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?

Solution to problem:

Pressure reducing valves require a minimum pressure differential across their ports to operate properly. This differential pressure required by most valves limits the minimum pressure they can obtain and hold accurately. The ABS supplier needed an unusually low pressure of 15 psi — far below the minimum 75 psi they could maintain with their current system.

Aside from its inability to accurately work at pressures down to 15 psi, the testing system was well designed and reliable. We suggested they just add a 65 psi check valve in line after the pressure reducing valve’s outlet to provide an additional pressure drop, thereby reducing the outlet pressure to the needed 15 psi. Such a simple idea, but it did the trick!