Piezoelectric Effect

The Piezoelectric Effect

Key Terms

oscillator
piezoelectric crystals
Piezoelectric Effect
Pyroelectric Effect

Introduction

Certain crystals and ceramics, when stressed, produce a voltage (potential difference) across their surfaces. That is, when you exert a force on a piezoelectric crystal, the ends of the crystal become electrically charged. The word "piezo" (pē-ĀY-zō) means pressure in Greek. This property was first observed by Pierre and Jacques Curie in 1880, but had little practical application because the voltage produced was rather low. Since the 1950's improvements in materials and instrumentation have made piezoelectric crystals a cornerstone in industry as a stress/pressure sensor.

A piezoelectric crystal in action (animation)

It is easy to see why external forces initiate charge separations by examining the animation below. The external stresses literally redistribute the arrangement of atoms so that one side acquires a net positive charge while opposite side acquires a net negative charge.

Applied forces produce charge separation (animation)

Turning force into electricity ... some practical applications

Accelerometers are devices that detect changes in motion (like a gyroscope). One is in your smart phone so it knows its orientation in 3D space (for compass apps and sky maps, for example). Tiny piezo-crystals are commonly used to sense when things move and which direction the motion is taking place. The same thing is found in Wii® gaming systems so you can out box the champ. A piezoelectric crystal is used to sense when a RF model airplane is changing direction. The next earthquake will likely be detected by a piezoelectric crystal mounted in an accelerometer. Note: Another way of doing the same thing is by using a system of capacitors whose plate separation changes when subject to accelerations. However, piezoelectric crystals are still at the heart of most accelerometers.
A digital tire gauge uses a piezoelectric crystal to measure tire pressure. As the tire pressure increases, it exerts a greater force on the crystal which produces a higher voltage. A microcontroller handles the rest. The same idea is used in digital scales.
Piezoelectric crystals make excellent tactile sensors (devices that need to know if physical contact is established). Since force initiates a voltage across the crystal, any force can be converted into an electrical signal which a microcontroller can use to make something happen. This is especially useful in the area of robotics. How does a robotic arm "know" when it has something in its hand? Simple ... use a piezoelectric tactile sensor.
Your car can tell when it is "knocking" by using a piezoelectric sensor. The vibrations push on the crystal which sends an electric signal to the computer.
The spark on some cigarette lighters is made by piezoelectric crystals. When you pull a lever, it stresses the crystal. The result is a high voltage spark ... similar to the spark you get in a spark plug. The same idea is used in the push-button igniter of a gas BBQ grill or fireplace.
These crystals are also used in microphones. A piezo crystal is connected to a diaphragm inside the microphone housing. When spoken into, the diaphragm vibrates which also vibrates the crystal. This creates a voltage that could be amplified.
Are you old enough to remember the days when music was played on vinyl discs? Phonograph needles sometimes used piezoelectric crystals. The idea was to convert grooves on the disk to an electronic signal.

Turning electricity into mechanical energy

Just as you have seen so many times in the course, a piezoelectric crystal can run in reverse. Look again at this animation:

Is the crystal changing shape because a voltage is applied ... or is a voltage difference set up because the crystal is changing shape? The answer is ... you can't tell because both can happen. This opens the floodgates for a whole new way to use these crystals. The idea is simple: Apply a charge to the crystal and it changes its shape.

Piezoelectric crystals can be used as fuel injectors (in cars) to push the fuel into the cylinders
Epson printers utilize piezoelectric crystals to propel ink out of the ink reservoir and onto your paper.
Ultrasound machines use piezoelectric crystals to generate the high frequency sound which penetrates the skin. Dentists use the same idea to loosen plaque off your teeth. Don't forget, fish finders and sonar all use the same idea. Note: The same crystals can be used to detect the echo signal since the incoming wave is converted back to electricity when it strikes the crystal.
Piezoelectric motors are small, but can produce a lot of torque in relation to their size. The vibrating crystals can be made to push a surface in one direction so certain arrangements of crystals can lead to linear or circular motion. Because they can stop within 1 degree of rotation they become useful in computer disk drives,
Piezoelectric beepers and buzzers are common in digital watches and anything electronic that goes "beep". They can even be made to play a song in a greeting card. You can play high quality music driven by piezoelectric tweeters in stereo speakers.
Acoustic wave touch screens utilize ultrasonic waves to determine where a person touches a screen. The waves are produced from the vibrations of a piezoelectric crystal. The receiver is also a piezoelectric crystal that converts the received vibrations and changes them into electric pulses. The waves travel along a wave guide and are split at each junction of the reflector (which reflects some waves and transmits the rest. See the animation to see how an undisturbed screen functions.

Acoustic wave touch screen (animation)

When a person touches the screen, it blocks some of the waves which would normally reach the receiver. The system is engineered so that the microcontroller will know exactly which wave path was disturbed by measuring the degree to which the signal is reduced. A similar grid is required in the horizontal direction to get an exact coordinate fix. The same technology can be done with infrared radiation instead of ultrasound.

