Ultra-Thin Force-Sensitive Resistors Enable Greater Command of Robotic Manufacturing

Every product endures a complex manufacturing journey that begins at assembly and ends at delivery. Especially in robotic manufacturing operations, this journey often involves programming a multifaceted, meticulously calculated sequence of manufacturing touch points to be executed in a precise order.

As with anything moving at high speeds, the potential for mistakes is a concern for everyone working in a high-volume manufacturing operation. Design engineers that develop robotic machines, control & diagnostic devices, and other smart manufacturing tools are challenged by growing demands for better production efficiency. These challenges are compounded when considering the quality and regulatory standards that are required with medical devices and products; where the slightest misstep in manufacturing could potentially impact a human life and cost lots of money.

With recent technological advancements, manufacturing systems embedded with force-sensing technology have been introduced to help operators feel what is happening on the production floor. These systems are optimized to deliver important insight on key aspects of the manufacturing process that could be going unnoticed.

Common Force-Sensing Alternatives Used in Automated Manufacturing Systems

There are several factors to consider when selecting an embedded force-sensing technology for a manufacturing application. Load cells, strain gauges, and force-sensitive resistors are common force measuring technologies used in robotic and other manufacturing systems, but each offer different benefits and limitations that will play a role in the final decision.

Load cells are the most well-known force sensor type used in manufacturing systems. They offer the highest level of accuracy among the other options, which can be important for operations where too much or too little applied force in a process can make a difference in product quality. However, load cells are heavier, larger, and often require more expensive electronics to operate than other alternatives. As a result, embedding load cells into certain machines or devices can be a challenge.

Strain gauges are much smaller than load cells, but yield measurements that are a result of an indirect force measurement drawn by correlating the strain of fine conductive wires with a load. This adds complexity to the electronics, and like load cells, strain gauges can be quite expensive to integrate and operate.

Piezoresistive sensors, or force-sensitive resistors, consist of semi-conductive material sandwiched between two pieces of flexible polyester that act as a force-sensing resistor in an electrical circuit (see Figure 1). Force-sensitive resistors need to be calibrated, and are not as precise as a load cell. However, force-sensitive resistors’ thin, flexible, minimally invasive properties may help address many other challenges in the embedding process. Also, force-sensitive resistors can usually be designed in a variety of shapes and sizes to meet specific application and device needs.

Force-sensitive resistors are used in a variety of applications, including:

Detecting and measuring relative changes in force or applied load.
Detecting and measuring the rate of change in an applied load.
Identifying force thresholds and triggering appropriate actions.
Detecting contact and/or touch.

Of the different types of force sensors, force-sensitive resistors offer several benefits that make them an ideal technology for embedding into robotic manufacturing systems, and other smart manufacturing devices.

This graph illustrates how a force-sensitive resistor works in an electric circuit. When the sensor is unloaded, its electrical resistance is high. As force is applied, this resistance decreases. This resistance change can be customized depending on the needs of the application.

Force-Sensitive Resistors Used in Robotic Manufacturing and Other Smart Systems

In today's data-driven world, there's no such thing as having too much information. Embedding force-sensitive resistors to capture critical force and pressure impact data will lead to greater efficiency and a superior end product.

The following application examples showcase ways that force-sensitive resistors have enhanced the manufacturer's ability to take control of critical manufacturing processes, specifically those involving automated technologies like robotics.

Connecting Man and Robotic Machine with a Sense of Touch. Historically, there have been limitations with grip measurement on pick-and-place robotic systems. As mentioned, traditional force-sensing methods like load cells can be difficult to embed into robotic systems due to their size, weight, and other limitations. For these reasons, robotic system designers have turned to force-sensitive resistors to help solve robotic grip measurement challenges.

Force-sensitive resistors have multiple applications for embedding into robotic systems. For instance, they can be positioned on key areas of the robot's grippers or arms to ensure safe, accurate, and more independent handling of products and equipment. Alternatively, in robotic applications where an operator is controlling the movements a robot makes, force-sensitive resistors embedded onto the control devices can help identify the level of grip forces the operator applies. This can simultaneously trigger an appropriate action by the robot.

