Frequently Asked Questions

General

How does a force-torque sensor work?


A force-torque sensor is an electronic device that converts the physical quantity of force and torque to a signal that can then be read by a human and/or a robot. An object can experience force and/or torque from various sources. In continuum mechanics, bodies are considered deformable. When a force is applied, deformation takes place. Deformation is directly related to force applied. One way to measure deformation is by attaching a film of variable electrical resistance to the deformed body surface by glue or other methods to share the same deformation. The resistance changes when the body elongates or compresses, due to force applied. This change is then captured by an Analog to Digital Converter (ADC) and is recorded as a digital counter. Then, it is processed and sent to the communication channel by a microcontroller to the network of the system for further processing. A simplified example of processing: When a force of 100 N is applied the digital counter records a change of 1000 counts. The most simple sensor design is to have a linear relation between force and change of counts. In this case the force is directly proportional to the change of counts by a factor of 10. If a change of 1500 is recorded then a force of 1500/10 = 150 N is applied. It is safe to say that if a change recorded is divided by 10 we can simply calculate the value of force applied. This is what happens on a digital weight scale. The only difference is that weight scales show the mass of an object by dividing force with the gravity acceleration of a specific location (usually where they are calibrated). For a 6-axis force torque sensor, three force and three torque components of the force/torque vector are applied and six signals from six different variable resistances can be recorded. Each one of these components affects less, or more, individual variable resistances that are located in different spots on the sensor body.




How do I choose a sensor?


Sensor selection is primarily dependent on the application. Each application is unique and requires a force torque sensor to have certain features, ranges, or software specifications. Check out our blog post on how to select a force torque sensor for more details. To get the best force torque sensing solution, we recommend contacting us and sharing your application details.




How do I calibrate the sensor?


Bota Systems sensors come pre-calibrated with a calibration matrix integrated into the device. The only calibration required is an offset calibration that can be done externally by gathering data while the sensor is steady and no change in the dynamic state is happening. After the data is averaged, it then should be subtracted from the current measurements.




Can my Bota Systems sensor become more sensitive?


Sensor sensitivity is directly related to the sampling frequency. The internal sensor filters can be utilized to increase the sensitivity. The user manual provides a table with all available filtering options and the resulted resolution after a filter is applied. Note: The internal filters are hardware filters optimized for a force-torque sensor signal. One can potentially run the sensor at full speed and apply their own filtering.




Rokubi force-torque sensor has a range of 1200N. Is this suitable for an application that requires a lower range?


It depends on the maximum force being applied and the resolution requirements of the application. The Rokubi sensor has 0.2N noise-free resolution in z-axis at 1000Hz and a range of 1200N. For example, if the maximum force to be applied in the z-axis for your application is 300 N and the resolution required is more than 0.2N then Rokubi is a sensor that can be used for your application.




What is the first thing I should do when I receive my sensor?


Unbox the sensor carefully and read the User Manual before operating with your FT sensor. We recommend navigating our FAQ, like you're currently doing, to understand how our sensors work and common concerns with operating FT sensors.





Technical

Do I need any extra accessories to install the sensor into my application?


Bota Systems sensors are designed to minimize integration efforts and have the smallest footprints and integrated flanges. Sensors include:

  • All necessary wires & cables.
  • Electrical adapters are NOT required. Our Serial and EtherCAT sensors come with all necessary accessories to mount on a USB or Ethernet port and directly connect with a PC.
  • Electronic boxes and signal amplifiers are NOT required. Bota FT sensors are integrated with all electronics for signal transformation. Check out our force-sensing kits for Universal Robots and Mecademic Robots.
Featured image: Bota Systems' Rookubi Force-sensing Kit for Mecademic's Meca500 robot.




What is drift? Why is it important?


Drift is when controllers behave as if force is being applied to a robot when it is not. If you want reliable and accurate force measurements, drift must be managed/minimized, and when managed properly, it increases a robot's productivity.




How to manage drift?


