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Accelerometer development at Stanford

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1996 Class. Quantum Grav. 13 A155

(http://iopscience.iop.org/0264-9381/13/11A/021)

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Class. Quantum Grav.13 (1996) A155–A158. Printed in the UK

Accelerometer development at Stanford

P W Worden and M ByeStanford University, Stanford, CA 94305-4060, USA

Abstract. Preflight prototype differential accelerometers for STEP are being developed atStanford under NASA funding. Subsystem development in progress includes work on thin-film superconducting circuits deposited on cylinders, SQUID-based superconducting positionmeasurement and electrostatic positioning and charge control. A thorough programme of testingand qualification of the subsystems is an essential part of the experiment. We have built a fluxmicroscope and magnetometer probe to study magnetic flux motion, one of the limiting factorsin the accelerometers; a position sensor study facility; a tipper table for testing and qualificationof bearings in three degrees of freedom and a ‘mechatronics’ lab for the manufacture of criticalcircuits on cylinders.

A rigorous testing programme is a necessary part of any space experiment because it ispresently impossible, or prohibitively expensive, to repair any failure after the experiment isin orbit. This is particularly true of fundamental physics experiments such as STEP and GP-Bin which the sensitivity of the apparatus depends directly on the absence of gravity. Thus theapparatus cannot be tested at full sensitivity until it is in orbit. The STEP development work atStanford is directed toward a partial answer to this problem, which guarantees that the instrumentwill at least function and tests it to the extent possible on the ground.

PACS number: 0480C

1. The Stanford programme

During the 1970s and 1980s preliminary research on differential accelerometers wasconducted at Stanford, partly to identify problem areas and partly to develop design andmeasurement experience. The biggest problem area that was identified was the occurrenceand motion of trapped flux in superconductors; the design work ultimately resulted in theSTEP science requirements document. Much more work is needed to develop a fully space-qualified accelerometer. This paper describes the work in progress at Stanford toward thisgoal.

We distinguish three component tasks in the development process. These aremanufacturing, testing and understanding the differential accelerometers and theircomponent parts. Properly coordinated, these three tasks form an iterative process.Components must be made which meet requirements that come from prior analysis, andmust be thoroughly tested. Understanding why the components are not ideal leads to furtheranalysis and an additional set of requirements. This process can converge on a space-qualified differential accelerometer for STEP. If we have the components of this processin place and operating before beginning a flight programme, we can minimize the cost,schedule and risk of the overall programme.

The STEP accelerometers are unusual in the number of new and existing technologieswhich need to be combined in a single instrument. These technologies include precisionmanufacture, SQUIDS and superconductivity, thin-film circuits on cylinders, electrostaticpositioning and sensing, and feedback and control. The test masses alone will challenge

0264-9381/96/SA0155+04$19.50c© 1996 IOP Publishing Ltd A155

A156 P W Worden and M Bye

the state of the art in precision manufacture. We are developing several facilities tomake and test these accelerometers. These include manufacturing facilities for makingsuperconducting bearings on cylinders to submicron accuracy, and for aligning them towithin a few arcseconds. We are completing a facility for testing the forces produced bythese bearings to a few tens of nanogees. Additional basic research is being performed inorder to understand some disturbances such as trapped flux and electrostatic patch effect,so that we can guarantee that they will not disturb the measurement.

The first goal of our work is the development of an infrastructure of test and supportequipment. First we must have the ability to produce differential accelerometer components,such as thin-film superconducting bearings, position sensor coils, heat switches andrelated circuit elements. We also need to develop techniques such as wire bonding forsuperconductors in order to connect components together. With some or all of these methodsin hand we can proceed with making ‘pre-protoflight’ differential accelerometers and testthem. The results of these tests can be used to iterate the process discussed above. Weenvisage initially producing accelerometers from a single-layer thin film of niobium, withmultilayer processes developed later. Niobium thin-film technology has been used in otherprojects but is untried for STEP. A backup technology exists in the proven but less precisehand-made wirewound technology.

Two sorts of tests of the accelerometers are needed. The first sort of tests are functionaltests which confirm the design and operation of the instrument. The second sort aremeasurements of performance and identification of potential error sources such as fluxmotion. What follows is a brief description of each of our facilities for the developmentprocess.

Our six active projects are a ‘mechatronics’ laboratory, a bearing test facility, anelectrostatic positioning system project, a flux microscope facility, a position sensordevelopment apparatus and a magnetometer probe. Each of these has its place in theoverall development.

The largest project is the mechatronics laboratory. Its purpose is to manufacturecomponents of accelerometers and develop processes that guarantee reliability. Themanufacturing process requires integration of mid-scale mechanical and electronic(superconducting) elements, and the laboratory comprises facilities for niobium sputtering,patterning and etching on cylinders. All of the processes we are interested in have beendemonstrated on planar substrates by other groups. Our facilities are specialized to addressthe problems of making niobium thin-film bearings and other circuits on cylinders, andmost of our work has been in developing a laser system which can expose a pattern tothe required precision and resolution. This system scans a modulated light beam acrossthe substrate to an accuracy of somewhat less than 1µm and resolution of about 10µm,with the possibility of improvement. It can also be used ‘in reverse’ for semi-automaticinspection of finished components.

