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The Cassini-Huygens Mission to Saturn and its Moons

By Nicole Knight,2014-01-20 04:15
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The Cassini-Huygens Mission to Saturn and its Moons

    The Cassini-Huygens Mission to Saturn and its Moons

    Calibrating and using a model of Huygen’s Tiltmeter

Introduction

    Figure 1 First colour image of Titan’s surface

    (Courtesy NASA/JPL-Caltech)

The NASA/ESA/ASI* Cassini-Huygens mission to Saturn and its moons began

    back in October 1997 with a launch from Cape Canaveral. Seven years later on

    Thursday 1 July 2004, having travelled some 2.2 billion miles, the Cassini

    orbiter with its Titan lander Huygens, went into orbit. On Christmas Day 2004

    the 318kg Huygens probe was released from the orbiter and entered Titan’s

    atmosphere on 14 January 2005 to land two and a half hours later on this

    moon’s surface. The first colour picture from the probe is shown in Figure 1. As

    can be seen, the landing was on a solid surface with various pebble size rocks

    or ‘ice’ blocks of water and hydrocarbon strewn around it.

     Figure 2 The Huygens probe

    (Courtesy Planetary and Space Sciences Research Institute, The Open University)

    * NASA National Aeronautics and Space Administration, ESA European Space Agency, ASI Italian Space Agency

    V1.2 Chris A Butlin July 2005

Here you will be modelling the Tiltmeter (TIL) part of its Surface Science

    Package as labelled on Figure 2. This package was a development led by John

    Zarnecki (see Figure 3) of the Planetary and Space Sciences Research Institute

    at the Open University in the UK. An interesting interview with him was

    conducted by the Planetary Society and can be found at

    http://www.planetary.org/news/2004/conversation_zarnecki_john_huygens_112

    9.html .

    Figure 3 John Zarnecki Principal Investigator for the Surface Science Package

    (Courtesy NASA/JPL-Caltech)

The tiltmeter was designed to measure any swing and spin as the probe

    descended through Titan’s atmosphere, any motion due to waves if it landed in

    a liquid, and its angle of tilt if it landed on a solid surface. Figure 4 gives an idea

    of its size.

Figure 4 The Tiltmeter being held by the SSP Programme Manager, Mark Leese, of the

    Planetary and Space Sciences Research Institute, The Open University. The two tilt

    sensors can be seen detecting movement along two perpendicular axes.

    (Courtesy Spectron Glass and Electronics Inc.)

V1.2 Chris A Butlin July 2005

     Your task will be to use simplified tiltmeters, calibrate them, and then use them to find the angle of tilt of an unknown surface.

The Tilt sensor

    Technically the tilt sensor is just the sensing part of the system without any electronics added. Once additional circuitry is added it becomes an inclinometer or clinometer. There are many types of tilt sensor in use in such applications as measuring the gradient of an oil or water pipeline, in attitude control of a submarine, in ensuring that a ship is adequately balanced on loading, in giving warning of a topple by a crane, providing correct alignment of wheels, measuring the sway of tall buildings, and in the guidance gyroscopes used on rockets and aircraft. They are commonplace devices. The type used on the Huygens probe was a Spectron Electrolytic Tilt sensor developed by Spectron Glass and Electronics Inc. of Hauppauge, USA. They were in fact cylindrical devices as you may have noticed in Figure 4, but their operation was very similar to the simplified form illustrated in Figure 5.

    Whilst the Surface Science Package tilt sensor (named TIL) measured the angle of tilt in two planes, you can see the two sensors arranged at 90? to each

    other in Figure 4, you will be using one which only senses in one plane. Its working is based on the change of resistance of a slightly conductive liquid between electrodes AB and BC as shown in Figure 5.

     Figure 5 Simplified view of tilt sensor (a) on level ground and (b) when tilted

    In Figure 5a with the sensor level, there would be no difference in the resistance between A and B and between B and C. However, if the sensor was tilted as in Figure 5b, then there would be less liquid than before between A and B and more liquid between B and C. This would result in a higher resistance between A and B and a lower resistance between B and C. These changes of resistance can then be used to give measurements of current and voltage at various V1.2 Chris A Butlin July 2005

known angles of tilt this is the calibration stage. Then, using your calibration

    data, you will be able to find the values of unknown angles of tilt.

