Figure 1. HT Micro PassiveMicro™ inertial G-switch. (Image: HT Micro)

Electrical switches of all types activate circuits, send information, and initiate actions. An inertial switch is one that is triggered to activate at a specific acceleration threshold. No power is consumed until the switch is ‘awakened’ by the relevant event, making it ideal for ultra-low power (ULP) and remote applications.

What Is an Inertial G-Switch and How Does It Work?

Miniaturized sensor manufacturing processes make it possible to design a switch triggered by a specific level of inertial force. Using microelectromechanical systems (MEMS), miniature acceleration switches can be designed to close a circuit solely based on a preset level of force experienced by the device. Typically referred to as an inertial or G-switch, it utilizes a proof mass suspended on a spring, acting as a movable electrode. The contact point is a stationary electrode. When an inertial force is exerted on the device, the proof mass moves toward the stationary electrode. If the force magnitude and duration are sufficient, the movable electrode will touch the stationary electrode, momentarily closing the circuit. The circuit will then be re-opened by the spring (k) (See Fig. 2).

There are numerous inertial switches in use today across a broad range of applications, and many different techniques can be utilized to realize an electrode configuration depending on the desired performance characteristics and activation threshold levels. The key parameters of an inertial switch include response time, contact time, and shock survivability. Response time is the time delay from the moment the inertial event is initiated until the movable electrode initially touches the stationary electrode. Contact time is how long the two electrodes maintain contact. Shock survivability is a measure of the maximum level of shock the device can withstand. Each of these characteristics can be controlled based on the topography of the device, how the springmass electrode configuration is designed, and the materials selected.

Figure 2. Schematic of the basic model of an inertial switch. (k = elastic coefficient of the spring; c = viscous damping coefficient of the spring; a = inertial acceleration.) (Image: Xu, 2022)

Key Functions of Inertial G-Switches

The design of the switch is determined by the needs of the application — for example, an airbag switch requires an immediate response time. On the other hand, the duration of the contact time may be the crucial variable to ascertain that an actual inertial event occurred rather than irrelevant noise. This is especially important in lower g-force conditions.

Since the basic action of the switch is a momentary closure, the circuit is turned off as soon as the spring retracts the proof mass. The functions triggered will depend on the rest of the circuit design and what outcome is desired upon triggering the circuit.

Inertial switches are ideal for functions including:

  • Wake-up sensing to initiate a process.

  • Shock detection to invoke a safety circuit or terminate a process.

  • Process monitoring to count inertial events.

Figure 3. (a) Sketch of the self-powered inertial switch (k is the elastic coefficient and c is the viscous damping coefficient). (b) Working principle of a wake-up system: potential application of the wake-up system in monitoring transportation of cargo. (c) Harvesting energy on the flat path. (d) Generating the trigger on a bumpy path. (Image: Xu 2022)

In a situation where you simply want an alert that an acceleration threshold was exceeded at some time in the past, a mechanical latching device would be required. In this case, instead of a momentary triggering of the circuit, the latching inertial switch design would prevent the spring from reversing the proof mass and it will maintain contact closure with the stationary electrode. For example, the threshold can be set high enough to avoid alerting with a normal movement (such as a portable EKG machine jostling atop a cart). Impacts above the threshold prompt a warning such as an LED light to alert users they should recheck the calibration.

Figure 4. Comparison of accelerometer and zero-power motion switch. (Image: HT Micro)

Counting of events can be accomplished by incrementing a register for each contact made. This information can indicate how many times a device exceeded the desired acceleration threshold. For example, a smart motor that counts the number of inertial events above an established safety parameter.

Use Case Example

A use case that illustrates an inertial g-switch’s functionality is a wake-up system for monitoring freight during transportation. If a truck hits a rough road and its freight experiences a shock load above a threshold, the movable electrode will strike the stationary electrode and wake the circuit. This releases a pulse signal to alert the driver that possible damage to the freight may have occurred. (See Fig. 3)

Zero-Power Inertial Switch Compared to Other Technologies

Other technologies used for inertial force detection include piezoelectric materials, electrostatic configurations, and accelerometers.

Piezoelectric devices are ideal for sound-based applications such as vibration sensors, speakers, microphones, and cell phones. The attract/repulse nature of electrostatic forces makes them suitable for air cleaners/filters, photocopiers, and laser printers. However, both of these have the disadvantage of requiring continuous power for operation.

Accelerometers are utilized in applications from smartphone orientation to biometric devices such as pacemakers. The construction of an accelerometer is similar to the inertial g-switch in that it uses the motion of a proof mass to calculate force and direction however, accelerometers must be continuously powered in order to perform their function of monitoring, collecting, and transmitting data.

The zero-power inertial switch offers the advantages of zero power usage until activated by a signal, as well as superior durability in extreme environments.

Application Possibilities

Advanced machining capabilities developed from MEMS technology have enabled design engineers to make existing applications smaller and more energy efficient, as well as inspiring next-generation applications, such as medical wearables and VR gaming. Here are just a few additional areas that could benefit from the zero-power g-switch:

  1. Theft detection. Ease of access and speed of removal has made it hard to stem the theft of catalytic converters. An inertial switch could be used to detect the vibration of a thief’s saw and instantly trigger an alarm or other deterrent.
  2. Micro-drive protection. Laptops equipped with shock sensors, typically accelerometers, retract the drive head (or shut down the drive completely) when a sudden acceleration or vibration is detected. A zero-power inertial switch could perform the same function without using any power until an acceleration event occurs.
  3. End-stop detection. In setting up a linear positioner, a home position typically needs to be established for absolute positioning. There are a variety of methods to determine home position. One method is to drive the motor into an end stop, then move to a prescribed position. An inertial switch can be used to quickly detect when the end stop is encountered, and quickly move to the home position.
  4. Remote monitoring. Industrial IoT and edge computing enable manufacturers to monitor equipment located in a different plant or even another country. Inertial g-switches can detect meaningful changes in acceleration without depleting battery power in difficult-to-access components. This can be useful, for example, for monitoring electric motors. As motor bearings begin to fail, an inertial sensor can detect the change in acceleration and alert the operator.
  5. Sports analytics. MEMS embedded in balls, bats, and rackets can transmit precise data — in some cases up to 500 times per second — about an object’s position, speed, and trajectory. Wearable sensors can be used to identify and track individual players, as well as monitor their biometrics such as temperature, heart rate, and oxygen levels. A zero-power inertial g-switch is an ideal way to activate these systems without draining battery life in between uses.

Conclusion

High Aspect Ratio Manufacturing (HARM), in which the vertical dimensions are larger than the lateral dimensions, makes it difficult to fabricate miniature devices such as the inertial switch. However, with advancements in microfabrication techniques, MEMS devices have become more readily available for many commercial applications. The passive nature of the microswitch opens up improvements in many mobile applications where miniaturization and low power consumption are required.

Additional Information

  1. Liu et al. (2022, February 24). Research Progress of MEMS Inertial Switches. Micromachines (Basel). 13 (3):359.
  2. Xu and Younis (2022, April 13). Micromachined threshold inertial switches: a review. Journal of Micromechanics and Microengineering. vol. 32, no. 6.

This article was written by Danny Czaja, CEO, HT Micro (Albuquerque, NM). For more information, contact Mr. Czaja at This email address is being protected from spambots. You need JavaScript enabled to view it. or visit here .