Components

A composite drawing of the device design is shown in Figure 5.1. Table 5.3 shows the device components. The contact pad (A) that contacts the forefoot is fixed to the top of the shear load cell (B). The shear load cell is attached to a slide (C) which moves superiorly and inferiorly along two vertical chrome steel shafts. The vertical force is transferred to the compression load cell (D) through the slide, whose inferior movement during loading compresses the load cell which in turn will measure this compression force. The slide attaches to a bracket (E) that is fixed to the shear actuator (F) via a mounting plate (G). The shear actuator sits on top of the compression actuator (H) and the two are fixed together via a connecting plate (I). The compression actuator is fixed via fixing brackets (J) to a base plate (K).

Figure 5.1 – Anatomy of loading device. A: contact pad. B: shear load cell. C: slide. D: compression load cell. E: bracket. F: shear actuator. G: mounting plate. H: compression actuator. I: connecting plate. J: fixing brackets. K: base plate.

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Table 5.3 – Details of main device components.

Component Supplier Part number Function

Vertical actuator. SMC Pneumatics (UK) Ltd.

MGPM32- 25A.

Vertical movement. Horizontal actuator. SMC Pneumatics

(UK) Ltd.

CXTM16- 25B.

Horizontal movement. Compression load cell. Applied

Measurements.

CDFM3- 500N.

0 – 500N measurement range. Shear load cell. Applied

Measurements.

OBUG-20kg. 0 – 200N measurement range. Load cell digitiser x 2. Applied

Measurements.

DSC-USB. Supplies power to the load cell and digitises the millivolt signal from it. Allows user to display and log the force values.

Pressure regulators x 2. SMC Pneumatics (UK) Ltd.

AW20- F02BCE.

Regulation of air pressure to cylinders. Solenoid valves x 2. SMC Pneumatics

(UK) Ltd.

SY5120- 6LOU-C6F- Q.

Control of actuator movements.

Limiter switch x 2 SMC Pneumatics (UK) Ltd.

D-M9BL. Detects actuator movements. Base plate, support;

mounting and sliding brackets; contact pad.

Ryder and Wallace Ltd.

SGCN 0433- 13A to 21A.

Aluminium supporting structure for device.

8mm diameter x 500mm long ground shaft

Hepco Motion. NIM08-500. Chrome steel shafts – elastic modulus 200 GPa.

Used in slide for movement of shear load cell to transfer load to compression cell.

Computer control box. Buswell Machine Electronics Ltd.

N/A. Control of loading sequences. Drives solenoid valves.

The target pressure, force and contact area data described above were used to assist in the development of the device components. These data were a starting point rather than a completed technical specification since there was no previous attempt to do this type of research documented, and thus some unseen challenges were expected in both the device and experimental protocol in its use. In order to deliver forces in both a vertical and horizontal direction, in a controlled and cyclical manner, the decision was made to use actuators powered by pneumatic cylinders. This was deemed a suitable and reliable alternative to using a manually driven device, as had been used in previous projects in the same department (Hashmi et al., 2013). The use of actuators would give a repeatable and adjustable means of applying forces to the foot at a safe magnitude. In order to have optimum control of the forces delivered to the foot via a probe impacting the plantar surface, pressure regulators were fitted to both pneumatic cylinders to allow adjustment of the amount of air pressure delivered to them, and therefore the force resulting from the resistance of the foot. A Jun-Air® Quiet Air 6-15 air compressor base unit was used to deliver pressure to the actuators. This system delivers a maximum of 8 bars (120 PSI) of

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pressure and has a 15 litre air tank which makes it ideal for delivering a large range of air pressure values. Velocity of the actuator movements was controlled by air flow restrictors located on the actuators.

It was necessary to have a method of measuring the amount of force delivered so that the air pressure to change the resultant load could be adjusted accurately for each study participant and target plantar pressures achieved with suitable accuracy and repeatability. The use of load cells that could be attached above the actuators and below the contact pad to measure the amount of force delivered by the actuators was deemed a suitable approach. The types of load cells required needed to reflect the types and range of the loads applied. A strain gauge button load cell with a maximum working value of 500N was chosen for compression measurement and a strain gauge single point load cell with a maximum working magnitude of 200N was chosen for shear measurement. Given the contact area of 0.00017m2, these sensors could measure up to 2,830.86 kPa of compression and 1,132.34 kPa; 2.3 and 5.6 times the expected maximum compression and shear pressures applied respectively. The compression load cell measures the vertical load applied directly to the cell, while the shear cell, which was positioned longitudinally, measures the amount of horizontal force delivered causing a bend through the strain gauge within the cell. Having developed an outline concept of what the device was required to achieve, the author consulted engineering design support to agree a technical specification, parts list, assembly schedule, and commissioning process. This allowed conversion of device specifications into practicality.