What happened to our shutter?

and

a brief description of our upstream beamline for those who were not around when it was built.

Target Moderators

Proton Beam

STARTING POINT

2

FNP DESIGN OVERVIEW

FNPB DESIGN -

5

UCN Guide

6

Beamline Under Construction

7

General Layout

Common vacuum for choppers and guides

Frame-overlap choppers located at

5.5 m, 7.5 m, 9 m, and 10.5 m

FNPB DESIGN -

9

Choppers Received & Tested.

10

Chopper Connectors & Cabling Installed.

11

Chopper Installation…

Shutter

Motor Drive

Step Motor Basics

A 1 -1 -1 -1 1 1

B 1 1 1 -1 -1 -1

Rotor 1 2 3 4 5 6 7 8

Real Step Motors

Step motor advantages: Easy Digital Control, High resolution (micro steps)

Step motor disadvantages: No positive indication of position (missed steps)

Motor Drive

Encoder

Rotary Optical Encoder

A & B channels allow the determination of the sense of rotation. Encoder has a resolution of >0.01° which is appropriate for our purposes.

NOTE: For safety reasons, the PPS system has an independent, redundant, system of “open/close indicators” that are much coarser than our shutter positioning system. A PPS “shutter open” indication does NOT mean the shutter is in the best position to transmit neutrons. The green light indicates the position is within a few degrees only.

The Design Concern

The graduated discs are made of glass. In the presence of radiation, glass can darken due to the formation of lattice dislocations (“color centers”). We could not find a suitable “rad hard” encoder. The best estimate for the longevity of an encoder in our environment was ~10 years…but we had no confidence in that estimate.

What do we think happened?

It appears that the encoder began to miss counts and thus we slowly began to get a cumulative error in our positions.

The missed counts seemed to occur on the side of the encoder that was closer to the beam so it may be due to radiation damage.

It is likely that we can no longer depend on the encoder to position the shutter.

This problem was anticipated and we have a “Plan B” which will be implemented over the shutdown.

PLAN B

Counting steps to the step motor has been shown to be quite reliable. But is subject to cumulative errors if occasional steps are lost.

In anticipation of the loss of the rotary encoder, the shuter was built with several hardware “home” position switches whose relative position was measured with respect to the optimal open position.

To implement this we must to rewrite the code that commands the shutter step motor. This is a routine step for the SNS motor control people. No disassembly of the shielding or shutter is needed. This can be done at any time before we need to open the shutter and can be done with the accelerator operational and the primary shutter closed.

Questions

If this has been a continuing problem, it may be that we have been seeing a progressive decline in flux as the drive system “lost touch with reality.” We will check this by going over the shutter logs which record every shutter opening and closing.

An additional question is whether a misalignment of the shutter could give rise to the structure that we see in the vertical phase space.