Monthly Archives: July 2013

Linear Variable Differential Transformers (LVDT)

Did you know that the innocent looking solenoid could be the basis of an extremely sensitive, accurate and repeatable measuring transducer? Of course, it does not remain as a simple solenoid anymore, two more coils are added to it, and its length may increase. That is about all the changes that are required to transform a solenoid into an LVDT or Linear Variable Differential Transformer.

This common form of an electromechanical transducer converts a linear or rectilinear motion of the object to which it is coupled mechanically, into an electrical signal that can be readily monitored on an oscilloscope. LVDTs not only measure movements that are only a few millionths of an inch, but can also measure positions that vary by +/-20 inches.

The LVDT has a primary winding that is sandwiched between two secondary windings that are identical. The windings are on a one-piece hollow form of a glass-reinforced polymer, which is thermally stable. The whole arrangement is secured within stainless-steel housing.

The moving element is a separate tubular armature core, made of a magnetically permeable metal. This core is moves freely within the hollow bore of the housing. As it is, an LVDT is more like a cross between an electrical transformer and a solenoid.

In operation, the primary winding of the LVDT is energized by an alternating current signal. Part of the flux generated is coupled to the secondary windings because of the core. If the core is exactly mid-way between the two secondary windings, the coupling is equal and an anti-phase connection between the two windings shows a null or no output on the oscilloscope. If the core moves to one side, the secondary winding on that side has a greater coupling, and its output increases, while the output of the other secondary coil falls because of reduced coupling. A corresponding output is visible on the oscilloscope.

The output of an LVDT is the differential voltage between the two secondary windings, and varies linearly with the axial positioning of the core within the hollow bore of the LVDT. In actual practice, the differential voltage is converted to a DC voltage or current, as these are easy to measure using conventional measuring instruments rather than an oscilloscope.

If the output of an LVDT is represented graphically, it is easy to see what makes the whole arrangement such a versatile and sensitive transducer. The null point is a highly defined position and very repeatable. Since the position of the core is defined mechanically, electrical power interruption does not cause the readings to change. The output is highly linear and does not require further conditioning.

Advantages of LVDT

• Since the operating friction is low, it is useful for many applications requiring light loading;
• Can detect very low displacements, is repeatable and is highly reliable;
• Long life due to minimal wear and tear – suitable for critical applications like nuclear, space, etc.;
• Safety from over-travel of the core – the core can come out of the hollow completely without damage;
• Sensitive only in the axial direction – not affected by misalignment or cross-direction movement;
• Core and coil assembly readily separable;
• Rugged, minimal impact of environmental variations, good shock and vibration immunity;
• Responds rapidly to changes in the position of the core.

What Are Proximity Sensors?

Those of you who use a mobile phone with a touch-screen may have wondered why items on the touch-screen do not trigger when you hold the phone to your ear while answering a call. Well, designers of mobile phones with touch-screen have built-in a feature that prevents a situation such as “My ear took that stupid picture, not me.” The savior in this situation is the tiny sensor placed close to the speaker of the phone, and this proximity sensor prevents touch-screen activity when anything comes very close to the speaker. That is what happens when your ear touches the screen as you are on a call, but does not generate any touch events.

So, what sort of proximity sensors do the phones use? Well, in most cases, it is an optical sensor or a light sensing device. The sensor senses the ambient light intensity and provides a “near” or “far” output. When nothing is covering the sensor, the ambient light falling on it makes it give out a “far” reading, and keeps the touch-screen active.

When you are on a call, your ear covers the sensor, obstructing the device to see ambient light. Its output changes to “near” and the phone ignores any activity from the touch-screen, until the sensor changes its state. Of course, the mobile phone considers more complications such as what happens when the ambient light falls very low, but we will discuss more on different types of proximity sensors instead.

Different types of proximity sensors detect nearby objects. Usually, the proximity sensor is used to activate an electrical circuit when an object either makes contact with it or comes within a certain distance of the sensor. The sensing mechanism differentiates the types of sensors and these can be Inductive, Capacitive, Acoustic, Piezoelectric and Infra-Red.

You may have seen doors that open automatically when you step up to them. When you are close to the door, the weight of your body changes the output of a piezoelectric sensor placed under the floor near the door triggering a mechanism to open the door.

Cars avoid bumping into walls while backing. The proximity sensor (a transmitter and sensor pair) used here works acoustically. A pair is fitted on the backside of the car. The transmitter generates a high frequency sound signal and the sensor measures the time difference of the signal bounced back from the wall. The time difference reduces as the car approaches the wall, telling the driver when to stop.

Computer screens inside ATM kiosks and the screen on your mobile are examples of capacitive proximity sensors. When you put a finger or a style on the screen, the device detects the change in the capacitance of the screen. The device measures the capacitance change in two directions, horizontal and vertical, or in x and y directions, to pinpoint the exact location of your finger and operate the function directly underneath.

When a security guard checks you out with a wand, or you walk through a metal detector door, the guard may ask you to remove your watch, coins from your pocket and in many cases, even your belt. The reason is the wand or the door has an inductive proximity sensor that will trigger in the presence of metals (mostly made of iron or steel).

Finally, the fire detector in your home or office is a classic example of a proximity sensor working on Infrared principles. Level of infrared activity beyond a threshold will trigger the alarm, and bring the fire brigade rushing.

