A supplier of anti-lock braking system (ABS) control modules was competing for a contract to supply the braking system for a newly designed hybrid vehicle. The vehicle manufacturer specified that the new design had to be cycle tested at several different pressures. This was a normal specification, but one of the test pressures had to be only 15 psi, with a tolerance of ±1 psi.

The ABS supplier’s existing cycle testing machine could only maintain a pressure accurately down to around 75 psi. It incorporated a proportionally controlled pressure reducing valve and a pressure transducer to provide a feedback signal to close the control loop.

The supplier’s designers researched various proportional pressure controls but could not find any brand that could meet their 15 psi needs. A consultant spent some time looking into the design and recommended using a bleed-off system incorporating a directly operated relief. However, this circuit was unreliable and sensitive to temperature changes and pressure spikes.

Any idea how this problem was solved?