Absolute encoders: SinCos® encoder or SinCos® scales: Absolute encoders offer the characteristics described for TTL or Sin-Cos encoders without the need for a startup sequence for phase commutation (see following paragraph). Also they avoid the need for a homing sequence to locate the axis zero point. These encoders and scales are very widely used.
The same limitations as previously mentioned apply to these devices. However, the advantage of this technology lies in the possibility of interpolating the sinusoidal output using an appro-
priate drive interface. Thus an encoder with 1 192-062011 N6
Fluid cooling
To avoid overheating due to environmental factors, or to achieve a higher continuous torque rating than is attainable with natural convection, it is possible to use fluid cooling.Fluid cooling specifications are based on two operating points:
Win
ding at 60°C.Maximum cooling (winding at 140°C) for obtaining the maximum continuous torque from the mot
or.
The outside of the stator is machined with a helical groove which forms a cooling circuit when the stator is fitted inside an outer housing. Circular grooves at each end of the helix form input TTL encoder or TTL optical scale: TTL encoders usually have a limited resolution (from 500 to 5000counts/rev.). However it is possible to find encoders with hollow shafts up to 50mm internal diameter and with resolutions between 15 000 and 20 000counts/rev. (multiplied by 4 after decoding).
The best solution in terms of resolution and accuracy is generally offered by optical scales with pitches from 0.5 to 5µm that can be attached to a hollow hub with the appropriate diameter. They can offer resolutions of 1 million counts/rev or more depending on the diameter.
The associated read heads have a maximum operating frequen-
cy which limits the speed for a given resolution.
Parker STK frameless motors have been designed for minimum torque harmonics when they are driven by a sinusoidal
brushless drive. Various positioning sensors are available, and the drive may use 4 types of positioning interface: Resolver: In a high-quality resolver (e.g. Parker RES FC6 72 32), the ac-
curacy is limited to around 1 arc-minute. In most of the
resolvers currently available, accuracy is of the order of several arc-mi-
nutes. It is necessary to check whether the desired positioning accuracy is compatible with the resolver accuracy.
On another hand, resolvers can limit the range of applications when they obstruct a hollow shaft.
Finally, resolvers are available with resolutions up to se-
veral hundred thousand counts per revolution, but such a high resolution is generally not useable because it is not compatible with the accuracy of the measurement system. 024 sinewaves per revolution used with x1 000 interpolation will produce one million counts/rev. The Sin-Cos optical scale has the advantage of producing high resolution either directly or by internal drive interpolation, together with an unrestricted hollow shaft, since it may be attached to an appropriate diameter hub.
Phase commutation
Permanent magnet
synchronous motors require a constant phase angle between the rotating fields of the stator and rotor in order to control the torque. The resolver provides the necessary phasing information and simultaneously gives the axis position (within one polar pitch). Absolute encoders also provide this phasing information, but this is not the case with
incremental encoders or scales.Encoder suppliers therefore offer specific ranges for
brushless motors including either:
a) Three-phase commutation rectangular signals (U, V, W, /U, /V, /W) in the case of a TTL encoder; these waveforms must have the same number of cycles per revolution as the number of pole-pairs in the motor.
b) Sinewave signals (1 cycle per revolution), sine and cosine, giving the absolute position within one revolution in the case of sin/cos encoders. The drive electronic interface multiplies this frequency by the number of motor pole pairs.In the case of an optical scale mounted on a hub, the information related to the phasing between the stator and rotor fields is not known. Therefore an initialisation sequence is needed during start-up; during that sequence the rotor will perform an indexing motion, or at least a small oscillation.
and output channels; the input and output portsin the outer housing must be axially aligned with thesegooves. Additional circular gooves at each end of the statorare fitted with O-rings to form a seal.Use either softened water with a glycol additive or a fluid approved for closed-circuit cooling in order to minimise the risk of corrosion and deposits.
Parker Hannifin GmbH & Co. KG Electromechanical Automation 11