Compact drives for hexapod micropositioning systems

The miniaturization of mechanical, electrical and electronic components is one of the key trends dominating research and development activities within the area of drive technology. Small yet high-powered motors are capable of exploiting new opportunities in automation technology. One area that has benefited considerably from miniaturized drive systems is micropositioning technology, designed to undertake high-precision tasks in the most varied areas of application. Micropositioning includes basic movements with resolutions in the sub-micrometre range and paths of just a few millimetres up to several hundred millimetres.

Drawing on over 30 years’ experience, PI (Physik Instrumente) has made a name for itself as a supplier of high-performance micropositioning systems. The product highlights include pioneering six-axis parallel kinematics micro-robots which are suitable for a variety of applications.
These specialist fields of application range from handling systems in electronics manufacture and product inspection in precision machine-tool production to medical technology and optical systems, e.g. space telescopes and satellite reception systems.

"Flight simulator" principle

Hexapod systems are based on six high-resolution actuators that control a single platform. This is the same principle as that applied in flight simulators, only much more accurate. Instead of hydraulic drives, hexapods are powered by high-precision drive spindles and precisely controllable electrical motors. Owing to the reduced mass of the moving platform, the settling time for positioning is considerably less than with conventional, stackable multiple-axis systems. The pivot, which can be freely defined using software functions, remains independent of movement, which is important in optical adjustments, for example.
The precise parallel kinematics micro-robots are provided in three basic versions for various applications. The M-850 is the ideal system for the full range of complex positioning tasks, where high loads of up to 200 kg in the vertical direction as well as an element of precision are of the utmost importance. Each axis can be positioned individually with a resolution of up to 0.005 µm. The M-840 parallel kinematics micro-robot has been developed for smaller loads and higher speeds. This allows loads of up to 10 kg to be positioned in any direction with a speed of up to 50 mm/s at micrometre accuracy. The most recent development, the M-824, works with "folded" drives and features a highly compact design due to the special arrangement of the drive and the spindle. One aspect that all three systems have in common is that they require drive technology tailored to the hexapod’s unique requirements. In particular, the drive components must be suitable for integration into the axes of the hexapods. In other words, the structural dimensions have to be as small as possible, but nevertheless be capable of supplying comparatively high power ratings of 3 to 19 Watt. In order to achieve the high positioning accuracy required, the drive systems must also work as backlash-free as possible over the lengthy operating period. Harsh environmental conditions also have to be taken into consideration, e.g. in outdoor applications. Micropositioning systems, for instance, are used in space telescopes and satellite reception systems installed in inhospitable mountain or desert regions.

Drives for hexapod micropositioning systems: small, robust and powerful

When it comes to challenging applications, FAULHABER’s standard range of DC precision motors is always primed for action. The classic bell-type armature motor with ironless rotor coil and precious metal commutation provides very favourable preconditions for such areas of application. The small, light-weight DC drives work reliably in adverse environmental conditions. They are ideally suited for ambient temperatures between – 30 °C and + 125 °C. With special design features, they are even able to deal with high levels of humidity of up to 98%. For PI, an important consideration in the selection of motors was the immediate and high torque start-up of the DC motors after a voltage is applied. This ensures direct reaction to the control signal. Thanks to the self-supporting copper coil, it is possible to construct particularly light motors with efficiency grades of 80 % and more.
Depending on the design and area of application, the hexapod micropositioning system combines DC motors with reduction gears. The low-play cylindrical gearing in an allmetal design is particularly conducive to uniform and low-noise operations. To avoid backlash, in most cases the gears are pre-tensioned. Motor and gears form a compact unit and are 60 mm long at a diameter of less than 25 mm. So even at rather tight installation ratios in the hexapod axes, the drive units can be easily integrated. Electrical connection is also particularly userfriendly. The DC geared motors can be controlled directly by PC cards without additional boosters. In some cases, however, manufacturers of hexapod micro-positioning systems have incorporated servo-boosters with inputs for pulse-width modulated signals, close to the motor in the basic board of the hexapods.

Position detection with magnetic pulse generators

For precise positioning, it is ab-solutely essential that one knows the actual position of the motors. Once again, FAULHABER came up with an effective solution. With the DC micromotors built into the hexapods, the current positions are recorded with magnetic pulse generators supplying 512 pulses per rotation. Resolutions of up to 0.005 µm are produced by means of quadruple interpolation, depending on spindle pitch. The pulse generators consist of a multiple-pole, low-inertia permanent magnetic disk, which is in-corporated either on the motor shaft or directly into the rotor of the motor, depending on the type of unit deployed. Magnetic sensors record the changes in magnetic flux. The output includes two 90° phase-shifted incremental output signals, which are sub-sequently processed by the system control of the hexapods. The supply voltage for pulse generators and the DC micromotor as well as the output signals are connected via a ribbon cable by means of a plug.

In kinematics, one can basically distinguish between parallel and serial kinematics. In serial systems, each actuator focuses on its own positioning platform and is assigned to one axis. This approach allows simpler mechanical structures and control technology. However, as runout errors tend to add up with the "stacked" systems, the de-gree of precision achieved is lower than with parallel systems. With parallel kinematics, unlike serial kinematics, all actuators act directly on the same platform. Alongside improved accuracy, this has a number of benefits: lower inertia of masses and thus superior dynamics, no moving cable that generates friction losses, and a more compact structure. The control of such systems is, however, very demanding and requires much more expertise.

Parallel vs. serial kinematics

In kinematics, one can basically distinguish between parallel and serial kinematics. In serial systems, each actuator focuses on its own positioning platform and is assigned to one axis. This approach allows simpler mechanical structures and control technology. However, as runout errors tend to add up with the "stacked" systems, the degree of precision achieved is lower than with parallel systems. With parallel kinematics, unlike serial kinematics, all actuators act directly on the same platform. Alongside improved accuracy, this has a number of benefits: lower inertia of masses and thus superior dynamics, no moving cable that generates friction losses, and a more compact structure. The control of such systems is, however, very demanding and requires much more expertise.

The drive systems used in hexapods have to withstand harsh environmental conditions, e.g. when being used in satellite reception systems and inhospitable mountainous regions or desert areas
This hexapod system operates with "folded" drives and is of a very compact design due to the special arrangement of the drive and spindle
Hexapod micropositioning systems are based on six high-resolution actuators that control a single platform