Force Torque Sensors for Robots: External vs. Built-In

Figure 1. Delicate deburring with force feedback. (Image: Bota Systems AG)

Force torque sensors are gaining more and more popularity in robotics applications — a clear trend is evident. The key drivers include the growing use of robots in unstructured environments, where they are required to perform more complex and demanding tasks, while working in cooperation with human collaborators.

Robot vendors frequently offer a built-in force torque (FT) sensor that is integrated into their robot as a standard part or offered as an optional extra. While this may be more than adequate for common applications such as pick-and-place tasks, a separate external force torque sensor offers superior precision and flexibility for more demanding and sophisticated use cases.

Integrated sensors are a fundamental part of every modern robot, so to be practical, they have to have lower cost than specialized external sensors. This in turn tends to result in their being lower in quality. Integrated sensors are therefore generally not as precise as external ones, so they require additional filtering and software algorithms to achieve the required accuracy, which then leads to undesirable latency.

What Is a Force Torque Sensor?

A robotic force torque sensor is a device that simultaneously measures the force and torque acting on a given surface area. This measurement is quantified, then recorded, or used to provide a real-time feedback signal, for example in human-robot interactions. The most widely used sensor of this type is the six-axis force torque sensor, where the forces and torques from all three Cartesian coordinates are measured simultaneously.

Today, force torque sensors are implemented in a wide range of industries and applications, though most commonly in collaborative robot (cobot) applications. Measuring signals in real time for feedback control enables robots to perform extremely challenging and highly precise human-machine interaction tasks. The most common applications are found in assembly, quality testing, robot-assisted surgery, teleoperation, and surface finishing. These applications are characterized by the force torque sensor acting as an interface mechanism between the robot and end-of-arm-tooling (EOAT).

Figure 2. Bota Systems force torque sensor for an EN ISO-9409-1-80-6-M8 mounting flange. (Image: Bota Systems AG)

A typical robot-tooling interface configuration utilizing a force torque sensor is shown in Figure 2. No mechanical adapter is required, since the Bota force torque sensor is mounted directly onto an EN ISO-9409-1-80-6-M8 flange.

The sensor features an M8 connector electrical interface utilizing an RS422, USB, or EtherCAT connection. When using the RS422 or USB option, the sensor is powered directly from a USB port and requires 5 V/1.5 W. With EtherCAT, the sensor is powered using an external 7 V to 48 V power supply with a power consumption of up to 1.5 W.

Force Torque Sensor Technology Explained

There are various materials and technologies that can be used to manufacture force torque sensors. Bota sensors are constructed utilizing resistive metal foil strain gauges, which are very robust components, providing consistent linearity, low drift, and good temperature stability. Bota strain gauges have been developed and optimized over a period of several decades to deliver superior results, surpassing other weighing methodologies such as optical, infrared, etc.

Bota sensors manufacturing takes steps to minimize hardware nonlinearities in order to eliminate accumulated errors that cannot be resolved via software algorithms. These steps include temperature and gain offset compensation, ratiometric voltage conversion, symmetrical loading, EMI shielding, parasitic capacitance, and inductance compensation, all of which minimize errors and maximize the signal-to-noise ratio.

The final quality of the signal and its latency greatly depend on the quality of signal conditioning and the data acquisition electronics. Bota sensors generate low-noise output signals, which coupled with their high data acquisition rate, reduce the need for additional filtering, which minimizes signal delays.

The sensor and interface electronics are fully integrated and shielded inside the sensor housing. This significantly reduces wiring, weight, complexity, and measurement uncertainty, while enabling advanced features such as inertial compensation.

A Comparison of Integrated and External Sensors

A Bota Systems force torque sensor was installed on a 20kg-payload robot and compared against the robot’s integrated force-torque sensor. Although this was a test on a specific robot, it is a typical application that, based on the company’s long-term experience, should provide objective comparative results.

Figure 3. Bota Systems FT Sensor fit for a 20 kg payload robot. (Image: Bota Systems AG)

Figure 3 shows that the Bota Systems FT sensor payload capacity fits the payload curve of the 20 kg robot, while the integrated/built-in sensor does not appear to be covering the robot’s entire area of operation. For example, a load of 100 N applied at a distance of 600 mm perpendicular to the reference frame of the tool flange induces a load of 60 Nm located outside of the operating range of the built-in sensor. Therefore, the internal sensor prevents the robot from performing force-sensitive tasks across its entire operating range, effectively limiting its payload and range capabilities in important applications such as surface finishing or assembly.

Table 1. Bota FT sensor vs. built-in FT sensor

The Bota sensor outperforms the built-in sensor across many criteria (see Table 1), where precision, noise, and accuracy are all substantially better. The maximum data sampling rate is also four times higher, at 2 kHz compared to 0.5 kHz.

An alternative approach for calculating the six-axis force vector at the robot’s end effector is by measuring the torque at the output shafts of the joints by using torque sensors (see Figure 2). Torque sensors are installed on each of the joints. The measured torque at each joint, combined with the joint’s position measurement and robot kinematics, is theoretically sufficient for calculating an estimate of the force vector at the end effector.

At first glance, this appears to be a neat and convenient integrated solution; however, in practice, it has several disadvantages. The noise level of the calculated six-axis force vector is not ideal since it is derived from the robot’s configuration. For example, when the arm is fully stretched out and certain components of the end effector force are absorbed by the robot’s structure, it cannot be detected by the joint torque sensors.

Precision Force Torque Sensors in Action

Robotics companies often focus on basic applications such as stacking pallets or welding, where built-in force torque sensors are generally good enough. However, there is a growing array of applications that require a higher level of sensing quality, for example, precision assembly applications or grinding and polishing tasks.

Surface finishing applications such as material removal, polishing, deburring, or deflashing, are some of the dirtiest, most unpleasant, and difficult tasks in the field of manufacturing. The use of force torque sensors and the implementation of direct force control enables the effective automation of these challenging processes. Rapid and accurate feedback is necessary to provide a consistently high surface quality for even the most complex shapes.

An additional benefit is that the use of external force torque sensors increases manufacturing flexibility, making it easier to adapt an existing system to create new applications by selecting a different sensor. This means customers can cost-effectively work at a smaller production scale, flexibly applying multiple variations of a similar part or even smaller batches of a specific part. It also makes it possible to modify a robotic system for a given time, for example, the installation of a different sensor for a short-run project running for just a few weeks.

Software, Support, and Tool Kit

Bota force torque sensors are provided as a complete kit, including integrated electronics, cabling, power supply, adapters, fasteners, etc. It also includes a USB stick containing all the required drivers and code for common platforms such as Python, TwinCAT, URcap, Stäubli Robot Studio, Matlab, LabVIEW, ROS, and Windows.

The package comes with sample code to save development time in setting up the most common applications. This code can be used to create the building blocks for various tasks, making it easier to write the complete program required by the customer.

Not All Sensors Are Created Equal

Robots need to be both smart and flexible, but without compromising on safety, which remains the number one priority. Force control addresses these key challenges, providing real-time actionable data from highly accurate and reliable force torque sensors.

It’s important to remember, however, that not all force torque sensors are created equal. External sensors have numerous advantages over standard built-in options, typically being far more precise and flexible.

This article was written by Klajd Lika, CEO, Bota Systems AG. For more information, contact Mr. Lika at This email address is being protected from spambots. You need JavaScript enabled to view it..