In medical device manufacturing — where fragile or potentially biohazardous materials can be present — having the ability to quantify a robot's performance ensures that the product or device is being assembled to exact specifications.

Stabilizing or Aligning Robotic Components. Robotic systems are programmed to perform a specific series of actions while responding appropriately to environmental feedback. However, it is not wise to assume that these systems will remain aligned over repeated use.

Whether used as part of a machine setup process, or to monitor real-time changes during production, force-sensitive resistors can help ensure that robotic systems are properly aligned. As vibrations or other impacts occur on the robot, force-sensitive resistors positioned on key contact points — such as a robot's arm — can provide force feedback that keeps the operator in sync with how the machines are performing. If misalignments occur, these systems can help the operator instantly pinpoint specific areas for adjustments that may not have been identified with other methods. This can be especially valuable for enhancing and streamlining quality control checkups.

Embedding Product Units for Feedback in Assembly Line Impacts. There are several twists and turns that a product endures on its way to completion. If a medical device or product is contained in fragile packaging or has fragile parts, too many force impacts can lead to damage and product loss.

Embedding force-sensitive resistors around product samples and sending them through an assembly line can record any impacts experienced by a product. This includes forces applied by robotic grippers, rail lines, and even impacts from adjacent product units. Data captured by these force-sensing product units can help operators take the necessary precautions to make sure their systems are running smoothly.

A similar application is currently used in bottle and can fill lines, where fabricated product units wrapped with force-sensitive resistors are used to wirelessly deliver force impacts to help limit product breakage.

Streamlining Inventory Management. Inventory management is a time-consuming aspect of any manufacturing operation. However, force-sensitive resistors can be used to capture actionable inventory data through changes in force.

Force-sensitive resistors embedded onto shelves, or positioned below storage bins, can provide instant force readings as materials are removed. With this application, data that would normally have to be acquired via on-the-floor checkups could be synched with the operation's inventory management system to provide real-time reports.

Force-sensitive resistors can be embedded into robotic assembly arms, such as the one pictured. They can either measure grip force applied by the robot itself, or provide force data on the physical grip applied by the robot's operator.

Taking this application one step further, these “sensorized” shelving or storage units could be drivers for an IIoT-optimized wireless communication system between a manufacturing operation and a third-party distribution center. As units are removed from the shelves, the tracking system could send an alert to the distributor to automatically schedule a new delivery of materials. This system can help supply managers stay efficient, save costs, and even protect against theft.

Smart Packaging for Secure Product Delivery. Today, most global shipping companies offer tracking services that provide real-time reports on where a package is at any point in transit. What these tracking systems currently lack is information on whether any potentially damaging impacts to the packaging occurred on its way to delivery.

Force-sensitive resistors embedded onto packaging, or positioned between product units, can ensure safe delivery of delicate medical devices, products, and even sensitive medications. Because force-sensitive resistors are thin, flexible, and only require simple electronics to operate, they can often be developed to conform to or around specific areas or shapes.

Putting Medical Device Manufacturers on a Path to Production Efficiency

As demand for automated manufacturing systems continues to grow, now is the time to consider how embedding force-sensing technology into robotics and other smart manufacturing systems can help give your design a competitive edge. When evaluating force-sensing technologies, there are four key questions to keep in mind:

What force sensing technology can effectively and economically be integrated into the product design?
What mechanical or electrical requirements or obstacles may impact the force-sensing technology?
Is there a specific force or sensitivity range that needs to be maintained by the robotic system?
Is the force sensor technology ISO 13485 certified, and backed with a support group of expert engineers with decades of experience working with medical device manufacturers?

There is a wealth of important force impact data that many of today's medical device manufacturing systems simply are not equipped to capture. This is an opportunity to win customers over by capitalizing on existing, proven technologies that enable greater control over key phases of the production process.

It may not be physically possible for customers to take the exact manufacturing journey their devices take, but developing systems andtools optimized with force-sensing technology will empower them with a quantifiable feel for their production landscape.

This article was written by Mark Lowe, Vice President of Sensors for Tekscan, Inc. (South Boston, MA). For more information, Click Here .