| 摘要 |
<p>A fail-safe pulse train presence detector for a microprocessor-controlled braking system having a number of interruptible safety devices including a control valve, consists of a pulse generator, interface device which receives a periodic trigger signal from the microprocessor and outputs a pulse train signal in response thereto, the pulse train signal is communicated to the base electrode of an NPN transistor and operates the transistor to alternating "on" and "off" states when in respective high and low conditions. A first current path is established during the transistor "off" state through which a first current flows and charges a first capacitor. A second current path branches off from the first current path, wherein a second current flows when the transistor is "on". A forward-biased diode in the first current path and an adjacent reverse-biased diode in the second current path block cross-flowing of currents between the respective paths. A second capacitor in the second current path is charged when the transistor is "on". The current in the second path flows as a result of the charged first capacitor in the first current path, this second current serving to energize the coil of a vital relay and to charge the second capacitor. When the transistor is thereafter turned "off", the second capacitor discharges to energize the relay. The charging and discharging times of the first and second capacitors in this alternating manner is selected to coincide with the duration of the high and low pulse train segments such that, a too long high or low segment prevents respectively, the charging of the second capacitor or discharging of the first capacitor. When the relay drops out, at least one set of contacts is used to interrupt a safety device as, for example, a brake pressure control valve to then apply the brakes.</p> |
