当设备屏幕关闭一段时间后，android传感器事件会变慢

I'm trying to collect accelerometer values at 50ms period and do something with collected values at every 1 minute. So i used alarm manager with 1 minute pending intent with RTC_WAKEUP flag and set sensor event to 50ms. It first seemed that sensor event was called well with exact time period and collected 1150~1210 accelerometer values at every one minute passed even when my device screen is off, but when i unplugged my device from computer and turn screen power off, it started to become slower, at the end it collected only 60~100 samples at every 1 minute. So i currently acquire Wakelock whole time and replaced alarm manager with timer to do timer taske at every 1 minute. But i'm worried of drain of my battery.I'm trying to collect accelerometer values at 5