The mechatronics laboratory is largely operational, but for the time being we areconcentrating our efforts on the bearing test facility. This facility will test the performanceof bearings and other products of the mechatronics laboratory, and can be upgraded to testentire single accelerometers.

In the 1980s we developed a quantitative method for measuring the effect of thetrapped flux in the prototype accelerometers. By tipping the apparatus we cancelled thetrapped flux forces against a component of gravity, and by doing this with the test massat different positions we were able to map out the trapped flux as a function of position.We then developed procedures for controlling it. The Dewar tipper servo used for thesemeasurements is being developed into a much more sensitive ‘tipper table’ that is the

Accelerometer development at Stanford A157

core of the bearing test facility. In the new system the weight of the test mass is heldby an electrostatic suspension system rather than directly by the bearing under test; thisconstrains the test mass to three degrees of freedom:X, Y and yaw. With this suspensionthe bearing can operate at low currents under conditions approaching those in orbit; themass is effectively zero-g in the horizontal plane. Based on current analysis and experiencewith the Dewar tipper, we should be able to achieve 10−8 g resolution or better. Thiscompares favourably to acceleration levels in non-drag-free spacecraft. We expect to beginoperating the bearing test facility very early in 1996. A more detailed description of thisfacility will be given in an accompanying paper.

An essential subsystem of the bearing test facility is an electrostatic positioning systemwhich both supports the test mass and measures its position in six degrees of freedom. Thistechnology development project is directed toward understanding some of the complexitiesand limitations of electrostatic systems and capacitative position measurements, becauseelectrostatic positioning is a major component of the STEP differential accelerometer design.It allows electrical forces on the test mass to be measured independently of magnetic forces,which is a distinct advantage in interpreting the data. The suspension system in the bearingtest facility can incorporate all of the features that we expect the flight system to need.

Our remaining projects are more nearly basic research rather than technologydevelopment. These aim to improve our understanding of the limitations of the technologyrather than to actually build a facility. The most technological of these is the position sensorprobe.

During the development in the 1980s we found that computing the test-mass–positionsensor-coil interaction was particularly difficult except in highly simplified cases. This wasparticularly so in estimating cross coupling (for example, yaw motion to axial output), andyet it is the cross coupling which ultimately determines the common-mode rejection ofthe accelerometers, and therefore the experiment’s performance. This makes it difficult tospecify requirements on the position sensor and test mass. The numerical algorithms wehave for calculating these couplings converge very slowly, and analytic methods are notmuch better. The position sensor probe addresses this problem experimentally. The goalof this project is to develop a description of the sensor from first principles, simultaneouslydeveloping analytic and numerical tools, using measurements as verification. The firstresults from this project are reported in an accompanying paper.

A more fundamental problem studied in the early development is that of flux trappingand motion. The prototype accelerometers were seriously limited by this disturbance,although we were eventually able to control it to a significant degree. Our flux microscopestudies the scientific basis of flux motion with the aim of understanding it. With thisunderstanding we will be better able to control the trapped flux and can have some confidencethat the control methods actually work.

The flux microscope directly images the trapped flux using a polarization technique.A magneto-optically active material (europium selenide) rotates the plane of polarizationof a light beam in regions of high magnetization, and the rotated light is imaged with ananalyser. Fluxons appear as bright spots. By measuring their behaviour as a function oftemperature, strain, applied field, composition of material and other parameters, we canconstruct a ‘phase diagram’ for the trapped flux and establish a zone of safe operationfor accelerometer components. Theoretical models can be tested against the data. Theinstrument can also be used to confirm that samples have acceptable behaviour for use in aflight accelerometer. This facility is operational but is waiting for usable samples.

The remaining project to be discussed is the magnetometer probe. This instrumentmeasures trapped flux motion in a similar configuration to an accelerometer—for example,

A158 P W Worden and M Bye

thin rings instead of thin-film planes. It measures bulk properties, unlike the flux microscope,but makes a more basic measurement than the bearing test facility which can only measurethe force due to trapped flux. In this instrument changes in trapped flux are directly measuredwith a SQUID, after cancelling the applied field. The results can be used in a similar wayto the flux microscope data, to determine a safe operating range for the material.

2. Summary

In summary, these development projects address the most critical elements in a programmeto build superconducting differential accelerometers. Many of the issues, such as precisionmanufacture and disturbing forces, would have to be studied in any programme to developan accelerometer with similar requirements. We expect to be able to reduce mission costs,improve the precision of the measurement, increase reliability and shorten the schedule bydeveloping the techniques to make the accelerometers now, instead of waiting until aflightprogramme is in progress.