The basic structure of a printing machine such as a sheet-fed offset printing press includes a main drive and an electronic control mechanism. Such a main drive and control mechanism is disclosed in U.S. Pat. No. 4,951,567, which is hereby incorporated by reference. The main drive typically contains three components: (1) an electric main drive; (2) an electromechanically operable brake; and (3) a tachometer generator. The three components of the main drive perform two primary functions, namely, driving and braking. To this end, the electric main drive typically contains a D.C. motor for driving the printing machine. The electromechanically actuable brake is assigned to this D.C. motor and is controlled by the control mechanism. The tachometer generator monitors the D.C. motor and feeds the value of the actual speed of the motor to the electric main drive. The electric main drive subsequently supplies the motor with current in such a way that the actual speed of the motor agrees with a desired value prescribed by the control mechanism of the printing machine. In other words, the tachometer and the electric main drive are typically connected in a feedback loop to control the operation of the D.C. motor.
The second component of the printing press, the electronic control mechanism, can be designed as one or more computers. The typical control mechanism has three components: (1) input sensors; (2) processing electronics; and (3) output electronics. Input sensors are used in the control mechanism to monitor the state of the printing machine. The processing electronics are used to determine whether the machine is operating in an acceptable fashion and to generate appropriate control signals if corrective actions are required. The output electronics relay the control signals generated by the processing electronics to the appropriate components of the printing machine.
In the normal course of operation it is sometimes necessary to bring a printing press to a stop. For example, it may be necessary to stop the machine to perform maintenance procedures or the like. Under such circumstances, the printing machine can be brought to a standstill at a relatively gradual rate. In certain emergency circumstances, however, it may become necessary to stop the printing press very rapidly such that the machine reaches a standstill within a predetermined maximum time period. This braking can be performed purely electrically by applying a braking armature current having a polarity opposite to the armature current which drives the D.C. motor or mechanically by actuating the electromechanical brake. Purely electrical braking is preferably when possible because it is generally less stressful on the components of the printing machine than mechanical braking.
Printing presses such as sheet-fed offset printing machines are often provided with multiple electrically secured guards and a plurality of emergency STOP pushbuttons positioned at various easily accessible locations on the machine. The electrically secured guards prevent an operator from coming into contact with moving or rotating machine parts. If such a guard is opened while the printing machine is running, a signal indicating the existence of an emergency condition is generated and the printing machine is quickly brought to a standstill. Similarly, if an operator or other personnel actuates one or more of the emergency STOP pushbuttons, the printing machine is rapidly brought to a stop. For this purpose, safety conditions exist which prescribe that, after an emergency STOP pushbutton is actuated or after a guard is opened when the machine is operating, the machine must have reached a standstill within a specific maximum time period.
In order to be able to carry out a multiplicity of movement processes both at lower and at higher rotational speeds, modern sheet-fed offset printing machines include, as a rule, D.C. drives with line-commutated power converters (e.g. 4-quadrant drives). This drive structure enables these presses to achieve a wide range of rotational speed settings, high power, and electrical braking. Thus, it is possible for these machines to respond to these emergency conditions (i.e., the opening of a guard with the machine running or the actuation of an emergency STOP pushbutton) by electrically decreasing the rotational speed of the D.C. drives.
However, additional security measures have proven necessary so that sheet-fed offset printing machines which include such a drive can fulfil the above-mentioned safety conditions for emergency stops, namely, bringing the press to a standstill within the prescribed maximum time period. Thus, for example, EP O 243 728 B1 provides a security system for a printing machine which is not only capable of controlling the actual rotational speed of the drive, but which also monitors and compares the desired and actual values of the rotational speed of the drive during braking and actuates an electromechanical brake if an impermissibly high deviation between these values is detected. This system, thus, first responds to an emergency STOP command by electrically braking the press and, if the actual value of the rotational speed does not reduce rapidly enough, by actuating the mechanical brake. The system uses a tachogenerator or tachometer to detect the actual value of the rotational speed of the drive. This system is disadvantageous because, in the final analysis, the reliability of the safety system depends on the reliability of the rotational speed detection which is to say, on the reliability of the tachometer. A further disadvantage results from the fact that printing machines represent a different load for the drive under different operational conditions and have different levels of freeness in their bearings. As a result, the predetermined rotational speed/time curve is monitored and matched to the minimum possible load torque in the case of an emergency STOP.
EP 0 187 247 B1 discloses another method for monitoring the braking of a printing machine. In this method, a device is used to detect movement of the motor and, the braking torque of the electromagnetic brake is determined based on these readings. However, it is not possible to monitor a purely electrical braking with this method.
EP 0 572 805 A2 discloses a method for monitoring the rotational speed of the drives in a printing machine operating at a creep speed and having a D.C. shunt motor. This method ensures that in specific operating states such as the creep speed state no impermissibly high rotational speed deviation occurs. For this purpose, the armature voltage of the motor is monitored and, in the event of an impermissibly high deviation, a control circuit is interrupted. Thus, a pure rotational speed monitoring of the drive is performed in this method, so that the observance of a required braking process can only be determined by means of additional measures.