As Industry 4.0 evolves, the use of more sophisticated robots, as well as advanced automation and control systems within industrial applications has become more common as companies continue their pursuit to increase efficiency and profitability.
The advancement of automated technology has helped drive the development of collaborative robots (cobots). They are robots that can work alongside or with humans. These cobots are intended to interact with and assist workers as opposed to being standalone automated equipment with little to no human interaction. Historically, this interaction was limited due to safety concerns and the fact that humans and robots were not able to work together safely, effectively, and efficiently.
Today, cobots offer much greater flexibility in the workplace since protective cages are no longer needed for safety purposes. They have been developed to work seamlessly with humans and can perform tasks together that neither humans nor robots could do on their own. This flexibility helps increase return on investment (ROI) due to increased productivity and reduced labor costs.
Cobots can also handle complex or dangerous tasks that humans either cannot complete or cannot perform safely. For example, cobots have the capability to perform surgery with a degree of precision and steadiness that is beyond the capabilities of even the best human surgeon. On the safety side, cobots can handle toxic materials and operate in environments that are not suitable for humans.
From product manufacturing operations to industrial packaging applications — and everything in between — cobots play a significant role in increasing productivity. As companies continue to find new ways to integrate cobots and the benefits of utilizing cobots become better understood, this is leading to enormous growth in their deployment in factories and manufacturing facilities. According to the International Federation of Robotics (IFR), 22,000 cobots were installed in 2020 — 11,000 more than in 2017. Additionally, global collaborative robot sales grew more than 25 percent since 2018 even though the overall industrial robot market observed a pandemic-induced slowdown. With this increase in the use of cobots, there has been a focus on regulations that can confirm a safe work environment and protect personnel working alongside them.
Protecting Cobots and Their Co-Workers
The trend of using torque sensors in cobot applications has been driven by the benefits that the sensors provide to the system. Torque sensors allow for a fast response time and improved accuracy, which leads to safer and more reliable cobot systems as well as increased human safety. Using a torque sensor in each arm joint allows direct detection of external force or torque exerted on the arm, rather than relying on complex calculations derived from motor currents. This enables a very fast stop on contact, and therefore can allow faster arm motion in collaborative situations where forces on any human contact must be limited. A further benefit is found when the cobot is operating in compliant mode. “Active” compliance allows the arm to be guided by hand (lead-through programming), simplifying the process of teaching the cobot new tasks. Direct detection of joint torque enables smooth and precise control of arm motion as it is guided.
A typical cobot arm is shown in Figure 1. The rotating joints, indicated as black bands, enable the arm to have six degrees of freedom: linear motion along the x, y, and z axes and rotation about each. Inside each of these joints is an array of functional components, including bearings, electronics for motor control, motor, brake, and gear box. There is also a safety torque sensor, which monitors the mechanical torque in the rotational pivot points on the cobot. It will cause the cobot to stop and retreat to a safety position if it senses unusual resistance.
The TE Connectivity (TE) safety torque sensor starts with a one-piece body that is designed to translate rotational torque into mechanical strain. The assembly starts with a solid metal disc that has four distinct areas. In the outer circle there is a series of bolt holes for attaching the sensor base to the cobot’s arm. Just inside of that is a circular area for mounting two circuit boards that provide connectivity and electronics. Next, is an area of extremely thin material called the flexure. The inner-most circle has another set of bolt holes for attaching the sensor to the motor drive’s gear box. In this way, the sensor assembly acts as a coupler, transmitting the motion from the gear box to the movable arm (See Figure 2).
The strain is detected in the flexure area when a turning motion (torque) is applied to the moveable arm. The strain gauges can detect flexure twisting of only a few microns. They measure the strain on the flexure caused by the difference in rotational force between the outer bolt circle and the inner bolt circle. The sensitivity of the sensor is determined by the thickness of the flexure — the thinner it is, the more sensitive — but the flexure has to be thick enough to drive the cobot’s load.
The red squares on the left-hand side of Figure 2 are the strain gauges, mounted to the flexure by an adhesive process TE calls Microfusing. They are assembled in a Wheatstone bridge configuration such that the mechanical strain can be translated into an mV/V output. On the next level, two small PCBs house an ASIC and other electrical components. Each of the silicon piezoresistive strain gauges, shown in the center of the Figure, output a voltage that varies with the amount of strain. The analog outputs of the gauges are wired up to the PCB just above them and then connected to the electronics on the next board. The electronics linearize the signals, temperature compensate them, and digitize them in an I2C format. The data is then output on a I2C bus to a system controller. Depending upon the application, a torque threshold can be established that will allow the cobot to shut down before any humans are injured or before any damage occurs.
The Benefits of Torque Sensors
TE safety torque sensors have been designed to provide accurate torque data while minimizing cross load errors (axial load, radial load, and tilting moment) and can act as a sealing wall in a gear box. To comply with functional safety requirements up to ISO13849 Category 3 PL d, the design is based on a dual channel system and includes other features to detect any safety related failure. TE’s torque safety sensor is leading the way to improved safety and reliability in cobots as those machines continue to drive the evolution of Industry 4.0.