Abstract:
An electric printer which allows sure printing which depends on identity  ween the orders given on the keyboard and the movement of the printing wheel. It includes means for uncoupling the drive shaft from the wheel and two sensors one of which indicates the position of the drive shaft and the other of which indicates the number of positions passed by the wheel before it stops. Application to a printer used for counting.

Description:
This is a continuation of application Ser. No. 906,303 filed May 15, 1978, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a printer including a plurality of electrically controlled wheels, said wheels printing figures or signs on a roll of paper. 
     BACKGROUND OF THE INVENTION 
     Prior printers print in response to an operator using a keyboard which sends electric signals to position a wheel from a starting position, the movement being controlled by a shaft driven by a motor and rotating through a complete turn in one direction then in the other. 
     A disadvantage of former machines resides in the fact that when they stop accidentally, they print the value at which they stopped. 
     Preferred embodiments of the present invention overcome this drawback. 
     SUMMARY OF THE INVENTION 
     The present invention provides a printer having a plurality of electrically controlled wheels, the wheels being driven by a drive shaft rotating alternately in a drive direction and then in a return direction, there being a pause for printing between the movements in opposite directions, each of the print wheels having typographical characters on its periphery, and being associated with a ratchet wheel having teeth at its periphery, said teeth being engaged by a stop lever which is released by an electromagnet in response to an electronic signal representative of the value to be displayed when the angular movement of said drive shaft corresponds to the value to be displayed, means for uncoupling the shaft from the wheel to separate said drive shaft from said wheel subsequent to the engagement of said stop lever in one of said teeth and electronic means for counting the number of positions through which each of said wheels rotates between a starting position and a stopping position to validate the position of the print wheels as set by the stop levers by comparision of each wheel&#39;s actual position with its intended position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention is described by way of example with reference to the accompanying drawings. 
     FIG. 1 is a schematic cross-section of a wheel of a printer in the rest position, with its associated components; 
     FIG. 2 is a schematic side view of several wheels of a printer; 
     FIG. 3 shows the same components stopped; and 
     FIG. 4 is a block diagram of the electronic circuitry of the printer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The printer is constituted by a plurality of printing columns. Each printing column such as shown in FIG. 1 includes an electromagnet 1 which attracts a stop lever 2 when the electromagnet 1 is energized, the stop lever 2 being returned by a spring 3 and a print wheel 4 which is integral with a ratchet wheel 5 which has teeth, on which the end of the stop lever 2 bears. The print wheel 4 has fifteen type characters for example engraved at its periphery, spaced apart by hollows 6 between teeth which are comparable to these of a toothed wheel. The ratchet wheel 5 has a catch 7 held by a spring 8. A drive shaft 9 which can rotate through 360° in both directions has a V-shaped notch in which the catch 7 engages. The assembly formed by the catch 7 and the spring 8 provides a friction drive of the print wheel 4 by the drive shaft 9. A check wheel 10 which has fifteen teeth, for example, is driven by the print wheel 4. A photoelectronic detector 11 disposed level with the teeth of the control wheel 10 detects as many teeth of the check wheel 10 as the number of characters through which the print wheel 4 passses before being stopped by the stop lever 2. It is sufficient to count this number of teeth to know the real rotation of the print wheel 4. 
     FIG. 2 shows an assembly of four print wheels 4, it being understood that this assembly could consist of a few tens of printing columns. A disc 12 which has notches on its periphery--the number of notches being the same as the number of type characters of the print wheels 4--is disposed on the drive shaft 9 outside the printing columns. A photo-electronic sensor 13 makes it possible to detect the notches of the disc 12. The sensor 13 delivers pulses representative of the position of the drive shaft 9. 
     FIG. 3 shows a print wheel 4 stopped on its second tooth 14, for example, the wheel having left the original position 15 of FIG. 1. This disposition is attained after one complete revolution of the drive shaft 9 in the direction of the arrow. When the drive shaft 9 rotates, it drives the wheel 4 by means of the catch 7 held by its spring 5 since the catch 7 engages in the V-shaped notch of the drive shaft (FIG. 1). In FIG. 2, the stop lever 2 drops due to the action of its spring 3 subsequent to the cut-out of the electro-magnet 1 and the head of the lever 2 moves to a position in front of a tooth 14 of the ratchet wheel 5. The print wheel 4 stops and as the drive shaft 9 continues to rotate, the catch 7 leaves the V-shaped notch of the drive shaft 9 and the shaft is uncoupled from the wheel. 
     While the ratchet wheel 5 is stopped in the second tooth 14 position, the drive shaft 9 continues to rotate in the direction of the arrow, then stops. After the printing operation, the drive shaft 5 rotates in the opposite direction to the arrow, the catch 7 engages again in the V-shaped notch of the drive shaft and as the shaft is coupled to the wheel, the print wheel 4 is brought to its original position. 
     In addition to being synchronised by the mechanical arrangement, the various operations are synchronised by an electronic arrangement. This arrangement, such as schematically shown in FIG. 4, includes an input memory 16 which contains the binary value which comes for example from a keyboard 17. This binary value corresponds to the required stop position of the print wheel 4 and remains in the memory during a complete cycle constituted by the rotation of the drive shaft 9 in both directions and until a new value is inserted. 
     The output 18 of the input memory 16 is sent firstly to a down counter 19 and secondly to a comparator 20. The contents of the memory 16 are transferred to the down counter 19 when a cycle start pulse 21 is received which reaches the pre-positioning terminal 22 of the down counter 19. The down counter 19 also receives on its down-counting terminal 23 the signals coming from the sensor 13 which senses the position of the drive shaft 9, which are shaped by a circuit 24 and transformed into rectangular pulses. The result of this is that the sensor 13 sends a pulse which decrements the counter at each position of the drive shaft. 
     The output 25 of the down counter supplies a logic state 1 when the value transferred by the input memory 16 is reduced to zero by the down counting. In the other cases, the output 25 of the down counter supplies a logic state 0. An RS bistable 26 whose operation is governed by the following truth table: 
     