The liquid that you will be using is a very dilute solution of copper sulphate with

    copper electrodes. The Huygens SSP Tiltmeter used a solution of N-propanol,

    methanol and potassium iodide with platinum electrodes. An alternating voltage

    source is provided so that problems of electrolysis and polarisation are avoided.

    The sampling rate for the real probe was 1Hz (1 sample each second) during its

    atmospheric descent and 2 Hz (2 samples each second) once it had landed.

    These rates were chosen as they were thought fast enough to detect the low

    frequency oscillations likely in these two situations. As we are now aware, the

    Huygens probe landed on a solid surface and so no wave motion was detected

    after landing.

After digitisation of its output the TIL sensor could detect changes of 0.03?. The

    data concerning movement during its descent and the final landing orientation

    was transferred by the Probe Data Relay Subsystem (PDRS) to the orbiting

    Cassini spacecraft and then back to Earth.

Stopping the liquid from solidifying was not a problem as the TIL sensor was on

    the top of the SSP and well within the Huygens probe. Heating of the probe

    was by its Radioisotope Heater Units (RHUs).

Activity: Using current readings to calibrate the tiltmeter/inclinometer and

     then using it to measure an unknown angle of tilt

    Figure 6 Set up of model tiltmeter for calibration and use

Connect the tiltmeter’s a.c. supply to the blue and green sockets of the tilt

    sensor, incorporating a milliammeter in series with them as shown in Figure 6.

    Set the milliammeter on it’s a.c. (~) 20mA current range. Pour 10ml of the dilute

    copper sulphate solution into the tilt sensor container.

V1.2 Chris A Butlin July 2005

Make a copy of the following table.

    Angle of tilt Current I/ICurrent IIBG GW BGGW

    /degrees /milliamperes /milliamperes

     0

    +10

    +20

    +30

    +40

    -10

    -20

    -30

    -40

    unknown

    Figure 7 Tiltmeter table of results

Take a tilt to the left (viewed from in front) as positive (+) and one to the right as

    negative (-). On the level is a tilt of 0?.

Switch ON the tiltmeter’s a.c. supply and leave it ON for a few minutes to

    stabilise. Using the ‘angle of tilt wedges’ record in your table values of the current I flowing between the blue and green electrodes for angles of tilt from BG

    0? through to +40? and -40?. Rearrange the circuit with the green and white

    electrodes connected instead and record the current I flowing between them GW

    for angles of tilt from 0? through to +40? and -40?. Switch OFF the tiltmeter’s a.c.

    supply. Calculate the values of I/I and record those in your table too. BGGW

Now draw a graph of Angle of tilt (X-axis) against I/I (Y-axis) complete BGGW

    with its best-fit line. You might use a computer graph plotter, spreadsheet, or

    simply do this by hand on a piece of graph paper.

Place the tilt sensor onto the slope of one of the ‘angle of tilt wedges’ of

    unknown angle (they have letters marked on them). Switch ON the tiltmeter’s a.c. supply and again leave it for a minute or so to stabilise, then record values

    of I and I in your table. Calculate the value of I/I and record that in BGGWBGGW

    your table too. Switch OFF the tiltmeter’s a.c. supply. Now, using the value of

    I/I and your graph, deduce the angle of tilt of the unknown slope. BGGW

Q1. What do you think was the angle of tilt of your unknown slope and what

    letter was marked on it?

Q2. What is the advantage of calculating the value I/I ? Explain. BGGW

In most situations values of voltage rather than current are required. This is

    because voltages can be fed directly into an Analogue to Digital (A to D)

    converter and digitised for transmission by radio. So, in the next activity you will

    see how a series of voltages can be obtained from this simple setup.

