UNIVERSAL TESTING MACHINE (UTM): How it works?

Rubber bands- they can be stretched by our hands. Ever wondered how other materials such iron, copper, superconductors etc. could be stretched? A universal testing machine (UTM), also known as a universal tester, materials testing machine or materials test frame, is used to test the tensile strength and compressive strength of materials. The "universal" part of the name reflects that it can perform many standard tensile and compression tests on materials, components, and structures.

-project and article contributed by Shantanu Bhosale, EEE, VNIT, Nagpur.

The specimen is placed in the machine between the grips and an extensometer can automatically record the change in gauge length during the test. If an extensometer is not fitted, the machine itself can record the displacement between its cross heads on which the specimen is held. However, this method not only records the change in length of the specimen but also all other extending / elastic components of the testing machine and its drive systems including any slipping of the specimen in the grips.

Components of UTM

Load frame - Usually consisting of two strong supports for the machine. Some small machines have a single support.

Load cell - A force transducer or other means of measuring the load is required. Periodic calibration is usually required by governing regulations or quality system.

Cross head - A movable cross head (crosshead) is controlled to move up or down. Usually this is at a constant speed: sometimes called a constant rate of extension (CRE) machine. Some machines can program the crosshead speed or conduct cyclical testing, testing at constant force, testing at constant deformation, etc. Electromechanical, servo-hydraulic, linear drive, and resonance drive are used.

Means of measuring extension or deformation - Many tests require a measure of the response of the test specimen to the movement of the cross head. Extensometers are sometimes used.

Output device - A means of providing the test result is needed. Some older machines have dial or digital displays and chart recorders. Many newer machines have a computer interface for analysis and printing.

Test fixtures, specimen holding jaws, and related sample making equipment are called for in many test methods.

Block Diagram of UTM

Display and Control Panel

Sensors Used in UTM:

Load Cell:

A load cell is a transducer that is used to create an electrical signal whose magnitude is directly proportional to the force being measured. The various types of load cells include hydraulic load cells, pneumatic load cells and strain gauge load cells. Strain gauge type load cell is used in this UTM.

                                                                  

 If a strip of conductive metal is stretched, it will become longer, resulting in an increase of electrical resistance end-to-end. Conversely, if a strip of conductive metal is placed under compressive force it will broaden and shorten. The strip can be used as a measuring element for physical force, the amount of applied force inferred from measuring its resistance. Such a device is called a strain gauge.  A strain gauge bridge circuit indicates measured strain by the degree of imbalance, and uses a precision voltmeter in the centre of the bridge to provide an accurate measurement of that imbalance.

                                              

In this circuit, R₁ and R₃are the ratio arms equal to each other, and R₂ is the rheostat arm has a value equal to the strain gage resistance. When the gauge is unstrained, the bridge is balanced, and voltmeter shows zero value. As there is a change in resistance of strain gauge, the bridge gets unbalanced and producing an indication at the voltmeter. The output voltage from the bridge can be amplified further by a differential amplifier.

                                  

Limit Switch:

 A limit switch is a switch operated by the motion of a machine part or presence of an object.  A limit switch is an electromechanical device that consists of an actuator mechanically linked to a set of contacts. When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection. They can determine the presence or absence, passing, positioning, and end of travel of an object. They were first used to define the limit of travel of an object; hence the name "Limit Switch".

Inductive Proximity Sensor:

An inductive proximity sensor is a type of non-contact electronic proximity sensor that is used to detect the position of metal objects. The sensing range of an inductive switch is dependent on the type of metal being detected. The inductive sensor is based on Faraday's law of induction. The sensor consists of an induction loop. Electric current generates a magnetic field, which collapses generating a current that falls toward zero from its initial trans when the input electricity ceases. The inductance of the loop changes according to the material inside it and since metals are much more effective inductors than other materials the presence of metal increases the current flowing through the loop. This change can be detected by sensing circuitry, which can signal to some other device whenever metal is detected. 

Variable Frequency Drive:

A variable-frequency drive  is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage.VFDs are made in a number of different low- and medium-voltage AC-AC and DC-AC topologies. The AC electric motor used in a VFD system is usually three-phase induction motor. The VFD controller is a solid-state power electronics conversion system consisting of three distinct sub-systems: a rectifier bridge converter, a direct current (DC) link, and an inverter. Voltage-source inverter (VSI) drives are by far the most common type of drives. The most basic rectifier converter for the VSI drive is configured as a three-phase, six-pulse, full-wave diode bridge. In a VSI drive, the DC link consists of a capacitor which smooths out the converter's DC output ripple and provides a stiff input to the inverter. 

The drive used in UTM is VFD-M series (0.75 kW, 230V – 1 Phase)

RS-485 communication protocol was used for sending commands to VFD by Atmega-32 microcontroller. 

         

 Working of UTM:

When the start button is pressed, input pin of Atmega-32 is triggered (Active low). Atmega-32 checks for all other parameters i.e. peak/instant, auto stop/on, tension/compression, limit-switch (if pressed, don't run the motor).

Atmega-32 sends the command to VFD to start with particular speed (can be set by Speed Adj.).

When motor starts, Atmega-32 receives input from proximity sensor (number of rotations), load cell (Voltage), limit-switch and it sends the data to Atmega-8 of each individual display using UART communication.

If Stop button is pressed, another input pin of Atmega-32 is triggered (Active low) and sends the command to VFD to stop the motor.

The 3-Phase Induction motor of 1390 rpm is connected to gear box to lower the speed and increasing torque. A bevel gear is used to change the axis of motion. This results in the vertical motion of the moving load-frame.

So, this is how an Universal Testing Machine works. 

Mention:  The model of UTM that has been presented above was as a part of project at IIT Kharagpur at the Cryogenic Engineering Centre.

-project & article contribution by Shantanu Bhosale, EEE, VNIT, Nagpur.