Bespoke testing on SATRA’s Pedatron
Investigating the value of specific analysis using this versatile machine.
by David Smith
The SATRA STM 528 Pedatron test machine is a versatile platform that can be used as the basis for a number of dynamic whole shoe durability tests. The latest models incorporate a uniquely accessible flooring holder with motorised height adjustment, which permits the use of a wide variety of different floor surfaces. These can be changed to provide a number of specific and challenging test environments.
In the article 'Using the STM 528 Pedatron to simulate running', we explained how the Pedatron can be adjusted to simulate the effects of running. In addition, another area of interest is having the Pedatron mimic the action of walking uphill or downhill. This is because walking on an incline promotes different and often accelerated wear, with additional strains on footwear due to an increased toe flex angle and different underfoot force profiles. To a certain extent, the same is true when inducing exaggerated inversion or eversion (rotation around the ankle joint) by means of walking on a laterally inclined floor surface.
One method of simulating the effects of walking uphill or downhill is the use of inclined flooring surfaces. During Pedatron testing, the wear pattern created by footwear on the floor surface is circular. Each footstrike lands so that the heel of the shoe hits the outer edge of the circle before the toe hits the centre of the circle, and then the whole floor surface rotates underneath the toe. Obviously, the relative height of the floor surface at the centre of this circle of wear is what creates the effect of walking uphill or downhill.
More pronounced soling wear can be achieved by inclining just one end of the floor surface used in the Pedatron to an angle of approximately 2.5°, while maintaining the standard pattern of rotation beneath the test footwear. This has the effect of causing the footwear to 'walk uphill' for a few steps, before the flooring rotates, and presents a surface that is tilted to one side. Another few rotations brings the incline to the heel end of the shoe, making the footwear 'walk downhill' for a few steps, before the rotation continues and the cycle begins again. This pattern is repeated as many times as the floor platform completes a full 360° rotation.
Angling the flooring surface in this way offers a more aggressive test scenario than the use of a standard, flat slab. It creates the effect of walking over varied and undulating terrain – increasing the strains on the upper or on any closure system, as well as increasing sole abrasion.
Using advanced footforms
The foot forms used on SATRA’s Pedatrons are an advanced prosthetic, designed for use by below-the-ankle amputees. They feature an internal supporting structure with a cushioned and compressible outer skin, shaped to mimic the contours and shape of a human foot – complete with toes and toenails.
This kind of foot allows SATRA to use the Pedatron to create realistic wear and abrasion to the interior and upper of footwear, in addition to the machine’s original function of creating realistic sole abrasion. Footwear tested with these prosthetic feet often shows lifelike and realistic indentations in the footbeds, with the imprints of toes clearly visible in the forepart.
This enables the Pedatron to be used for additional bespoke testing, where the focus is on, for example, the footbed, upper material or even the hose worn with the footwear. Use of prostheses allows any of these components to be compared and assessed for durability. This is often done in a 'host' or control shoe, in a realistic and repeatable manner – without recourse to human wear trials, which can be time-consuming and inconsistent. The degree of wear can be quantified visually, or by measurements such as mass loss.
All surfaces covered
The standard flooring surface within the machine can be replaced with a number of novel surfaces to create bespoke test scenarios (figure 1). Footwear has already been tested by ‘walking’ over such materials as gravel, artificial turf, surfaces contaminated with food products and shallow trays of water, as well as across a simulated tool edge. Surfaces with greater or lesser slip resistance can be used, which can promote unusual movement of the foot. This can even cause the footwear to slip on the floor surface in a very lifelike manner, achieved by increasing the horizontal force applied by one of the pneumatic cylinders.
SATRA is often called upon to develop bespoke tests. So, during the course of our research work, the Pedatron has been used for a number of non-standard tests.
For instance, bespoke testing on footwear intended for use in the food industry can be conducted on the Pedatron, examining key properties such as stain resistance and durability of finish under realistic test conditions. In one example of previous work, a homogenous, ubiquitous and controllable material (baked beans) was selected to cover the Pedatron's floor surface, as it was both readily available in large quantities and it possessed staining properties. A tray can be filled with foodstuff like this and loaded into the Pedatron to be walked through, prior to carrying out a series of staining resistance assessments and tests to establish the ease with which the footwear can be cleaned afterwards.
Specialised pressure-mapping testing
The effectiveness of protective or cushioning elements added to the footbed or outsole that are designed to reduce feel-through and discomfort from underfoot surfaces can also be assessed by means of bespoke testing on the Pedatron. SATRA uses pressure-mapping equipment inside footwear to record the effect on underfoot pressure of various types of shoe and boot. Specifically, a metal bar is affixed to the standard 'floor', so that the midpart of the Pedatron's foot strikes it with every step (figure 2). This creates a pressure hotspot beneath the foot that is measurable on the plantar surface of the prosthetic foot inside the shoe. By monitoring underfoot pressure with pressure-mapping equipment while a test like this is active, it is possible to quantify the reduction in underfoot pressure (and the corresponding assumed increase in comfort), that any innovative sole or footbed confers (figure 3).
Assessing footwear likely to get wet
Interestingly, footwear intended for use in and around water does not always need to be waterproof, but it does need to be resistant to the effects of prolonged immersion or walking in water. For example, footwear intended for use in angling and some watersports such as surfing or kayaking could be required to protect the foot against abrasion or provide thermal insulation, but not necessarily be required to keep the foot dry.
SATRA has tested several types of footwear designed to be worn in and out of water, through a combination of flexed water immersion tests and periods of prolonged wear on the Pedatron while still wet. A recently developed test method – SATRA TM446:2015 – 'Resistance to waterborne abrasive particles' is a particularly aggressive test that can be used to assess footwear for watersports. This immersed flexing test serves to accelerate any abrasion and wear resulting from the presence of water and waterborne particles within the footwear while it is flexed. Combining this with the Pedatron test further increases the realistic wear patterns caused by walking motions and forces likely to be encountered in wear. For this kind of application, footwear is kept moistened to keep it wet during Pedatron testing.
Ensuring that bespoke testing is both realistic and repeatable involves recording and verifying the underfoot forces within the Pedatron. To confirm and record forces created during custom testing, a slimline force plate is secured into the flooring area of the Pedatron, using a specially made holder. Forces are then measured during the full gait cycle of the Pedatron, from heel strike to toe-off. It is also sometimes useful to measure and confirm internal pressures within the shoe with the use of in-shoe pressure-mapping sensors, especially to look at increased forepart pressures.
This force plate can also be used to measure forces, including torque forces, under the shoe when a variety of different flooring samples are fitted to the surface of the force plate. Floors with differing slip resistance properties can then be used to evaluate rotational slip under a variety of conditions, and comparisons made between different sole types.
Photograph at top of page shows artificial turf being loaded into the Pedatron as a flooring surface for footwear testing.
How can we help?
SATRA can offer a variety of tailored testing solutions for footwear products, using the Pedatron with advanced prosthetics, different floor surfaces and advanced measurement tools. Members interested in discussing ways that footwear can be assessed using bespoke test methods are invited to email firstname.lastname@example.org
This article was originally published on page 36 of the September 2016 issue of SATRA Bulletin.