an electrical signal. Similarly, in speakers are also transducers.
• Thermocouple / Resistance Temperature Detectors (RTD) are used in industries to monitor and control system temperature.
• In medical industry, acceleromyograph, pneumograph, digital thermometer are the use transducers.
• In processing industries, various kind of transducers viz. Frequency, Reactive Power, Voltage, Ampere, Power Factor Transducers are used in control room. It is also used to sense the mechanical pressure of gas / liquid.
• In communication system, electromagnetic wave gets converted into electrical signal using transducer in Antenna.
• Photovoltaic cell is an example of transducer which converts sun light into electrical signal.
• Transducers are used in Robotics and Automation to convert movement into electrical signal.
• Nuclear radiation is measured by using transducer which transforms the radiation energy into electrical signal in Direct Reading Dosimeter (DRD).

Types of Transducers

Transducers are of two types –

1. Analog Transducers.
2. Digital Transducers.

These are explained as below –

1) Analog Transducers –

Analog Transducers are those whose output is continuous. This means that, the electrical output signal will be continuous function of time. RTD, Thermocouple, LVDT, RVDT, thermistor etc. are examples.

2) Digital Transducers –

Transducers which convert the input quantity into an electrical output signal in digital form is called Digital Transducers. Here, the output is not continuous rather it is in the form of pulse which means that it is discrete. Shaft Encoder, Limit Switch, Digital Tachometer etc. are digital transducers.

Load cells are the elements of an electronic weighing machines and other transducing devices. These are comprises of strain gauges in combination. Different types of load cells are used in electronic weighing scales to suit different purposes.

The working principle of a load cell used in a weighing machine is shown in figure.

When a weight is placed on the platform of weighing machine, strain produced in the the load cell get the element to deform. This causes change of resistance in the strain gauge wheatstone bridge circuit. This produces an electrical signal in the circuit. The signal is then read by an electronic device and convert it into weight scale.

This change in resistance is proportional to the applied weight / load / force on the platform.

• Load cells, used in weighing machines are of following types –

Pneumatic load cell dealt with air pressure. It consist of an elastic diaphragm attached to a platform for placing the object to be weighed. The scale works by using pressurized air to balance the weight of the object, and the amount of air required is therefore used to the weight of object. The pressure gauge then converts the reading into an electrical signal.

The hydraulic load cell works by using a fluid such as water or oil instead of air as in pneumatic load cells. When an object is placed on the platform, the piston applies increased pressure to the diaphragm which is proportional to the applied weight. After calibrating the pressure, reading is converted to an electric signal.

Strain Gauge

The strain gauge is most important part of a load cell. It works on the principle of change in electrical resistance when strain is induced in a conductor. This change is proportional to the strain developed in the load cell.

It is made by pasting of a resistive foil on a non conducting backing gauge plate as shown in figure.

A load cell gives very accurate measurement because, 4 strain gauges inside the load cell are connected in a wheatstone bridge configuration. This is called a full bridge circuit.

The construction of a strain gauge is shown in figure.

Capacitance load cell work on the ability of the system to store electrostatic charge. This load cell consists of two parallel flat plates forming a capacitor which can store charge. The amount stored charge or the capacitance depends on the size of the gap between the plates.

When an object is placed on the platform, the gap between the plates of the capacitor will decrease which cause a change in the capacitance. This change in capacitance can then be calculated into weight measurement.

Pressure Transducers

A pressure transducer or a pressure transmitter is a transducer that converts pressure into an analog electrical signal. Although there are various types of pressure transducers, one of the most common is the strain-gage based transducer. The device is also fitted with a burdon tube pressure gauge to record the mechanical pressure.

Working principle of a pressure transducer is shown in figure.

The conversion of pressure into an electrical signal is achieved by deformation of strain gages mounted on diaphragm. The strain gauges are bonded into the rubber diaphragm of the pressure transducer and wired into a wheatstone bridge configuration. Pressure applied to the pressure transducer produces a deflection of the diaphragm which develops strain into the gages. The strain will produce an electrical resistance change proportional to the change in pressure.