Tech Briefs Magazine - June 2022 - Sensor-15

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
confi rm 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 benefi ts 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 benefi t 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.
Inner Workings
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.
Sensor Technology, June 2022
How it Works
Basic function
1
2
3
4
Attach the sensor with
bolts to the robot arm
Add a drive shaft from a motor or gear
box to the center flange
Apply a torque and twist the shaft
The output signal represents the twisting
force applied thru the torque sensor to
move the arm
1
2
3
4
5
6
20
40
60
80
APPLIED TORQUE (Nm or ft-lbf)
100
120
Figure 3. The TE Connectivity Torque Sensor - how it works. (Image: TE Connectivity)
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 fl exure. 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 fl exure
area when a turning motion (torque) is
applied to the moveable arm. The strain
gauges can detect fl exure twisting of only
a few microns. They measure the strain
on the fl exure 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 fl exure - the thinner
it is, the more sensitive - but the
fl exure 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 fl exure by an adhesive process
TE calls Microfusing. They are assembled
in a Wheatstone bridge confi guration
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.
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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.
This article was written by Pete Smith, Sr.
Manager Sales Support, TE Sensor Solutions.
For more information, contact Mr. Smith
at pete.smith2@te.com or visit http://info.
hotims.com/82322-163.
15
ST TE Conn Tech Leaders Feature 0622_1.indd 15
Cov
ToC
5/18/22 3:02 PM
OUTPUT SIGNAL (Data)
http://info.hotims.com/82322-163 http://www.techbriefs.com http://info.hotims.com/82322-850

Tech Briefs Magazine - June 2022

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