THE
GOODYER SKIN RHEOMETER
USERS MANUAL
NOVEMBER 1999
The Goodyer Skin Rheometer
Contents
1. General Description of system
2. Installation
2.1 Fitting the probe
2.2 Mechanical installation
2.3 Software installation
3. Operating the system
3.0 The main menu
3.1 Take measurement; the Readings menu
3.1.1 Taking a reading
3.2 The review readings command
3.3 The Calibrate menu
3.3.1 Calibrate active probe
3.3.2 Change active probe
3.3.3 Enter new calibration factors
3.3.4 Exit
3.4 Exit
3.5 Show usage
1. Outline description of the system
1.1 What it does.
The system is designed to measure the dynamic spring rate (DSR) of human skin in vivo by applying a varying lateral force to the skin and measuring the resultant displacement. The force is normally varied sinusoidally over one cycle, though more than one cycle can be applied if required.
Because of hysteresis in the elastic properties of the skin, the force/displacement curve is normally an ellipse. The system calculates the slope of the major axis, and displays this as grams(force) per mm, or in other units if desired. The magnitude of the hysteresis is indicated by the area of the ellipse.
1.2 How it does it.
From the measuring head a light stiff probe protrudes. The far end of this is bent through a right angle and has at its tip a small round disc, which is attached to the skin under study by means of double-sided adhesive tape. Inside the measuring head, the probe is attached to a load cell, which is moved along the probe axis by a motor-driven lead screw. An LVDT is used to monitor the position of the load cell and probe.
The desired force/time cycle, which is normally a single sinusoid, is initially calculated from an equation, and then stored in memory as a table of values. The actual force applied to the probe is compared with the desired value in the table 1000 times per second. A feedback loop is used to control the motor which moves the load cell in such a way as to minimise the discrepancy. The force applied thus follows closely the desired force/time cycle The control loop uses an algorithm with proportional and integral terms, whose relative weighting can be varied.
The actual force values are plotted against the corresponding displacement values and displayed on the screen. The curve is usually a narrow ellipse. The slope of the major axis is displayed as an indication of skin elasticity, and the ellipse area as an indication of hysteresis.
The software will store the calibration data for up to ten different DSR probes. When the system is switched on, by default it assumes that the probe that is in use is the same as in the previous session.
The power supply for the measuring head is derived from the PC's 5 volt supply; this is converted within the head to ± 12 volts.
2. Installation
2.1 Fitting the probe to the measuring head.
This operation needs to be done with care, since it is possible to damage the load cell if excessive forces (over 20 gm) are applied to it.
Switch off the instrument before removing or replacing the probe. The rheometer has a spring gripper, and the probe is simply pushed into place. The spring is quite stiff, so a degree of force will be required, therefore it is essential that you hold the collet firmly whilst inserting the probe.
2.2 Hardware.
The measuring head is driven from a computer. The recommended minimum requirement is a 25 MHz 486 PC or later. A vacant slot is needed to accommodate the interface board (GSI Ltd Analogue Co-Processor). This is a half-size ISA card, and requires a half length 16-bit slot. It will be supplied to you already set up.
The card is inserted into the slot. A PC interface cable is supplied attached to the rheometer, with a 25 way D-type connector at the other. The instrument lead is plugged into the mating D-type connector on the analogue card.
Should you wish to use a different analogue data acquisition card then please contact us to determine if it can be used.
2.3 Software.
The first time the system software is installed, create a directory on the hard disc and copy into it the files from the diskette supplied. The following files are required:
UNIDSR.EXE
UNI.DAT
CAL.DAT
HELVB.FON
3. Operating the system
Before starting to use the system, ensure that the probe is free to move. Having selected the directory in which the operating software is stored, type SKINDSR (return). The main menu will appear on the screen:
3.0 The main menu
1. Take Measurement
2. Review Measurements
3. Calibrate
4. Exit
5. Show usage
Active probe: n
At the foot of the screen is an indication of which probe is in use; the calibration data for this probe will be used in the data processing. The probe number can be chosen by the user; it must be in the range 1 - 10.
By using the up and down arrows on the keyboard, select the heading you require and press (return), or just enter the menu entry number.
3.1 Take measurement: the "Readings" sub-menu.
When you select item 1 on the main menu, a message appears on the screen: "Calculating New Control Table...Please wait." During a period of some five seconds, the computer calculates a look-up table of time and force values to be used in the generation of the required force/time cycle applied to the probe. After this, the Readings sub-menu appears.