         ______________________________________      S    R      Q______________________________________      0    1      0      1    0      0      1    1      1______________________________________ 
    
     receives on its R input the signal coming from the output 25 and on its S input the &#34;cycle start&#34; pulse which sets the bistable 26. The bistable 26 is positioned in the state 1 on its Q output by the &#34;cycle start&#34; pulse 21. It feeds the electromagnet 1 via an amplifier 27. The bistable 26 changes only when the down counter 19 is at zero (i.e. when the output 25 supplies the logic state 1). This de-energizes the electromagnet 1 and immobilizes the wheel 4. 
     The &#34;cycle start&#34; pulse 21 is also sent to a reset to zero input 28 of a counter 29 which is incremented by one unit at each pulse which reaches the counting input 30 of the photo-electronic detector 11 whose signals are shaped by the stage 37. 
     The comparator 20 receives on its input 31 the data of the counter 29 and indicates at the output 32 whether there is identity between the input memory 16 and the counter 29 which expresses the wheel position considered. A signal with a logic state 1 appears at the output 32 of the comparator in the case of said identity; the signal is sent on one of the two inputs of an AND gate 32, the other input receiving a validation pulse 33 generated at the end of the rotation of the drive shaft 9. The output signal 34 of the AND gate 32 allows the roll of paper to be printed on when its printing wheel 4 is stopped. If there is no identity inside the comparator 20, the output 32 will be in the logic state 0 and nothing will be printed. 
     The printer in accordance with our invention provides greater reliability due to the two sensors, one of which indicates the position of the drive shaft and the other of which indicates the number of positions passed before the wheel stops. 
     The invention relates to printers used for counting in which part of the calculation is performed mechanically and another part of the calculation is performed electronically.