Limit Switches

After comparing a variety of limit switches online, I decide to stick with the original plan of using mechanical limit switches. There are some larger ones I found on farnell that could stand near 2 newtons of force, they are a bit more than £6 each. Without a registered account, a minimum of £20 per purchase is required, meaning I need to buy 4. Considering that, I started searching for smaller options. I have found out that Maplin have some small ones that could fulfill my need and are priced at around £1.5 each. I went to purchase 2 and will look into options of how to secure them in position. This may involve some redesigning of the backing. I am also going to extend the cover so less components are exposed.

These limit switches are normally normally closed SPDT, with the middle leg as the common. They also features two connecting holes suitable for M1 screws.

Data Collecting

I have tried to make a phantom with gelatine and gelly. As there is no information online for the ratio of the two, I have tried combinations of 1 part of gelatine to 5 parts of gelly to pure gelatine. More gelly would offer the benefit of bounciness, but soft and fragile. If only gelatine is used, the outcome is firmer. Comparing the phantoms with my stomach, pure gelatine gives the best matching result.

Considering its softness, magnification of sensitivity of the load cell is required, I have tried to achieve this by increase the gain of the differential amplifier. The resistors I am using currently are 260Ohm and 560kOhm, it brought a 4V offset while magnifying the output from the load cell. Once read by the controller, the freeload voltage is 875 in 16 bits.

For the data collection, I have placed the probe on my own stomach, and judging with my own experiences of undergoing an abdominal ultrasound test. When the maximum level of force is applied, the reading rise to 880. The change is not very significant, one of the reason could be that the probe is not secured onto the backing properly, the force is not fully transferred to the tip of the probe. This point will need to be considered when reviewing the mechanical design.

 At this point of time, it is hard to see any difference caused by the orientation of the system, thus an accelerometer may not be necessary.

Abdominal Phantom in Ultrasound Examination

Tissue-mimicking phantom serve an important role in ultrasound research and development without the required to use human or animals in experiment.Ultrasound phantoms are generally of two types.  One mimics the acoustic properties of tissue (with regard to the speed of sound, average attenuation, etc.). The main purpose of the other is to approximate the sonographic appearance of tissue, aiding biopsy training. Comparing the two types, biopsy phantoms are simpler in construction, contain simulated cysts or masses, and are either echogenic or sonolucent. Typical values of soft tissue characteristics are the average speed of sound is 1540 m/s while attenuation coefficients range from approximately 0.5 to 3.3 dB cm−1 MHz−1; they are as  low as 0.18 dB cm−1 MHz−1 for blood. Backscatter coefficients in tissue typically range from 10−5 to 10−1 cm−1 sr−1.

The following popper user interface control may not be accessible. Tab to the next button to revert the control to an accessible version.

Destroy user interface control
Various additives were added to a gelatin base to provide realistic acoustic and optical properties. For example forty-micron, spherical silica particles were used to induce acoustic scattering, Intralipid® 20% IV fat emulsion was employed to enhance optical scattering and ultrasonic attenuation, while India Ink, Direct Red 81, and Evans blue dyes were utilized to achieve optical absorption typical of soft tissues.

Rather than these characteristics, what I am interested in is purely on the physical properties of the phantom. This would ease the process of making a phantom however there is rarely any information can be found on it. I have decided to experiment a bit with combinations of gelatine and gelly aiming for a phantom mimicking the physical properties of human abdomen.

Referenced from:

Acoustical properties of selected tissue phantom materials for ultrasound imaging

Zell, K ; Sperl, J I ; Vogel, M W ; Niessner, R ; Haisch, C

Physics in Medicine and Biology, 2007, Vol.52(20), pp.N475-N484 [Peer Reviewed Journal]

Ultrasound Speed of Polymer Gel Mimicked Human Soft Tissue within Three Weeks
Nur Shakila Othman, Muhamad Suhaimi Jaafar, Azhar Abdul Rahman, Ernee Sazlinayati Othman, and 

Aifa Afirah Rozlan

Tissue-mimicking phantoms for photoacoustic and ultrasonic imaging

Jason R. Cook, Richard R. Bouchard,and Stanislav Y. Emelianov

Ultrasound skin characterization : an in vivo study of intra and 
inter individual variations 
M. Lebertre1
, F. Ossant
1,2
, J. Bouyer
1
, L. Vaillant1,2
, S. Diridollou
3
 and F. Patat1,2
1
GIP Ultrasons / LUSSI EA2102,Tours 
2
University Hospital, Tours 
3 Pierre Fabre Research Institute, Toulouse, France.

lebert_m@med.univ-tours.fr

Problems with Prototype 1

1. Casing of the load cell on the lid is very very tight, the load cell is not perfectly horizontal.
2. Nut slots on the base is  extremely tight that I had to file the nuts a bit.
3. The spindle axis failed to align with the base, I had to use a coupling with a smaller diameter.

A redesign of the top of the load cell holder is required.

Assembly Schedule

Start with the base, put slider into the set of vertical holes and secure with two short bolt that sinks into the opposite side of the backing.

Take a 3mm diameter shaft, put through the hole on the horizontal part of the backing, secure with two nuts on both the top and bottom of the backing. Put a washer on top of the nut before assemble the base of the load cell holder through the shaft. Align the three holes of the base and the backing with 2.5 diameter bolt preferably coach bolt whose threads are only at the bottom. Use the shaft as centre axis, locate the load cell and the top of the cover, rotate until the other set of three holes align with the base, secure with bolts and nuts. The nuts are sunk into the bottom of the base which can be used to help tighten the bolts. Put the coupling into the circular solt.

Put the motor into the cover and secure with four short bolts diameter 2.5mm. On the other hand, allocate the slider base into the cover and tighten a 3mm bolt into the nut sunk within the cover. Slide the slider base into the slider on the backing. Lastly, screw the spindle through the motor and push the 3mm diameter no thread side into the coupling. Tighten the screws on the coupling to prevent rotational motion. Apply glue into the vertical slots within the top of the load cell cover to secure the coupling in location.

Housing