Tech Briefs Magazine - May 2024 - Sensor-11

TECHNOLOGY LEADERS Sensors
Force Torque Sensors for Robots:
External vs. Built-In
F
orce 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 pickand-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 inSensor
Technology, May 2024
Range (Fx, Fy, Fz)
(Mx, My, Mz)
Noise 1σ (Fx, Fy, Fz)
(Mx, My, Mz) at 100Hz
Precision
Accuracy
Data Sample Rate
Bota Systems
+/- 2000, 4000N
+/- 70, 70Nm
70mN, 50mN
2mNm, 0.7mNm
<1%
<1%
Up to 2 kHz
Table 1. Bota FT sensor vs. built-in FT sensor
teraction 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).
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
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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
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Tech Briefs Magazine - May 2024

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