How Does the Touch Screen on a Mobile Phone Work?

The mobile phone is an amazing piece of work. Earlier you had to press buttons, now you just touch the app on your screen and it comes to life. You can even pinch your pictures to zoom in on a detail or zoom out to see more of the scene. The movement of your finger in the screen causes the screen to scroll up, down, left or right.

The technology behind this wizardry is called the touch-screen. It is an extra transparent layer sitting on the actual liquid crystal display, the LCD screen of your mobile. This layer is sensitive to touch and can convert the touch into an electrical signal, which the computer inside the phone can understand.

Touch screens are mainly of three different types – Resistive, Capacitive and Infrared, depending on their method of detection of touch.

In a resistive touch-screen, there are multiple layers separated by thin spaces. When you apply pressure on the surface of the screen by a finger or a stylus, the outer layer is pushed into the inner layers and their resistance changes. A circuitry measuring the resistance tells the device where the user is touching the screen. Since the pressure of the finger or the stylus has to change the resistance of the screen by deforming it, the pressure required in resistive type touch-screens is much more than for capacitive type touch-screens.

Capacitive type touch-screens work on a principle different to that of the resistive touch-screens. Here the change measured is not in terms of resistance but of capacitance. A glass surface on the LCD senses the conductive properties of the skin on your fingertip when you touch it. Since the surface does not rely on pressure, the capacitive touch-screens are more responsive and they can respond to such gestures as swiping or pinching (multi-touch). Unlike the resistive type screens, the capacitive screen will only respond to touch by a finger and not to stylus or a gloved finger, and certainly not to fingers with long nails. The capacitive touch-screens are more expensive and can be found on high-end smartphones such as from Apple, HTC and Samsung.

As the screen grows larger, such as for TVs and other interactive displays such as in banking machines and for military applications, the resistive and capacitive type technologies for touch sensing quickly become less than adequate. It is more customary to use infrared touch screens here.

Instead of an overlay on the screen, infrared touch screens have a frame surrounding the display. The frame has light sources on one side and light detectors on the other. The light sources emit infrared rays across the screen in the form of an invisible optical grid. When any object touches the screen, the invisible beam is broken, and the corresponding light sensor shows a drop in the signal output.

Although the infrared touch-screens are the most accurate and responsive among the three types, they are expensive and have other disadvantages. The failure rate is high because diodes used for generating the infrared rays fail often.

What is an oscilloscope and how does it work?

An oscilloscope enables the visual display of a voltage that varies with time. One of the two input points is generally connected to the chassis and grounded, but this is not always the case.

A probe, attached to the input port of the oscilloscope, is connected to the voltage source. Some oscilloscopes have two or more input ports. Oscilloscopes with multiple ports can enable simultaneous viewing of waveforms, say, at the input and output of a circuit, for comparison and measurement, etc.

Analog and Digital Oscilloscopes

The analog oscilloscope uses a Cathode Ray Tube, and is also called a Cathode Ray Oscilloscope. In an analog oscilloscope, a thermally heated electron gun emits electrons, and an applied DC voltage causes the electron beam to impinge upon a fluorescent screen as a bright spot. A control grid results in axial movement of the electron beam and controls the number and speed of electrons in the beam. The momentum of electrons impinging on the screen decides the brightness of the spot. Applying a more negative voltage causes fewer electrons to impinge and is used for intensity control. A variable positive voltage on the second anode adjusts the trace sharpness. On applying an input voltage, the electron beam deflects proportionately, creating an instantaneous trace on the screen.

If a voltage input is applied to the vertical deflection plates and the horizontal deflection plates are grounded, the spot on the screen moves only up and down. On interchanging the signal to vertical and horizontal plates, the spot moves from left to right. If two signals of same frequency and in synchronization are applied to the two pairs of deflection plates, a trace results. The bright spot must repeat the same trace at least 30 times a second for the human eye to see it as a continuous trace.
By contrast, a digital oscilloscope first samples the waveform, and converts it into a digitally coded signal by an analog-to-digital converter. The oscilloscope processes this digital signal to reconstruct the waveform on the screen. Storage in a digital format enables data processing even by connected PC’s. In this oscilloscope, stored data including transients can be visualized or processed at any time, a feature not available in analogue oscilloscopes.

Displaying a Waveform

Whereas in analog oscilloscopes, continually varying voltages are used, in digital oscilloscopes, binary numbers are employed and these correspond to the input voltage samples. An ADC or analog to digital converter changes the measured voltage into its digital information. A series of samples of the waveform are taken and stored, until there is enough to describe a waveform. The information is then reassembled to be shown on the Liquid Crystal Display.

Unlike an analog oscilloscope, which uses a time-base and a linear saw-tooth waveform to display the waveforms repeatedly on the screen, a digital oscilloscope uses a very high stability clock to collect the information from the waveform.

Types of Digital Oscilloscopes

There are three types of digital oscilloscopes and they are classified as digital sampling oscilloscopes, digital phosphor oscilloscopes and the digital storage oscilloscopes.

In conclusion
Oscilloscopes, both analogue and digital, are among invaluable measuring and diagnostic tools in the electronics industry with newer applications continuously evolving with innovations in technology.