Piezoelectric oscillators

Since these crystals are completely reversible in the way they interchange mechanical vibrations and electric pulses, they have become useful as oscillators. If you have ever played on a swing you know what an oscillator is. On the swing you are constantly converting gravitational potential energy to/from kinetic energy, but in a very rhythmic way. You learn at a very early age at what point of the swing path a push is required to keep the swing moving with this natural motion. Another type of oscillator is found when a weight is suspended by a spring. Intuitively you know when to apply slight pushes to keep the system bouncing up and down (or side to side). The common link here is a system where energy can bounce from one form to another. In these cases, there is a natural oscillation pattern that the system resonates in. You can keep the system "swinging" by adding energy at just the right time and place.

The crystal timer

Let's start with a piezoelectric crystal which is vibrating. If you have ever played with a tuning fork you learn that once it is set in motion, it continues to do so in a natural pattern. I'm referring to the natural resonance frequency. Guitar strings, rocking chairs, bungee cords, even swaying buildings all have this natural tendency to move in distinct patterns. The vibrating piezoelectric crystal is no different.

What is special here, however, is that along with the natural pattern of vibration in the crystal comes a pattern of voltages as well. The key is to find a way to make this crystal continue to oscillate. Here is one way you can do that: Tap the electrical energy produced by the crystal and amplify it. Next feed it back to the crystal (just the right amount and at the right time ... think about the swing example). This makes the crystal want to vibrate some more ... which makes more electricity ... which is amplified and fed back to the crystal ... which (I think you get the idea) ... an oscillator is born. These become perfect metronomes to beat out the passage of time and are the heart of all crystal watches. Earlier computers also used the beating of a piezoelectric crystal to define the clock speed of the CPU. However, there are even more ways a piezoelectric oscillator can become useful:

Piezoelectric chemical detectors. Basically, you coat a piezoelectric crystal with a chemical agent (A) which will react with some different chemical (B) you wish to detect. The crystal is set into natural vibration and it vibrates at a known frequency. However, if this crystal comes in contact with the chemical (B) it is designed to detect, that chemical will react with the coating (A) and add (or subtract) mass to the crystal itself. This change in mass results in a change in the resonance frequency of the crystal (just like tuning forks of different masses will produce different sounds) ... which a microcontroller picks up and signals an alarm.
The resonate frequency in some piezoelectric materials are temperature dependent. That is, the rate that they naturally vibrate depends on the temperature. This can be a problem in most other applications (you don't want your quartz watch to run at different speeds ... do you?), but not if you want to make a thermometer out of these materials. A microcontroller converts these slight shifts in frequency directly into digital temperature readouts.
Piezo-driven fans are useful for cooling CPUs and microcontrollers (which generate unwanted heat). The idea is to attach thin plates to the surface of an oscillating crystal which act to circulate air (and heat) away from the source. Since they are reliable and quiet they can be found in cell phones and computers.

The Pyroelectric Effect

Another interesting property exhibited by certain polarized crystals (like zinc oxide) is the Pyroelectric Effect. Crystals with this property produce electric charges on their surfaces when heated. That is, when the crystal undergoes a temperature change, it acts very much like a piezoelectric crystal when it is stressed. In fact, all pyroelectric materials are piezoelectric (but the converse is not true). So any crystal that displays the pyroelectric effect can be used as a heat flow sensor.

To understand why these crystals show this behavior, you must first understand the principles behind the piezoelectric effect and carry it one step further. Suppose we have one of these crystals and heat it up suddenly. As these crystals are changing temperatures, internal stresses are initiated within the crystal itself. You can see this when you drop an ice cube in a glass of water. Often the ice will suddenly crack as it is placed in the water. This is because heat is suddenly transferring into the ice cube ... heating it up and expanding the outer surface first. The inside of the cube remains colder so it does not expand. This produces internal stress which is enough to make the cube crack. When a pyroelectric crystal is heated suddenly, it behaves the same way ... initiating internal stresses (which act just like external forces on a piezoelectric crystal). This initiates a charge separation (voltage) across the crystal. Can you see why the crystal only produces these charges when it is heating up (or cooling down)?

Pyroelectric motion sensors

The human body emits infrared radiation. This is sometimes called "radiant heat". Pyroelectric crystals are sensitive to this kind of radiation. When exposed they absorb the incoming waves and heat up. This changes the crystal shape which then initiates an electric charge (which a chip is constantly monitoring). Motion sensors consist of a pair of pyroelectric crystals and a lens (to focus an image of the room).

The chip looks for patterns of signals from both crystals. If the room suddenly becomes dark (because a cloud covers the sun) both sensors are triggered at the same time and the chip is tuned to ignore these effects. However, if a person enters the room, the motion creates a pattern as it first reacts with one sensor and then the second sensor. This pattern is enough to tell the chip to issue a warning or turn on a light. In addition, the lens has a filter on it that only allows infrared with wavelengths from 8-14 mm because humans emit radiation strongest in that bandwidth. This helps cut down on false alarms.

Pyroelectric motion sensor (animation)