Using pressure mapping to create better furniture

SATRA uses pressure mapping to assess the comfort of furniture products such as seating and bedding, as well as cushions and mattresses.

Pressure is a very useful indicator of comfort. Anything that presses into us when we sit on a surface exerts a pressure, whether due to a poorly stitched seam or insufficient cushioning, and this can be quantified to enable comparisons of comfort to be made.

Utilising two flexible, conformable blanket-type sensors – which lie on the surface of furniture in such a way that they do not interfere with its normal use – we gather pressure and force data from underneath a subject as he or she sits or lies on the test piece. It is a versatile system, which can be used to instrument both the seat and the back of a chair, or laid end-to-end to instrument a bed or mattress.

The system’s sensors are made of a stretchy and conformable material, so that when laid over the product to be tested, they hug its contours and move with the subject as they sit or lie on it. Being flexible, they eliminate potential bridging or ‘hammocking’ (dipping in the middle) effects that might result from using sensors made of more rigid material. As each square sensor is 470 mm square and contains 1,024 sensing elements, we can gather a wealth of data that help us to quantify some of the furniture’s comfort characteristics, whether it is a seat, mattress or cushion. We can effectively map the distribution of force and pressure under a subject’s body while sitting or lying on the furniture, and use this information to quantify the comfort of the item under test.

Areas of low pressure appear on the software display as a dark blue colour, while areas of potentially uncomfortable high pressure appear as red zones. This technology is particularly useful for the assessment of pressure-relieving medical cushions and for verifying the effectiveness of comfort-enhancing pressure-relief strategies in other seating or bedding.

Assisting product manufacturers

With increased interest in the development and testing of furniture for patients undergoing bariatric treatment (for severe obesity) and other products for assisted living, the assessment of pressure-relief should be very relevant to manufacturers in this market. Using this approach to quantifying pressure and comfort lets us pinpoint which areas of the user’s body are most at risk of developing pressure sores (and other medical problems), and to identify where most load is situated – indicating that pressure-relief features should be focused at these points. This can be achieved by recording as a reference the seated pressure distribution of a subject sitting on a hard flat surface, in order to establish the worst case scenario for pressure-related discomfort, before going on to record seated pressures on the test furniture to establish whether it reduces pressure in the areas of concern.

SATRA can conduct pressure mapping before and after periods of wear on our furniture test machines, allowing us to simulate continuous use and to measure any loss of effectiveness of pressure-relief features and comfort after a period of usage.

The data collected from furniture tested in this way allow us to make judgements about the design of the furniture – in terms both of the suitability of the cushioning and the influence of its design and construction on overall comfort.

A pressure mapping system records such factors as force, pressure and total contact area. This is very useful in understanding how a subject’s weight is distributed across the surface of a seat or mattress, and how cosseting the material of the furniture is. When plotted against time on a graph, it is an excellent way of recording and monitoring changes in comfort over time.

In addition to recording force and pressure over time, we can also plot movement, and change in ‘centre of force’. This describes where a subject’s force is centred on the sensor – the centre of force for a subject sitting slightly lopsidedly will obviously appear off to one side, while a subject leaning forward will display a centre of force towards the front of the trace. Of course, this centre of force will move throughout the duration of a pressure recording, and will be affected by both the shape and structure of the furniture under assessment. For example, wedge-shaped wheelchair cushions will shift the subject’s centre of force either forwards or backwards, depending on their purpose. By comparison, cushions filled with high-density foam will be less compressible than those filled with low-density foam, and could therefore exaggerate any natural tendency for uneven loading that a subject may have.

In addition to recording short-term scenarios at a high sampling rate (up to a maximum of 100 Hz), there is also validity in recording long-term test scenarios at a low frequency sampling rate. Longer pressure recordings can be taken lasting for several minutes or hours (dependent only on the amount of storage memory in the system), where fidgeting, shifting of position and discomfort show up as a series of linear traces on the software. These can be analysed to provide a measure of the stability and long-term comfort of the furniture. Fidgeting or constant shifts in position register on the software as a series of hashed lines, with varying lengths relating to the distance moved by the subject during each small change in position.

As well as obtaining this numerical data, we can also use the system to pictorially identify pressure hotspots under the body of the seated or prone subject. This will highlight any potentially uncomfortable areas of the furniture, along with the anatomical areas most at risk.

Although subjective feedback from users is useful when assessing the comfort or ergonomics of furniture, such an approach can be vague and dependent on many other factors, such as subjects’ physiology, tiredness, and their current mental state. Using our state-of-the-art pressure mapping system, we are able to get objective data describing their comfort, backed up by the interpretive expertise of our furniture department.

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