HeavyDuty Absolute Encoders for Drives
with Large Shaft Diameters
The authors:
Ralf Schenk is Director of Marketing and
Bernhard Hiller is Director of Research &
Development at Baumer Hübner GmbH in
Berlin, Germany
Ralf Schenk, Bernhard Hiller
Up to now, drives with a large diameter hollow shaft, such as
torque motors, have often been fitted with encoders for the acquisition
of the rotor position, which are mounted on the side and driven
through toothed belts. But this design is heavy on maintenance
and not very robust. Absolute encoders that are mounted directly
on the shaft offer a solution that is both maintenance-free and stands
up to rough environmental conditions. The example of an encoder
installed in a high-power shredder plant is used for a detailed explanation
of their advantages. The components that are required for a
hydraulic solution make it very expensive: it needs an electric motor to drive the hydraulic
pump, a hydraulic motor to drive the actual machinery, as well as piping
and hose installations for hydraulic oil and the cooling water feed. Not only that, but
hydraulics is heavy on maintenance and has a poor level of efficiency.
Drive units fitted with asynchronous motors and output gearboxes, which are
used in large numbers, also have their weaknesses. This is because the material
being shredded frequently contains massive lumps that can only be broken
up with great difficulty. Again and again, they suddenly bring the shredder rotor to
a complete standstill – even though the The shredding of waste material and scrap
that is required for recycling is a high-tech operation that makes heavy demands on
the solidity of all the drive components used in the plant. In order to get an idea
of the stresses that have to be withstood by the encoders that are used, we will first
of all explain the method of operation and the drive concept for various commercially
available systems [1]. The heart of a shredder plant is usually a
large-diameter rotor shaft made from solid steel, which can weigh several tons. The rotor
body is fitted with replaceable tool bits, with a size, shape and quantity adapted
to the material to be shredded. The shaft is mostly driven by an electromechanical
drive, consisting of an asynchronous 3-phase motor, a turbo-clutch and a
belt drive or universal-joint shaft with a gearbox. Very large systems generally use
an electro-hydraulic drive. However, both concepts involve disadvantages.
ENCODERS AND SENSORS
drive motor is adequately dimensioned. These blockages can indeed be removed
by an automatic reversal built into the machine control system, so that the rotor
moves backwards and forwards repeatedly until the interfering lumps have been
broken up. But one can easily imagine that the drive components (motor, gearbox
and clutch) suffer severe shock loading in the process, as fast-rotating components
have to be braked rapidly to avoid damage to the machinery. And this freeing cycle
generated by the automatic reversing puts a heavy strain on the drive components.
New drive concept for shredder plants
2: HeavyDuty absolute encoder on the torque motor in the shredder
achieve reliable acquisition of the position of a hollow motor shaft with a diameter of
up to 300 mm. This is described in detail below.
Up to now, drives with a large diameter hollow shaft have often been fitted with
absolute or incremental encoders for the acquisition of the rotor position, which are
mounted on the side and driven through toothed belts. However, this solution
involves several disadvantages. The belt is subject to heavy wear, especially in
arduous operating conditions. It also has to be replaced at regular intervals, thus
causing a down-time for the system. In addition, suitable tensioning equipment
has to be brought along for the maintenance work. The position information
required for commutation is thereby lost, and has to be re-established with the help
of special equipment before the system is commissioned again, thus making
the maintenance operation even more involved. And there are well-founded fears
that, in extreme cases, the shock loading could cause the toothed belt to jump over
several teeth on the pinion. Furthermore the rotary movement is transmitted to the
shaft of the encoder, and its ball bearings are subject to above-average wear, because
they have to withstand high radial forces. (Fig. 1). Two tracks (elastomer strips) that
have embedded magnetic patterns are to be found on the outer face of the encoder
wheel. These are detected by sensors that are mounted in the sensor head. Fig. 2
shows the encoder that was built into the shredding plant. Integrated signal processing
is used not only to derive the absolute information, but also to simultaneously
generate incremental sin/cos signals for dynamic control of the system. If necessary,
the elastomer strips can be covered by a special stabilizing bandage, to provide
additional protection. 1: Encoder wheel and sensor head
A new drive concept is implemented in the plant shown in the title picture. The
picture is to be interpreted as follows: the scrap material is fed in from above
on a conveyor belt, which distributes it between two separate shredder units. The
shredded product is passed on to conveyor belts that are fitted at the side. The new
feature of this machine is: the application of specially designed torque motors (with a
torque up to 15000 Nm) that can apply the necessary torque directly to the drive shaft
over the entire speed range. This saves having a gearbox, in other words: a major
heavy-wear item is no longer required. Since the magnetic field between the
armature and the housing of the torque drive does not form a rigid mechanical
connection, shocks are no longer fed back to the drive. The shredder plant can be run
much more economically with a dynamic torque motor, but the permanently excited
multi-pole synchronous drive requires precise, uninterrupted and absolute position
information for commutation. And so a new challenge has appeared: how to
Although the encoder has a very simple construction, it nevertheless required quite
a bit of development work to match up the materials (encoder wheel, elastomer strip)
and the magnetization method to one another in such a way that it not only ensured
the mechanical robustness that was required, but also permitted a generous
clearance between the encoder wheel and the sensor head. For instance, the encoder
type that is used in the shredder plant permits a radial gap of 1 mm and an axial
displacement of +/- 1 mm. This is considerably more than for other commercially
available magnetic encoders, but in this case it was absolutely necessary in order to
allow for the thermal expansion and bearing movement that arises in operation. In
the final result, there must be no possibility of a crash between the encoder wheel and
the sensor head, and the measurement acquisition must not be interrupted if the
path of the encoder components drifts a little. Initial fears of a possible weak spot
proved to be unfounded: stray magnetic fields do not, in practice, appear to be
A lot of know-how went into it A maintenance-free and much more
robust solution is provided by the absolute encoders developed by Baumer Hübner,
with a large-bore hollow shaft and magnetic sensing. They consist of an encoder
wheel, which is mounted directly on the drive shaft, and a separate sensor head that
is bolted firmly onto the motor housing Innovative encoder
solution ENCODERS AND SENSORS any problem. For although these encoders
have, in the meantime, been fitted to a considerable number of large torque
motors, always being mounted close to the motor end-plate, not a single case
has become known where measurement acquisition has been disturbed by the
magnetic field of the motor, or where the measurement has been falsified. Only in
the event that ferromagnetic particles are present would it be advisable to provide
a protective covering, to prevent such particles from accumulating on the outer
surface of the encoder wheel. Features of the magnetic absolute encoders at a glance
- suitable for very large shaft diameters - compact dimensions, very short axial fitted length
- very robust and free from wear, as they have no bearings and the electronics is
completely embedded in the sensor head - extremely high vibration and shock resistance
- insensitive to dirt, high enclosure protection level up to IP 68
- wide operating temperature range - large permissible spacing between the encoder wheel and the sensor head - simple, fast installation - easy adaptation to existing fittings
motor end-plate. This initial problem was cured by the additional use of alignment pins.
In addition to shredder plants, the absolute encoders presented here are also, for
instance, in operation on test beds and injection molding machines. Furthermore,
they are predestined for drive machinery with large-hole hollow shafts, such as are
indispensable for tool clamping devices or rods.
