Weighing with simplicity and precision

Written by: Tom Shelley | Published:

Weighing systems tend to be based on either springs and mechanical linkages or load cells with strain gauges

Problem: – neither system being perfect. Mechanical systems are prone to friction and wear while strain gauge load cells incur a significant cost penalty and tend to be temperature sensitive.

Solution: Imperial College MEng student, Jack Orman, has arrived at an alternative as he has devised a low cost capacitance-based weighing system.
Working at Sensatech in Brighton, during an eight week Shell Technology Enterprise Programme (STEP) placement, the design brief for the system demanded that it should weight 0 to 16kg, work within a 0 to 50°C temperature range and with a precision of ±50g.
His design uses a platform supported at each corner on four nylon domes mounted on springs, each with an integral capacitance height sensor. Since capacitance-based height sensors are dependent on the spacing between two plates, they are essentially very reliable, low cost and impervious to the effects of temperature and humidity.
The prototype had a direct materials and labour cost of £50 – about a tenth of that associated with previous designs. It also features complete insensitivity to eccentrically applied loads and the minimal frictional forces experienced through the use of nylon domes removes any associated hysteresis.

Applications: The design can be adapted to both greater and lower load capacities by simply modifying the spring dimensions in the Mathcad worksheet he used.
The project earned Orman a runner up position in this year's STEP Solutions Best Electronic Engineer 2002 Competition and there has since been interest in the product from companies in the UK and Bulgaria. Advances from Sensatech, using capacitance-based techniques, have been revealed in Eureka on a number of occasions in the past. TS


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