To manage drift, you must first discover the source. This can be dependent on the type of application, type of sensor, and the environment. Depending on your drift source, you should respond with an appropriate solution. Below are types of drift sources and ways to manage drift. Check out our comprehensive guide on how to manage sensor drift for more details, or contact us to discuss solutions. Drift Sources:

  • Heat
  • Mechanical creep
  • Sensor design
Tips and solutions :
  • Offsetting the sensor measurements from the bias regularly, or even every couple of seconds, depending on your needs.
  • Warming up a sensor before using it.
  • Stop, reset the sensor, and once stabilized, start again.




Does my force-torque sensor need IMU?


Force torque sensors are mainly used to measure the contact forces when a robot interacts with an object. Besides contact forces, inertia forces/torques are commonly applied to the robot's end-effector while it is moving. Gravitational forces being one of them. Even in quasi-static motion, these forces are present and are measured with the contact forces, leading to inaccurate contact force measurements. To compensate for additional forces, a calibrated IMU is required to give a clear contact force signal.
For sensors with IMU, check out our Rokubi EtherCAT and SensONE EtherCAT product pages.




What is the difference between Serial and EtherCAT?


Venn diagram showing the difference between Serial and EtherCAT sensors Serial Our Serial devices are ideal for force-torque sensing beginners because a significant amount of documentation is available to support them, and they are easier to code. EtherCAT EtherCAT devices are generally more expensive and include extended features compared to their Serial counterpart. One may consider the EtherCAT version as a premium industrious sensor. EtherCAT protocol supports high bandwidth, real-time communication with robots, and can connect to a huge network of devices.

  • Supports high bandwidth- can theoretically support and communicate with up to 65,000 devices.
  • Supports real-time communication with robots (can serial not do this)
  • A power supply range of 7 to 70 V
  • Suitable for mobile applications (is serial not?)
  • Has integrated IMU, which allows the user to use the Bota Dynamic Measurements.
For example, at Bota Systems and Robotic Systems Lab of ETH Zurich, we use EtherCAT for high-speed control of quadrupedal robots with up to 16 actuators, four sensors, and many other peripherals where synchronization is a critical factor to making the robot balance successfully. Because EtherCAT is powerful, it needs a lot of complex coding to parse data correctly, and this can make integration complex if not familiar with the coding process. That is why we support our devices with ready-to-use code, examples, and are continuously working on new development to facilitate integrator's needs. Serial & EhterCAT Both include:
  • Temperature sensors
  • Hardware filtering options
  • Adjustable sampling rate and resolution
  • Dustproof and water-resistant
  • Similar in size and weight




What is repeatability, and why is it important for force-control applications?


Repeatability is the ability a sensor or system has to regenerate a signal under the same loading and environmental conditions. For robotic force control production applications, the ability to replicate a product tens and thousands of times is important for the business to operate smoothly. For example, a polished orthopedic implant should be consistent in surface texture, so that the finished product is both compliant and safe for the patient. For surface consistency, a robot performing the polishing process needs to apply the same force for thousands of units. For this to be possible, a sensor with high repeatability in measurements is necessary. Note: Repeatability (precision) and accuracy are two different things. Accuracy is a metric that can be calibrated and calculated, but repeatability is connected to the sensor's design and manufacturing. accuracy vs. precision illustration





Software

What should I do to take N and Nm units from the sensor?


Nothing! The sensor outputs data directly in N and Nm.





Installing

How do I install my sensor?


Installing FT sensors can be tricky. That is why Bota Systems sensors are highly-integrated with embedded electronics, which means, other than cabling, you do not need to work with any extra components outside of the sensor's housing. We recommend referencing our Bota Systems FT Sensor User Manual, where you will find step-by-step instructions on how to install our sensors , or by heading over to our YouTube Channel. Looking for software installation instructions and more information? You can find this on each of our sensor product pages.




What should I do to minimize drift from temperature change?


  • Surround the sensor housing with a thermal isolation material like rubber.
  • Do not allow the heat to flow from multiple directions.
  • Do not operate under shocking temperature changes.
  • Calibrate the temperature compensation when the sensor is mounted to the robot or tooling. By mounting, this will take into account the heat conduction capabilities of the sensor and thermal conductors surfaces. If mounting to plastic, or other thermal isolation, an adapter is an option for minimizing heat transfer.