Brett Hansen and Dr. Stephen Schultz, Electrical and Computer Engineering
There are various applications for shock sensors, ranging from cell phones, to postal packaging, to automobile safety designs. These devices are designed to indicate when a predetermined threshold of impact force has been experienced. The designs of these sensors are varied. Some employ electrical stimuli while others utilize magnetic force. Much of the fabrication complexity and cost comes from the packaging and need to supply power to the sensor. To make practical the wide spread, every day use of shock sensors (i.e. postal packages), the sensors must be very cheap to make and require no electrical power. Therefore, there is a need to create zero power shock sensors. By making shock sensors that require zero electrical power, the connection and packaging will be practically eliminated.
We have been successful in designing a prototype shock sensor as is shown in Figure 1. It is possible to achieve a zero power shock sensor through use of a bistable compliant mechanism. A compliant mechanism is a device made from a single component that is designed to be flexible. The device is bistable, meaning that like a light switch, for example, the device will have two states at which it can rest. When an external force is applied to the device, it will switch positions, and will then stay in that position to allow it to be read. We fabricated this prototype by using an experimental procedure of laser-cutting.
Upon beginning this project, we had several specific objectives. First, we desired to experiment with different types of plastic in an effort to determine which type of plastic would be most appropriate for this design. Next, we planned to learn more about altering the dimensions of different parts of the design, with the goal of achieving optimal specifications. Finally, we wanted to implement a simple indicator for the system that would make it a simple matter to ascertain if the system had experienced enough impact force to switch to its alternate state.
After experimenting with various types of plastic, we found the best kind of plastic to be used in this type of design is called Delrin. This plastic stands up relatively well to laser-cutting, and so provides a high yield of devices during the fabrication process. However, it was discovered that the stress that the design applies to a plastic can have some adverse consequences. When plastic is stressed, it can exhibit a behavior known as creep. This means that the plastic becomes less stiff and so its behavior will be altered. It is the equivalent of a spring that becomes overstretched beginning to lose some of its characteristic behavior. This problem is currently being investigated in two ways. First, by use of computer models and further experimenting, we are developing ways to predict the creep that Delrin will exhibit. Second, we are also investigating ways to fabricate this design in metal. Metal is known to exhibit less creep, but creates manufacturing obstacles with a design that features such fine dimensions. As such, other types of fabrication methods are currently being explored. Both avenues look promising with further study.
We also have studied the effect of changing various dimensions of the device, including such dimensions as thickness, leg length, and leg angle. These dimensions change based on the material that is used. A balance must be struck between the stiffness of the material, and the amount of stress that the material can take before breaking. With delrin, successful sensors are fabricated that are a less than a tenth of an inch thick. Simulations predict an even thinner sensor will be possible with metal designs, but with an increased size in surface area. In short, we believe that we have found optimal parameters for our plastic design, but the metal design is continuing to evolve.
Finally, we have investigated how the system can be used on a large scale that will allow the average person to use it. Originally, we planned to use a color-coded indicator (as seen in Figure 1). While this was effective, there was a need to make it even more simple to read if this were to be employed on a mass scale. We have introduced RF ID tags onto the sensor. These tags basically transmit different data based on the position that the sensor. They also still maintain a zero power environment. This allows users to simply scan over the sensor, without actually having to see it, and determine if it has experienced large impacts. This is extremely useful in applications such as shipping and freight.
In conclusion, we have developed a shock sensor that requires no electric power, and that is able to be used easily by the average worker. Research is ongoing into different materials that can be used for bistable compliant mechanism designs, with the goal of achieving a design that is easy and inexpensive to fabricate, but also minimizes the amount of creep that is seen by the system.