Motor feedback systems: Precise path measurement in mechatronic servo technology

Electric power instead of oil, servo actuators instead
of hydraulics – machines designed to reshape, press, or join workpieces in a
fully automated manner are increasingly being
equipped with mechatronic drive technology. As well as considerably
lower operating costs, users also see the infinitely variable control
characteristics as a key benefit. Motor feedback systems such as the SKS36 from
SICK Stegmann, which Ortlieb Präzisionssysteme uses in its SERAC servo
actuators, ensure precise and highly available path measurement and position
control.

Processing workpieces and blanks of
different strengths on one machine, performing several process steps in the
same actuator movement, starting different machining positions steplessly and smoothly – there are numerous
scenarios in which servo technology offers key advantages over hydraulic and
pneumatic drives.

“Mechatronic systems with motion
control functionality are therefore becoming more popular,” explains Niels
Selig, Drive Technology Product Manager at Ortlieb Präzisionssysteme GmbH &
Co. KG in Kirchheim/Teck.

“Here, we are seeing a change in attitude in the market both with new systems
and retrofits, especially as servo technology offers a clear advantage compared
to hydraulics and pneumatics when it comes to operating costs.”

Precise and highly available path
measurement systems – e.g., SKS36 rotary motor feedback systems with HIPERFACEinterface from SICK Stegmann – are used in servo actuators, such as those
in the SERAC series from Ortlieb. This
means that mechatronic systems can perform the reshaping and joining process
with the highest level of accuracy and synchronisation
is possible when several servo axes are used
in one machine.

Ortlieb: ‘PRECISION COMPANY’
for clamping and servo spindle technology

For
more than 100 years, Ortlieb in Kirchheim/Teck has considered itself to be a ‘precision
company’ for efficient and precise solutions in workpiece and tool clamping, as
well as in drive technology.

“A
series of important DIN standards for clamping technology was developed on
the basis of our developments and patents,” states Niels Selig. “From
the standard collet right through to the one-off production of special clamping
solutions, we offer flexible standard and customer-specific options from a
single source.”

The
second, increasingly important pillar of the company is mechatronic assemblies
and drive systems. In spring of 2017, this branch will move into one of the
most innovative company buildings in Baden-Württemberg with the highest standards
in terms of architecture and energy
efficiency. The emphasis is on complete ready-to-connect and ready-to-install solutions made up of
linear actuators which can be configured
in a modular way.

“The
motor, spindle, encoder, and, if necessary, even a retaining brake, are fully
integrated into the electric cylinder,” explains Niels Selig. The company’s
knowledge of spindle technology is behind the exceptional performance of the
SERAC servo actuators.

SERAC servo actuators
combine knowledge from space travel and mechanical engineering

True
to the slogan ‘electric power instead of oil’, with its SERAC servo actuator
series, Ortlieb provides innovative solutions for replacing hydraulic and
pneumatic cylinders. There are different versions available with regard to housing design, motor version,
anti-locking protection, or mounting options. What makes them so special is
their high force density and stability. They stand the test – under the
harshest conditions – in an extremely wide range of industrial punching,
reshaping, and joining applications, as well as when pressing small parts.
Special planetary roller screws, originally developed for space travel, form
the technology behind the SERAC electric cylinders. They feature a reduced
weight and make fine positioning possible without additional reduction gear. Ortlieb developed the ASCA servo spindle
and prepared it for series production for use in mechatronic systems using the
global patent from the DLR (Deutsches Zentrum für Luft- und Raumfahrt e. V. –
German Aerospace Center) as a basis. The option of using the actuator spindles
of many different SERAC versions with small increases of just a few millimetres makes it possible to start up
operating points very precisely with the electric cylinders. This requires encoders which support suitable
accurate path measurement and positioning, like the SKS36 motor feedback
system.

SKS36 motor feedback system
controls fine positioning

SKS36 motor feedback systems with HIPERFACEinterface
from SICK Stegmann are used by Ortlieb in all SERAC product
families. According to Niels Selig, one important constructive advantage is
that: “Their compact design enables short overall motor lengths.” In fact, the
housing diameter is just 36mm thanks to the clever system construction – and
the height without shaft is exceptionally
small at just 37mm. As a result, this motor feedback system, which weighs in at
just 65 g, is one of the most compact encoders of its type. As a singleturn encoder system in the digital
control circuit of the Ortlieb servo actuators, the SKS36 provides values for
commutation and speed, as well as absolute position information, via a
rotation, which is broken up into 128 sine/cosine periods. And it does all this
with the highest level of reliability and accuracy, which is in part thanks to
the very small code disc positioned in the middle of the rotation axis and
which therefore prevents all types of eccentricity errors. On the code disc,
there are 128 lines which are scanned by a specially developed opto-ASIC in such a way that sine signals arise
with high linearity and interpolation capabilities.

“This
means that it is possible to measure the feed rate of the SERAC spindle very
precisely and then use it for fine positioning,” explains Niels Selig. The
reason behind this is that the planetary roller screws do not function using
positive locking, but rather friction locking.

According
to Niels Selig: “As a result, much higher forces can be transferred without impairing the service life of the spindle.
The slip which arises due to the principle used while the spindle is travelling requires an additional linear
measurement system next to the encoder. Therefore, as a result of the
interaction between the two encoder systems, positioning accuracies
< 0.01mm and repeatabilities of ± 0.01mm are possible.”

In
this respect, the SKS36 does not just
stand out because of its high level of accuracy, but also its high availability
and ruggedness.

“The
maximum permissible operating temperature in the servo cylinders is
considerably less than the specified 125 ¡C of the encoder,” states Niels
Selig. “Vibrations and impacts which can arise when punching, reshaping, and
pressing do not stop the SKS36 from
functioning reliably. SERAC KH cylinders with these motor feedback systems can
perform more than 40 million cycles at an interval of three seconds
without any problems when reshaping sheet metal parts, for example.”

Multiturn and safety
solutions for mechatronic systems

In
Ortlieb electric cylinders, the SKS36 singleturn
motor feedback system comes out on top thanks to its precision, easy
integration, and availability. For tasks that require repeated rotary path
measurement and positioning, the SKM36 multiturn motor feedback system provides
the option of measuring up to 4,096 rotations with the help of the integrated
micromechanical drive and displaying in a resolution of up to 16,777,219 steps.

When
taking into account the safety considerations for mechatronic systems and drive
technology solutions, the SKS36S and SKM36S SIL2 motor feedback systems from
SICK STEGMANN are also highlighted as
safe encoders in dynamic and precise servo control circuits. Both systems can be used in applications which comply with the
SIL2 classifications according to EN 62061 and IEC 61508, as well as the
performance level (PL d) according to EN ISO 13849-1. They, therefore, provide
certified functional safety – in other words, the option of monitoring
functions and function chains of servo actuators and mechatronic systems in a
safety-oriented manner with regard to
speed, direction of rotation, and downtime. As a result, motor feedback systems
from SICK Stegmann offer numerous options for path measurement and positioning
in mechatronic systems – and not just in Ortlieb electric cylinders.

Written by Kathrin Kritzer,
Motor Feedback Systems/ Motion Control Sensors Product Manager, SICK STEGMANN
GmbH, Donaueschingen

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