Magnetic Micro Relay

Since several years, micro-actuators are becoming a growing field of interest. Information processes and communication devices are some of the most promising application areas for micro actuators, since they require only a small amount of energy to handle information signals. Optical switches have already been developed and put into practice. They also find application in large force actuators, such as motors, relays and valves. The miniaturization of mechanical relays is an even more promising application and will have a much larger market size than optical switches.

Relay

A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier. The figure below shows a simple relay.

Main types of relays

Basically there are two types of relays

ElectroMachanical Relays (EMR) Solid State Relays (SSR)

Electromechanical relays (EMR) remain widely used for a number of applications including automotive control circuitry, test equipment, and the switching of high frequency signals. Historically the manufacturing process of electromechanical relays has been serial, i.e., devices are built one at a time, which can result in production bottlenecks and make it difficult to produce large relay arrays.

Solid state relays (SSRs) have been one solution to this production problem. SSRs allow for devices to be batch fabricated; however, in some cases they may also have higher offset voltage injection, lower maximum off-state resistance, and higher contact power

dissipation than their electromagnetic counterparts. Micromachined relays are produced by the application of batch fabrication techniques to electromechanical relays in an attempt to combine the best attributes of both electromechanical relays and SSRs. Thus, 90% of the relays are ElectroMachanical Relays and 10% are Solid state relays.

Types of micro relays

Different actuation principles for micro relays are Electrostatic Electromagnetic Thermal Piezoelectric

Of which the major ones are Electrostatic Electromagnetic

The former exhibits advantages in low force with small gap and high-frequency applications, the latter being more suitable for high force, large gap applications. In addition, magnetic actuators are very robust in harsh environment (dust, humidity) and can be driven by common low-cost, low-voltage controllers.

A Micromachined relay combines the best attributes of both EMR and SSR. Further, introducing MEMS technology to mechanical relays greatly reduces size, cost and switching time. It also allows the combined production with other electronic components. These advantages are even greater when we consider a group of relays such as matrix switches, in telecommunication systems.

The main requirements of relay operation are that there should be sufficient contact force to provide stable contact and should have reliable opening.

Principle of operation

Figure 1

Figure 2 below shows the structure model and equivalent magnetic circuit for this microelectromagnetic relay. The magnetic flux is generated when a current passes through the exciting coil. Most of the flux is concentrated in the permalloy magnetic core. The permalloy cantilever is attracted by the magnetic force and then bends down to connect to the fix contact. The active armature is also a conductor; hence, the current will flow from the active armature to the fixed contact when the active armature connects to the fix contact, and then the relay will stay at the “ON” state. The active armature will be forced to leave the fixed contact by the spring restoring force when the exciting current is cut off, and then the relay will stay at the “OFF” state.

Figure 2

Applications of relays

Relays are used to and for:• Control a high-voltage circuit with a low-voltage signal, as in some types of

modems or audio amplifiers, • Control a high-current circuit with a low-current signal, as in the starter solenoid

of an automobile, • Detect and isolate faults on transmission and distribution lines by opening and

closing circuit breakers (protection relays)

• MEMS pressure sensors, accelerometers are now established commercialized products.

• Electromechanical microrelay is amongst the predicted MEMS-based products to be commercialized in the next five years.

Fabrication process

Micromechined electromagnetic relay