    V1.2 Chris A Butlin July 2005

Activity: Using voltage readings to calibrate the tiltmeter/inclinometer and

     then using it to measure an unknown angle of tilt

    Figure 8 Set up of model tiltmeter for calibration and use

Connect the tiltmeter’s a.c. supply to the blue and green sockets of the tilt

    sensor, incorporating a resistor in series with them and connect a voltmeter

    across the resistor as shown in Figure 8. Set the voltmeter on it’s a.c. (~) 2V

    voltage range. If not already done, pour 10ml of the dilute copper sulphate

    solution into the tilt sensor container.

Make a copy of the following table.

    Angle of tilt Voltage V/VVoltage VVBG GW BGGW

    /degrees /volts /volts

     0

    +10

    +20

    +30

    +40

    -10

    -20

    -30

    -40

    unknown

    Figure 9 Tiltmeter table of results

Take a tilt to the left (viewed from in front) as positive (+) and one to the right as

    negative (-). On the level is a tilt of 0?.

Switch ON the tiltmeter’s a.c. supply and leave it ON for a few minutes to

    stabilise. Using the ‘angle of tilt wedges’ record in your table values of the

    voltage V across the resistor for angles of tilt from 0? through to +40? and -40? BG

    when the blue and green electrodes are in the circuit. Now record in your table

    V1.2 Chris A Butlin July 2005

values of the voltage V across the resistor for angles of tilt from 0? through to GW

    +40? and -40? when the green and white electrodes are in the circuit. Switch

    OFF the tiltmeter’s a.c. supply. Calculate the values of V/V and record BGGW those in your table too.

    Now draw a graph of Angle of tilt (X-axis) against V/V (Y-axis) complete BGGW with its best-fit line. You might use a computer graph plotter, spreadsheet, or

    simply do this by hand on a piece of graph paper.

Place the tilt sensor onto the slope of an ‘angle of tilt wedge’ of unknown angle

    (these have letters marked on them). Switch ON the tiltmeter’s a.c. supply

    again leaving it for a minute or so to stabilise, then record values of V and BGV in your table. Calculate the value of V/V and record that in your table GWBGGW

    too. Switch OFF the tiltmeter’s a.c. supply. Now, using the value of V/V BGGW

    and your graph, deduce the angle of tilt of the unknown slope.

Q3. What was the angle of tilt of your unknown slope and what letter was

    marked on it?

    Q4. What is the advantage of calculating the value V/V ? Explain. BGGW

As you might already know, if a current I flows through a resistance R then the

    voltage V produced across it is given by V = I ? R. So in this last activity the voltages that you recorded were simply the result of the currents flowing

    through the resistor.

You will probably have found that these simple setups are not very sensitive

    little change of current or voltage resulted from a change of slope. It was also

    somewhat awkward to have to alter the circuit to get two values for each angle

    of tilt, although this could have been done with the aid of a switch. To

    overcome these difficulties commercial tiltmeters/inclinometers are usually

    arranged in what is called a Wheatstone Bridge network. This circuit is named

    after the English physicist Sir Charles Wheatstone (18021875) who is best known for his work on the development of the electric telegraph and with

    acoustics, the science of sound. Oddly the Wheatstone Bridge was not his

    invention, but that of Samuel Hunter Christie of the Royal Military Academy.

    Wheatstone just made this arrangement famous.

    Figure 10 Sir Charles Wheatstone

V1.2 Chris A Butlin July 2005

Activity: Using a Wheatstone Bridge to calibrate the tiltmeter/inclinometer

     and then using it to measure an unknown angle of tilt

Figure 11 Set up of model tiltmeter in a Wheatstone Bridge circuit for calibration and use

Connect the blue, green and white sockets of the tilt sensor to the blue, green

    and white sockets on the Tiltmeter box. Connect a voltmeter set on it’s a.c. (~)

    2V voltage range across the Tiltmeter box’s yellow sockets. If not already done,

    pour 10ml of the dilute copper sulphate solution into the tilt sensor container.

Make a copy of the following table.