Cycle Time 3.0
Maximum Force 3.0
Gain 1.0
Study Number
Subject's Name
Visit Number 1.0
Number of Readings 1.0
Take a Reading
Quit - press ESC
By default, a cycle time of 3 seconds and a maximum force of 3 grams are used. These values can be changed by selecting the appropriate item, using the up and down arrows as before, pressing (return), inserting the new value, and pressing (return) again. If the values are changed, there will again be a pause while the control table is re-calculated. The cycle time can be up to 5 seconds, and the maximum force up to 20 grams.
A ‘STUDY NUMBER’ for the results can be inserted by selecting this item, pressing (return), entering the number, and pressing (return) again. The results file will be now named ‘Rhnnn.DAT’ where nnn is the study number. If you do not enter a study number, the results will be stored in a file called RH000.DAT. If the file already exists then new results are appended to the existing results.
In the same way the subject's name can be entered, and will be displayed on the screen when the data is stored or reviewed. Successive readings in the same file and under the same subject name can be recognised by the dates and times at which they were made, which are automatically stored with each curve.
The gain field should normally be left at 1. If for any reason the probe is unable to move, for example the subjects skin is extremely stiff, then the gain can be increased. Similarly delicate more movements can be obtained by reducing the gain.
3.1.1 Taking a reading
The operating system initially centres the probe position, so as to allow the greatest possible displacement in both directions. While this is being done, the probe must be free to move.
The entry "Take a reading" initiates a reading cycle.
The display now shows the instantaneous force acting on the probe. It is not possible to zero this out, nor is it necessary; all readings are taken relative to the starting point. The purpose of this display is to confirm that the subject is not moving significantly; the force reading should be less than ±3 g and should be steady to within ± 0.25 g.
At this point in the proceedings, the probe should have been attached to the skin by means of a small disc of double-sided adhesive tape.
Press any key when you are ready to start. The probe will then move sinusoidally about the start point, as it follows (as closely as possible) the desired force/time curve. The corresponding displacements are measured.
A real-time display of force versus displacement is then shown on the screen. The DSR and the area of the ellipse fitted to the data are calculated and displayed. You are then asked if you wish to save the results. If you enter YES, then the results are appended to the file named in the Results Filename field of the "Readings" sub-menu.
If you selected more than one operating cycle (see section 3.5), then the system will check to ensure that none of the readings gives a DSR value which differs by more than 30% from the overall mean DSR. If this deviation is exceeded, then the results are rejected and you are invited to take another measurement. This is to guard against erroneous results caused by movement of the subject. If the results meet this criterion, then the mean values are displayed as the final results.
3.2 The Review Readings Command
This enables the user to examine previously made measurements held in a file on the hard disc. When this command is selected, the words "Enter filename" appear on the screen. When you enter the filename, it is not necessary to add the suffix ".DAT" The screen will then display the first results stored in that file, showing the force/displacement curve and the numerical results for DSR and ellipse area, as well as the date, time and subject's name. The data points shown are those obtained after preliminary smoothing of the raw data.
By pressing any key except X, the next results in the file are presented until you reach the end of the file. After viewing the last entry in the file, the system will return you to the main menu. You can exit from the file at any time by pressing X.
3.3 The Calibrate Menu
In order to calibrate the probe, you will need the calibration unit. This uses a spring attached to a Maywood D2000 digital indicator to generate two accurately defined forces. It also uses a Mitutoyo digital micrometer attached to a rod to indicate displacement precisely. These two instruments are mounted on a turret which can be turned through 180° in order to present one or other to the measurement head. The calibration routine moves the load cell inside the head so as to produce two different forces, when the head is attached to the spring, and to produce two different displacements when it is attached instead to the rod.
When removing the probe, and when inserting the rod or the spring of the calibrator into the collet of the measuring head, use extreme care and follow the instructions given in 2.1.
The calibration sub-menu enables the user to re-calibrate the measuring head, referred to as the probe in this sub-menu. On selecting this item from the main menu, the following display appears on the screen.
1. Calibrate active probe
2. Change active probe
3. Enter new calibration factor
4. Exit
Active probe = X Force Slope = xxx.xxxx Displacement slope = xxx.xxxx
3.3.1 Calibrate Active Probe.
Item 1 on this menu gives instructions for performing a full calibration of the probe. When it is selected, the following display appears:
RECALIBRATION - LOAD POINT 1
ATTACH SPRING NOW
ALLOW SKIN RHEOMETER TO WARM UP FOR 5 MINUTES
PRESS ANY KEY WHEN READY
Place the end of the spring into the hole in the collet into which the probe is normally fitted. When you have done this, press any key; the following line appears at the foot of the screen:
ENTER ACTUAL FORCE NOW IN GRAMS
Enter the number indicated on the Maywood D2000, and press (return). The following line appears:
VALUE X.XXXX - IF OK PRESS RETURN
If the entry is not correct, pressing any key will highlight the entry position and enable you to enter the correct value. When you press (return), the following display appears:
RECALIBRATION - LOAD POINT 2
PRESS ANY KEY WHEN READY
When you do so, the following appears:
ENTER ACTUAL FORCE IN GRAMS
The load cell inside the measuring head has moved, so that a new value of force is produced. Enter the reading, and press (return)
VALUE = X.XXXX - IF OK PRESS RETURN
10 readings are taken, and the results are averaged in order to obtain the new calibration factor.
