In the actual commissioning process, sometimes there is such a situation, a software system from the theoretical extrapolation can fully meet the process requirements of machinery and equipment, and in the operation process in any case can not be put into normal operation, in the process of system commissioning, in addition to the consideration of software design methods, but also from the following aspects to find a solution.
1 scan cycle and response time
PC design a control system, one of the most important parameters is the time, the PC to execute all the instructions in the program to use how much time, (scan time) there is an input signal after how long the PC to have an output signal (response time) to master these parameters, the design and debugging of the control system is undoubtedly very important.
When the PC starts running, it serially executes the program in memory. We can divide the scan time into four parts. Common parts, such as clearing the time monitor and check the program memory; data input, output; execution of instructions; execution of peripheral device instructions.
The time monitor is a timer used to measure the scan time inside the PC, and the so-called scan time is the total time spent to execute the above four parts. The amount of scan time depends on the acquisition of the system, the number of I/O points, the instructions used in the program and the connection of peripheral devices, when the hardware design of a system is finalized, the scan time depends mainly on the length of the software instructions from the PC receives an input signal to the output side of the time required to output a control signal, called the response time, the response time is variable, for example, at the end of a scan cycle, the receipt of an input signal, the end of the next scan cycle, received an input signal, the next scan cycle a start, the input signal to play a role, at this time, the response time of this input signal is the shortest, it is the input delay time, scan cycle time, the output delay time and the three, if the start of the scan cycle received an input signal, the input signal will not play a role in the scan cycle, only If an input signal is received at the beginning of a scan cycle, the input signal will not work during the scan cycle, but only until the next scan cycle, when the response time of this input signal is the longest, which is the sum of the input delay time and output delay time:
The minimum response time = input delay time + scan time + output delay time, the maximum response time = delay time + 2 × scan time + output delay time.
From the above response time estimation formula can be seen, the response time of the input signal is determined by the scan period, the scan period depends on the hardware configuration of the system on the one hand, on the other hand, by the instructions used in the control software and the number of instructions to decide, in the block molding machine automatic control system debugging process occurs in such cases, the automatic push plate process (the process of sending blocks from the molding table to the conveyor) to start, The process of transporting bricks is completed and the process of sending boards is completed at the same time, which informs the control system that the next forming process can be completed.
However, in the process of debugging, it was found that the actual control timing of the system was such that when the first molding process was completed, the automatic board pushing process was not carried out, but the next molding process was started directly. After repeatedly checking the program, no error is found, and only then is it considered that the response time of the command may have created a problem. Block molding machine control system is a huge system, its software control instructions up to five or six hundred. Forming process start signal, from a forming process end signal and have a board signal, then, will produce such a situation, in a certain scan cycle to scan the HR002 signal, in the implementation of the reset push plate process, directly to the next forming process, which may be caused by the response time of the input signal is too long, in this case, because the hardware configuration can not be changed, the number of instructions can not be In this case, since the hardware configuration cannot be changed and the number of instructions cannot be changed, the problem was solved by trying to make adjustments in the software to make the end of the molding process signal occur sooner.
2 software reset
PLC program design in the use of the most common one is called the internal relay to maintain the relay. PLC to maintain the relay from HR000 to HR915, a total of 10 × 16, the other is a timer or counter from TIM00 to TIM47 (CNT00 or CNT47) a total of 48 (different models of PLC to maintain the relay, the timer points are different). Among them, keep the relay is to achieve the function of memory, memory of the mechanical system operation, the normal timing of the operation of the control system, in the control of timing, in order to achieve the safety of control, timeliness, accuracy, usually used when a mechanical action to keep, the control signal (generated by the keep relay) used to terminate the previous mechanical action at the same time, the next mechanical action to start the time relay If the hold relay is not forced to reset before power on, it will produce mechanical equipment misoperation, the system design, the usual method is to set the hardware reset button, when needed, can make the hold relay, timer, counter, high-speed counter forced to reset, in the control system debugging found that if the use of hold relay, timer, counter, high-speed counter, if the system is not reset, it is not possible to reset. high-speed counter too many times, the hardware reset function will often not work, that is, the hardware reset method sometimes can not be accurate, timely PLC internal relays, timers, counters reset, resulting in the normal operation of the control system, in the debugging process, artificially set the software reset signal as the internal signal, to ensure that the relay to maintain effective reset, so that the system in any In the commissioning process, the software reset signal is artificially set as the internal signal to ensure that the relay is effectively reset, so that the system can operate normally under any circumstances.
3 Hardware Circuit
The PLC is composed of the control system hardware circuit. When a two-wire sensor, such as photoelectric switches, proximity switches or limit switches, etc., as the input signal device is set to the input of the PLC, the leakage current may cause the input signal is ON, in the commissioning of the system, if the occasional false action, it is possible that the leakage current generated by the wrong signal. In order to prevent this from happening, in the design of the hardware circuit, the input terminal is connected to a parallel resistance, parallel resistance calculation formula.
Among them, different models of PLC leakage current values can consult the product manual provided by the manufacturer, in the hardware circuit to do this treatment, can effectively avoid the false operation due to leakage current.