The present invention relates to a copying apparatus which forms a latent image electrostatically on a photosensitive element in the form of a belt or a sheet by projecting a light image thereonto and then developing it and transferring the developed toner image onto a recording or copy sheet.
A copying apparatus of the type described conventionally employs a photosensitive element which may take the form of a drum, a belt or a sheet. Like a photosensitive drum, a photosensitive belt or sheet is driven continuously at a constant speed while undergoing various major copying steps such as charging, exposing, developing and transferring. A problem inherent in the use of a photosensitive belt or sheet is that slippage tends to occur between the belt or sheet and rollers adapted to feed it. The slippage is liable to prevent the actual processing timing in each of the copying steps from coinciding with predetermined ones. Complete recording may fail even after the predetermined timings are over or the recorded image may have deviations in small and/or large sections.
In a known system, a photosensitive element drive train is provided with a timing pulse generator (encoder) made up of a slotted plate and a photosensor. This system counts timing pulses from the generator from a start of copying operation and energizes various elements at timings based on the counts of the timing pulses. However, even though the mechanical elements may operate properly, a slippage whether temporary or continuous between the elongate photosensitve element and rollers will shift the actual charging, exposing and other timings either partly or entirely from the operating timings of the mechanism. This prevents a desired image pattern from being reproduced. During a series of continuous copying cycles, the shift or deviation will remain within the range of each copy if the count is reset copy by copy. If this count is not reset, however, the deviation in timing will accumulate and become more critical as the copying cycle is repeated.