Present monitors

SIMULATION OF OSCILLATING ARM
WIRE MONITOR MECHANICSDRIVEN BY A STEPPER MOTOR

x In 0.87/72/590 MeV beam lines.
x Oscillating arm driven by DC motor.
x Wire speed 0.75 m/s, not sufficientfor full beam current at 0.87 MeV.
x Wire position determined bypotentiometer.

R. Dölling
Paul Scherrer Institut, 5232 Villigen, Switzerland

The present oscillating arm wire monitors at HIPA operate with wire
speeds of 0.75 m/s. Based on basic dynamic simulations of mechanics andmotor, we discuss possible variants of this design using stepper motors inopen loop control. The results suggest that 4 m/s can be reached with
sufficient position resolution, when using a predefined step sequence
customized to the mechanics. This speed should be sufficient to measure
the full proton beam current in the injection line.

Lower-inertia setups

Simulation procedureThree setups simulated, all with same basic dimensions:
1. The present monitor, but with stepper. Worm gear reduction 20:1.
2. Moderately less-inertia. No potentiometer. Spur gear reduction 3:1.
3. Ditto, but gear reduction 1:1 (direct drive).

The simulation of monitor movement includes:x Basic geometry and estimated inertias of moving parts.
x Stepper motor speed-dependent pullout-torquecharacteristic.
x One of the three preset time-dependent ‘input
characteristics’a) rotor angular speed,b) rotor angular acceleration,c) used fraction of pullout-torque fracD.
x fracD depends on phase difference Mtooth = Mstat_cur - Mrot
between rotor and stator field:
fracD = sin Mtooth nteeth), (‘cosine potential’),with nteeth the number of teeth at the rotor
x Viscous loss due to ‘field friction’ at different speeds ofstator field and rotor.
And the ‘disturbing conditions’x Missing to shift the driver microstep sequence by Mtooth.
x A possible deviation to the assumed pullout-torquecharacteristic.
x Phase shift due to impedance of motor and cable,microstepping and driver PWM frequency.
x Error of full-step positions due to manufacturing
inaccuracies of the teeth of the rotor.
x Loss in worm gear.

Simulation resultspresent monitor (20:1)

The simulation is split into three runs, each done intime steps of 0.2 Ps.
1. A ‘preset trajectory’ is calculated from the ‘input
characteristic’.
2. The driver microstep sequence is calculated,including ‘disturbing conditions’.
3. The ‘resulting trajectory’ is calculated, including‘disturbing conditions’.

In the ‘resulting trajectory’, oscillations of the rotor
angular position in the ‘cosine potential’ of the teeth
are visible, caused by the ‘disturbing conditions’ andnot well adapted ‘input characteristics’.
Angular positions, speeds and kinetic energies of themoving parts and the torques between them arecalculated. The error of wire position caused by themotor characteristics and the ‘disturbing conditions’is predicted from the difference between ‘presettrajectory’ and ‘resulting trajectory’. For the profile
evaluation it is corrected by the predictable part ofthe effects of PWM and, in case, ignoring Mtooth.

lowerinertia(3:1)
CWx6gear play shifted awayfrom beam center

lower inertia (1:1)

Conclusions
Simulations imply that oscillations in ‘cosine potential’ of
the teeth are reduced by (in this order)
x taking into account Mtooth_prex providing a (preferentially smooth) input characteristic,
not interfering with the eigenfrequency of oscillations
x the presence of damping hloss >0 Js/rad2
x microstepping at least at lower speeds
x small other disturbing conditions
x reduced motor current, if torque is abundant.
Thereby the accuracy of wire movement is improved, andthe used fraction of pullout-torque reduced.

x An ultimate limit for maximum wire speed isgiven by the available motor torque.
x However, oscillations in ‘cosine potential’ willlead to a loss of synchronisation already atsomewhat lower speeds. This is described by thesimulation.
x For quantitative predictions, at least, a more
precise determination of hloss is required.
x Experimental tests are needed to check thepredictions of the simulation. A simple test casecan be a motor loaded with an additional inertia.

x With an adapted input characteristic, the simulations
suggest that 4 m/s are well in reach.
x Electronics able to provide a microstep sequence from alook-up table to the step-direction input of a commercial
driver is required for this.
x The predicted accuracy of wire movement, defined asband width of xwire_dev2pre_corr, is somewhat better at gear
ratio 3:1 (~50 Pm) than at 1:1 (~150 Pm).
x With lower wire velocity the accuracy does not improve.
Here, a higher gear ratio can provide better accuracy.
With 20:1 ~3 Pm is predicted, which is smaller than theeffect of the to be expected mechanical play of themechanism.

x A test stand is under preparation to evaluate theperformance of monitor setups as the ones
discussed here.
x Furthermore, other aspects, as, e.g., the lifetime of
the bellows, the stiffness of the MA and wire
vibrations will be studied.