Bota Systems Medusa Pricing & Product Details

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 the 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, the individual variable resistances that are located in different spots on the sensor body. By linear combination of the six signals through an individual calibration matrix, the physical quantities force and torque are obtained.

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.

Bota Systems sensors come pre-calibrated with a calibration matrix saved on 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 can be subtracted from the current measurements.

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.

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.

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.

Drift is when sensors 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.

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 :

• Mount the sensor according to the specifications, use all screws and pins.
• Operate the sensor in steady environmental conditions, warming up a sensor before using it.
• Offsetting the sensor measurements from the bias regularly, or even every couple of seconds, depending on your needs.
• Stop, reset the sensor, and once stabilized, start again.

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 these additional forces, a calibrated IMU can be used to give a clear contact force signal. Furthermore, an IMU can provide other tactile information like vibrations during slippage and complement the force torque measurements. Often, IMUs are also used for state estimation and it provides directly the sensor orientation with respect to gravity.

All our EtherCAT sensors come with an integrated IMU. Have a look at our product pages to find the model fitting your application.

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
• A power supply range of 7 to 70 V
• Suitable for mobile applications
• Has an integrated IMU for dynamic compensation

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 move 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

Repeatability is the ability of 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 timaes 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.

We define noise-free resolution as the 6 sigma (σ) of a signal. σ stands for the standard deviation of the signal. It is calculated by recording 10 sec of measurements when the sensor is in static conditions. 6σ defines the peak-to-peak range that the noise level will remain below 99.73% of the time.

At Bota Systems, we calculate a sensor’s noise-free resolution, the peak-to-peak values of the signal, as a multiple of the standard deviation σ. We then calculate the standard deviation by recording one second of continuous measurements in a stable environment when the sensor is not loaded. Assuming the noise signal follows a normal distribution.

For example, 6σ means the resolution will have a probability to exceed nominal peak-to-peak values for 0.27% of the time.

Did you know, in most robotic systems, noise is inevitable? However, noise corruption is unacceptable for applications, especially those that are safety-critical, like robotic-assisted surgery.

Unbox the sensor carefully and read the Quick Start Guide and 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 resolve common concerns about operating FT sensors.