Monthly Archives: July 2024

Industrial Safety Devices

The market is overwhelmed with various safety devices, making the design of machine safety a daunting task. Selecting an optimum safety device often depends upon understanding its proper use for a specific design.

There are different types of industrial automated equipment. They can be as simple as a pneumatic cylinder, or various automation components working simultaneously. Irrespective of the complexity of the system, it is imperative to consider the safety of the maintenance staff, integrator, or operator. Depending on the system they are installed in, safety systems can be very simple or complex. Moreover, the complexity of the safety system depends on the automated system, increasing with the latter’s complexity. The availability of different types of safety systems on the market makes the choice rather difficult.

As automated equipment have been around for so long, documentation on standard design principles, best practices, and guidelines for safety systems are available in plenty. When designing safety systems, these documents are a great resource, and it is necessary to consult them to ensure that the equipment is safe.

SIL or Safety Integrity Level is a measure of the failure rate of equipment expressed in terms of the probability of its failure. Typically, a safety-rated equipment will most likely have a published SIL number. This number is not a rating, but rather a guideline to the type of system with which it can be used. For instance, if the system has a SIL rating of 3, then all devices within the safety system must also have a SIL number of 3. There are four SIL levels and level 4 is the highest, meaning it has the lowest probability of failure.

The E-Stop or Emergency-Stop button is the most common safety device. It is the first safety device typically added to a system. Typically, the push button comes with one normally open contact for monitoring and two normally closed contacts for de-energizing. The button is colored bright red and has a yellow label. Its basic purpose is to stop all sources of motion or hazards by de-energizing the power within the system. For pneumatic equipment, engaging the E-Stop results in venting the stored pressure to the atmosphere, and turning off any STO signals for motion devices.

While there are many ways of using an E-Stop button, the most common is to couple it with a safety relay. Typically, two monitor circuits of the safety relay pass through the dual contacts on the E-Stop button. Pressing the button opens the contacts, causing a break in the redundant safety circuit, thereby triggering the safety relay, which then opens its contacts.

A safety controller or safety PLC using special safety inputs can also be useful for monitoring the state of the E-Stop button. The PLC program should incorporate opening the output contacts when the emergency stop button has been pressed. For simple systems, passing the STO signals or the control voltage for contactors through the E-Stop contacts should suffice.

The inexpensive E-Stop button is a simple way for easily stopping and de-energizing hazards within a system. It is possible to integrate them easily into either simple or complex safety systems.

Position and Distance Sensors for Accurate Tracking

Many engineering processes like security systems, feedback control, and robotics rely on position and distance sensors for machines to operate safely and accurately. These sensors provide vital information in real-time about the position and displacement of an object. The coordinates of an object relative to a known reference give a measure of its position. The movement of the object from one location to another with a determined angle and distance, is its displacement.

The history of position sensors begins with the potentiometer. Johann Poggendorf invented the potentiometer in 1841. With a change in resistance, it measures the position of a movable contact on a resistive track. Later, the field of position sensing was taken over by magnetoresistive sensors, which measured the change in a material’s resistance due to a magnetic field.

Soon, new position sensors appeared. These were based on solid-state electronics and included LVDTs or Linear Variable Differential Transformers. Later came digital position sensors, which offered high-resolution measurements suitable for integrating into computer systems. The latest trend is to miniaturize position and distance sensors.

Position and distance sensors provide real-time information by detecting changes in physical properties like magnetic field, inductance, capacitance, and displacement. There are various position and distance sensors, including level, thickness, radar, capacitive, gravitational, and potentiometric sensors.

Thickness and level sensors are useful in measuring the thickness and level of powders, solids, and liquids. Primary technologies they employ include optical, laser, and ultrasonic. For instance, they calculate the thickness of a material based on distance measurement between an object and the sensor within a referenced space. In the same way, level sensors measure the height of the liquid in a container to produce a level reading. Industries using thickness and level sensors include manufacturing, pharmaceutical, chemical, and food processing.

RADAR or Radio Detection and Ranging technology locates objects using radio waves. They detect objects by transmitting radio waves at specific frequencies and listening for echoes of the signals as they bounce back from objects. By analyzing the time and frequency of the returned signal, radar can determine the object’s size, speed, and location.