Verification that the reading is correct, and correction of incorrect results is as before. When you press return, the following screen appears:
RECALIBRATION - POSITION 1
RELEASE SPRING AND ATTACH ROD WITH CARE
PRESS ANY KEY WHEN READY
Release the spring from the collet, rotate the turret and insert the rod into the collet with the precautions set out in 2.1. When you press any key, the following message appears
ENTER ACTUAL POSITION NOW IN MM
When you do so, the system asks for confirmation that the value is correct, as before. When you confirm, the following screen appears:
RECALIBRATION - POSITION 2
PRESS ANY KEY WHEN READY
When you do so, the system prompts you for the actual position in mm, and for confirmation as before.
5 readings are taken, and the average is used as the new calibration factor.
When both the load cell and the LVDT have been calibrated the following message appears:
CALIBRATION COMPLETE
UNCOUPLE ROD WITH CARE
LEAVE WITH SPRING TOWARDS PROBE
THEN PRESS ANY KEY TO EXIT
The system then returns you to the calibration menu.
3.3.2 Change Active Probe
When you select item 2 on the calibration menu, the message
ENTER ACTIVE PROBE NO.
appears. On entering the probe number, the system prompts for confirmation, and if this is given, it returns you to the calibration menu. The system will then use the calibration data for the selected probe, which is stored on the hard disc.
3.3.3 Enter new calibration factor
When you select item 3 on the calibration menu the screen reads:
ENTER NEW FORCE SLOPE
After the force slope figure is entered, it is displayed for confirmation. The previously used routine for confirmation or correction of erroneous results follows. The screen then reads:
ENTER NEW DISPLACEMENT SLOPE
Again entry of the number, confirmation and/or correction are possible. After this is done you are returned to the calibration menu.
3.3.4 Exit
Selection of this item on the calibration menu returns you to the main menu.
3.4 Exit
Selection of this item on the main menu causes an exit from the operating program.
3.5 Show Usage
When this item is selected, a help screen appears. This shows how the command, SKINDSR used to initiate the operating software, can be extended to pre-set some of the operating parameters, namely:
Maximum force (default 3 gm)
Cycle time (default 3 seconds)
Subject's name
Number of operating cycles (default = 1)
Display fitted ellipse (default = off)
Display fitted line (default = off)
Name of results file
The extensions to the command are added immediately after it, separated by slashes. The SKINDSR command alone, without extensions, will operate the software using the default settings indicated. At the top of the display, the version number of the software is given, together with the Copyright declaration.
Other options may be added at a future date. These will be listed by the SHOW USAGE option on the main menu. The current available options are
/A ANALOGUE CARD TEST FOR TEST PURPOSE ONLY
/Fnn PRESET MAXIMUM FORCE TO THE INTEGER VALUE nn
/D DEBUG MODE EXERCISE SOFTWARE WITHOUT A RHEOMETER
/R REVERSE THE MOTOR DIRECTION
/L DRAW THE BEST FIT STRAIGHT LINE
/E DRAW THE BEST FIT ELLIPSE
/M MOTOR DRIVE OUTPUT TEST FOR TEST PURPOSES ONLY
/P KICKS THE MOTOR DRIVE OUTPUT TO FREE A ‘STUCK’ PROBE
/Q KICKS THE PROBE SHUT
/Gnn ENTER A NEW GAIN
/H SELECTS DISPLACEMENT CONTROL (EXPERIMENTAL ONLY)
/I ENTERS A NEW INTEGRAL TERM, FOR TEST PURPOSES ONLY
/Sabc ENTERS A SUBJECTS NAME
/C SELECTS TIMED DISPLAYS
/Ynn ENTERS A MOTOR DEAD-BAND, FOR TEST PURPOSES ONLY
/Tnn ENTERS A NEW CYCLE TIME
Any other text that is not preceded with a / symbol will be used as the results file name. This name will be prefixed with the letters RH
For more information please contact us on (44) 1509 844473, or write to Eric Goodyer, 20 Gladstone Street, Hathern, Leicester, LE12 5LE.