    Angle of tilt Voltage across yellow sockets V YY

    /degrees /volts

     0

    +10

    +20

    +30

    +40

    -10

    -20

    -30

    -40

    unknown

    Figure 12 Tiltmeter table of results

Take a tilt to the left (viewed from in front) as positive (+) and one to the right as

    negative (-). On the level is a tilt of 0?.

V1.2 Chris A Butlin July 2005

Switch ON the tiltmeter and leave it for a few minutes to stabilise. Turn the

    blue potentiometer (variable resistor) knob until the voltmeter displays as small

    a voltage as possible: it probably will not be 0V. Using the ‘angle of tilt wedges’ record in your table values of the voltage V across the yellow sockets for YYangles of tilt from 0? through to +40? and -40?. Switch OFF the tiltmeter’s a.c.

    supply.

Now draw a graph of Angle of tilt (X-axis) against V(Y-axis) complete with its YY

    best-fit line. You might use a computer graph plotter, spreadsheet, or simply do

    this by hand on a piece of graph paper.

Place the tilt sensor onto the slope of an ‘angle of tilt wedge’ of unknown angle

    (these have letters marked on them). Switch the tiltmeter ON and again leave it

    for a minute or so to stabilise. Note in your table the voltage V. Also just rock YY

    the tilt sensor back and forth as if it were being rocked by ocean waves and

    note the effect on the voltage V. Switch OFF the tiltmeter. Now, using the YY value of V and your graph, deduce the angle of tilt of the unknown slope. YY

Q5. What problem did you encounter in trying to deduce the angle of tilt this

     time? Explain.

Q6. How might you solve the problem that you encountered? HINT: Although

     not the commercial solution, tilt switches might help. A tilt switch is shown

     in Figure 13.

    Figure 13 A mercury-based tilt switch

Tilt switches are very simple devices, usually made of glass and containing a

    conductive liquid such as mercury. Inside are two electrical contacts separated

    from each other and a small quantity of mercury. As soon as the device is tilted

    the mercury flows and covers both electrical contacts. Similar versions use

    gold-plated ball-bearings instead of the mercury. You might well have come

    across one on a pin-ball machine to detect any tilting.

Q7. If the Huygens probe had landed in an ocean on which there were waves,

     as the team had hoped, how would you deduce this from your tiltmeter

     data?

Q8. What other factors do you think the Huygens SSP team had to take

     account of with their tiltmeter? Explain.

V1.2 Chris A Butlin July 2005

The following section will tell you a little about the Wheatstone Bridge. There

    are d.c. as well as a.c. versions and they have a wide range of uses.

The Wheatstone Bridge

    Figure 14 A Wheatstone Bridge circuit

You may have wondered how the Wheatstone Bridge tiltmeter worked.

    Consider the circuit shown in Figure 14. It is much the same as the one that

    you used and which is marked on the tiltmeter box. The only difference is that

    the two resistors R and R are separate, whilst in the box they are part of the 12

    potentiometer (variable resistor), separated by the contact point which you

    changed the position of, and so their resistances, by turning the blue knob. On

    the tilt sensor the resistors R and R represent the resistances of the liquid BGGW

    between the blue and green, and green and white electrodes respectively.

On what is known as a ‘balanced’ bridge the voltmeter would read 0V and no

    current would flow between B and D. To do this the voltage at B and at D must

    be equal to each other. Then the current I flowing through R must be the 11same as that flowing through R. Similarly, if the current flowing through R is 2BGI, then that same current must also be flowing through R. 2GW

    So the voltage V across R is given by V = I ? R, the voltage V across AB1AB11ADR is given by V = I ? R but, as V = V, we can write BGAD2BGABAD

    I ? R = I ? R and by rearranging have I/I = R/R112BG12BG1

    Similarly you should be able to see that the voltage V across R is given by BC2V = I ? R and the voltage V across R is given by V = I ? R. BC12GWGWGW2GWHowever, V = V and so we can write BC GW

    I ? R = I ? R and by rearranging have I/I = R/R 122GW12GW2

Therefore I/I = R/R = R/R and again by rearranging we have 12BG1 GW2

    R/R = R/R 12BGGW

    V1.2 Chris A Butlin July 2005

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