Capacitive sensors measure distance by measuring the change in capacitance due to the proximity of objects to the sensor. The basic sensor has two conductive plates with a dielectric material separating them. As an object moves closer to one of the plates, the capacitance of the sensor changes. This generates a voltage signal proportional to the distance of the object from the plate.

The advantage of capacitive sensors is their formidable accuracy and resolution. For instance, capacitive sensors can measure displacement in the nanometric range. Harsh environments do not affect these sensors. However, outside interference in the form of humidity and magnetic fields can affect their performance.

Potentiometric sensors typically measure linear or angular displacement. For this, the sensor employs a resistive element. The basic construction consists of a thin resistive wire of film wound around an insulating element like a ceramic rod. A metallic wiper moves on the resistive element. As the wiper moves, its resistance changes with reference to one end of the resistive wire. An electronic circuit quantifies the resistance to produce a voltage output, indicating the displacement of the wiper attached to the measured object.

Skin Effect in Conductors

Alternating current distribution is non-uniform in real conductors with finite dimensions and rectangular or circular cross-sections. This is because the alternating current flow creates eddy currents in real conductors leading to current crowding, following Faraday’s laws.

AC currents, being time-varying, produce non-uniform distributions across the cross-sectional area of a conductor. The conductor offers a high-frequency resistance, and for the approximation, we can assume the current to flow uniformly in the conductor, in a layer one skin deep, just below the surface. This phenomenon is known as the skin effect. However, this is only a simple explanation, with the actual distribution of current being much more nuanced, even within an isolated conductor.

For instance, what is the current distribution within a cylindrical conductor with a diameter 2.5 times greater than the skin depth at the frequency of interest? For the answer, it may be necessary to look closely at the physics of skin effect, and the way skin depth is typically derived.

Skin effect is caused by a basic electromagnetic situation. This is related to the propagation of electromagnetic waves inside a good conductor. Textbooks typically examine the propagation of a plane wave within a conducting half-space.

Euclidean space is typically three-dimensional, consisting of length, breadth, and height. A plane can divide this space into two parts, with each part being a half-space. Therefore, a line, connecting one point in one half-space to another point in the other half-space, will intersect the dividing plane. Plane waves propagate along the dividing plane in the conducting half-space.

Now, plane waves consist of magnetic and electric fields that are perpendicular to the direction of propagation and each other. That is why these waves are also known as transverse electromagnetic or TEM waves. Moreover, within a plane wave, all points on planes perpendicular to the direction of propagation, experience the same electric and magnetic fields.

For instance, considering the electric field (E) is in the z-direction, the magnetic field (H) will be in the x-direction, while the wave propagates in the y-direction. Therefore, assuming a plane wave propagation, the electric and magnetic fields remain constant along the x or y direction, and change only as a function of y.

Moreover, for a good conductor, the electric field and the current density are interrelated to the conductivity of the conductor. Using these parameters allows us to calculate the current density, and the skin depth, by solving Maxwell’s equation.

Maxwell’s equation tells us that the amplitude of the current density at the skin depth decreases at the surface of the conductor. It also gives an initial idea of the change in current density at any instant in time as we go deeper into the conductor.

The equation allows us to relate the skin depth to the wavelength within the conductor. The attenuation constant and phase constant of a good conductor are inversely related to the skin depth. It is easy to see that a single wavelength within the conductor is about 6 times larger than the skin depth. This also means the current density will attenuate significantly at a distance of one wavelength.

Coin-Sized MEMS Rocket Thruster

Thrusters, in addition to engine and control systems, typically require hydrazine rocket fuel, which they must store in tanks, making the entire setup physically rather large. Recent innovations are under development, and they use ion and electric propulsion systems. Although the newer thrusters are physically smaller, they are still too large for Nano and Pico satellites, which weigh between 1 and 10 kg, and between 0.1 and 1 kg respectively.

These small satellites require miniature satellite thrusters. These are rocket engines with combustion chambers of 1 mm size. They use only electricity and ice to create thrust. Manufacturing such tiny coin-sized thrusters requires MEMS fabrication techniques.

Miniaturization of electronics is leading to increased accessibility of orbital launch capacity. In addition, small satellites are experiencing fast growth. But, along with electronics, many other things need to shrink too.

For small satellites, thrusters and other equipment for stabilization must also proportionally shrink in size. Although satellites for special purposes are getting smaller, some key components, especially thrusters, have not kept pace with the downsizing.

