壓電換能器之主動減振控制

Abstract

本論文提出一主動減振控制方案，其適用於車用距離偵測系統之壓電換能器。近年來車用電子領域已日益受到重視。壓電換能器為此領域中一種重要的感測器，其利用超音波來偵測車輛周遭的物體以避免可能發生的碰撞。但是壓電換能器的偵測範圍會受到其本身機械振盪衰減速度的限制，故本論文希望藉由引入主動減振控制來提升此感測器振動的衰減速度。 發送超音波的過程包含了對壓電換能器的驅動以及減振。壓電換能器的最佳操作頻率可根據驅動電路的操作由其共振頻率來決定。此外，由於壓電換能器通常具有極高的頻率選擇性且其特性容易因環境變化而改變，使得要持續有效操作壓電換能器在同一頻率有所困難。因此壓電換能器在進行操作前需先行追蹤其共振頻率。 為了實踐所提出的主動減振控制方案，在本論文中製作了數個驅動電路。第一個電路使用方波電壓來驅動壓電換能器以追蹤其串聯共振頻率。第二個電路則是藉由偵測其可達到零電壓切換的頻率區間來追蹤並聯共振頻率。最後主動減振控制被實現於第三個電路中。透過實驗結果可知，藉由使用主動減振控制，壓電換能器振動衰減速率的理論時間常數可由368.86 μs 改善為117.37 μs。

This dissertation proposes an active damping control method for the piezoelectric transducer in a distance measurement system of a vehicle. In recent years, automotive electronics have become more and more popular and important. Among the devices of automotive electronics, the piezoelectric transducer is one of essential sensors. It utilizes ultrasound to detect the objects surrounding a vehicle to prevent possible collision. However, the detectable range of the measurement system with piezoelectric transducer is restricted by the decay rate of its mechanical vibration, and thus the active damping controls are introduced to enhance the decay rate of the vibration. The process of transmitting ultrasound with active damping controls includes driving and damping a piezoelectric transducer. The optimum operating frequency of a piezoelectric transducer should be determined by the resonant frequencies according to the operation of the driving circuit. Moreover, because a piezoelectric transducer is usually highly frequency-dependent, and its characteristics are easily influenced by the variation of environment, it is difficult to continuously operate a piezoelectric transducer efficiently at a constant frequency. Therefore, the resonant frequencies of the piezoelectric transducer should be tracked before the transmitting process starts. To realize the proposed active damping controls on a piezoelectric transducer, several prototype driving circuits of the piezoelectric transducer were constructed. The first circuit drives the piezoelectric transducer with square-wave voltage to track the serial resonance frequency. As for the second circuit, the frequency at which zero-voltage-switch is achieved is detected to track the parallel resonance frequency. Finally, the active damping control is implemented in the third circuit to transmit ultrasound. The experimental results show that the theoretical time constant of the decay rate is improved from 368.86 μs to 117.37 μs with active damping control.