Enter The Imperial College of London, where a team has designed a new micro thruster especially meant for Nano and Pico satellite applications. The ESA or European Space Agency, who tested the new thrusters, has dubbed them as ICE-Cubes or Iridium Catalyzed Electrolysis CubeSat Thrusters. ICE-Cube thrusters use the process of electrolysis to separate oxygen and hydrogen from water.

The thruster then recombines the two gasses in a combustion chamber less than 1 mm long. The miniature size of the chamber requires a MEMS fabrication process to create it. In laboratory tests, the thruster delivered 1.25 millinewtons of thrust, and it could sustain it for an impulse of 185 seconds.

Although a fast-growing category of space vehicles, Nano satellites are a relatively new breed. While 2012 saw only 25 launches, a decade later, there were 334 launches in 2022. By 2023, that number has nearly doubled.

Being tiny, Nano satellites have little room to spare. That means, conventional tankage carrying corrosive and toxic propellants, such as hydrazine, is no longer practical. While there are forms of propulsion available on a smaller scale, and they typically use compressed air, ions, or steam, these are neither energy-efficient nor do they offer sufficient lifetime. The highest energy efficiency comes from using oxygen and hydrogen in a combustion system.

Nano satellites typically store their propellant as water-ice, because it is safer and less expensive as compared to holding it in liquid or gaseous form. The electrolysis process requires only 20 Watts, which storage batteries or solar cells can easily produce. Therefore, the satellites typically convert solar energy into thrust using ice.

The Imperial Plasma Propulsion Laboratory of the college fabricates the above devices in-house, using their own MEMS process. They create the shape of the device with a reactive ion etching technique using a refractory metal. Then they sputter-deposit an indium layer, which acts like an ignition catalyst, while simultaneously creating a protective oxidation layer for the walls of the device.

The college laboratory has developed two types of micro thrusters—the ICE-200 producing a design thrust of 1-2 N, and the ICE-Cube, generating a thrust of 5 mN.

Difference Between Protection and Control Relays

Today’s industrial revolution is paced fast enough to require the safety of people and processes through protection systems. There are many types of electronic protection relays, and they are different from standard control relays. Being an essential element of industrial control engineering, it is impossible to conceive of machine control without relays. Traditionally, almost all relays have ON/OFF features. However, relay technology has advanced, and now protection relays can have more than the customary features.

For instance, protective relays can measure specific process variables like voltage and current and switch the output based on the measured values. On the other hand, control relays do not monitor anything. Rather, by detecting the presence of an electrical signal at their input terminal, they change their output contact state.

For integrating into a machine, it is necessary to specially wire protective relays. Control relays, being simple, do not require complex wiring.

In an industrial scenario, the main objective of protective relays is to prevent electrical and electronic systems from exposing any type of hazards. Control relays can only switch applications, like switching from one state to another, but they do not inherently protect from hazards.

Some protective relays can indicate the value of the variables they measure, such as current or voltage. However, control relays cannot do anything like that, only outputting the state change.

Many types of protective relays are available on the market. Their selection depends on the nature of the hazard from which the protection is desired.

For instance, an earth fault relay offers protection in a system where current is passing through an earth terminal. In a system, current passing through an earth terminal represents a significant fault in the wiring. It can damage not only the wiring but also the connected electronic and electrical components. The earth fault relay recognizes the current flowing through the earth terminal and protects the system from this hazard.

When the earth fault relay detects current flow through the earth terminal, it trips the connected circuit and sounds an alarm to protect the system from the hazard. To detect the current, it uses a current transformer connected to the earth terminal.

For instance, moisture can short the phase terminal to the earth terminal. This causes current to flow from the phase terminal to the earth terminal, creating an earth fault. On detecting such a current flow, the earth fault relay typically trips the circuit breaker, cutting off supply to the connected circuit. Simultaneously, the relay indicates the fault with the alarm flag. Medium to high-voltage applications typically use earth fault relays. These include substations, transformers, and distribution panels.

Voltage relays protect the system against voltage fluctuations. Two major voltage relay types are undervoltage and overvoltage relays. While overvoltage relays protect the connected system from voltage supply greater than a certain level, an undervoltage relay protects the system from voltage drops below a certain level.

This is necessary to protect equipment from the hazards of voltage supplies below and beyond the specified level damaging electronic and electrical components. Voltage relays are available for AC and DC systems, and voltage values range from 5 to 500 V.