Square D Company ®Norpak Control System part2

13·2 DEVELOPING NOR LOGIC CIRCUITS

Several methods can be followed in developing static control cir­ cuits. Listed below are some of the most practical in use at this time.

The first method is to convert relay circuits directly to NORs. To convert a relay circuit into NOR logic, substitute for the relay contacts according to the chart shown in Fig. 13·11. After all

the relay functions have been converted, the NOR circuit should be simplified by eliminating redundant functions. Figure 13·11 can be expanded to meet multiple-input circuits as shown in Fig. 13·12.

The relay circuit in schematic form of Fig . 13· 13a can be con­ verted by direct substitution to NOR logic. During the conversion process it is best to work line by line from right (outputs) to left (inputs). With only a general inspection (Fig. 13·13b) we might say that CR 1 is a two-input AND circuit consisting of inputs X

and Y. First we draw a two-input AND, using NORS (Fig. 13·13c) . X and Y would be the inputs, and CR 1 would be the output. Y, however, is a two-input OR with one input being A and the other CR 1 (Fig. 13·13d). X, of course, is the same as now the pieces can be put together to show the entire circuit (Fig. 13 ·13e).

A closer look at the circuit reveals that one of the inputs, CR 1, is also the output CR 1. Some type of feedback or MEMORY circuit is therefore involved. The approach to this circuit could be simplified . MEMORY circuits should be recognized by a hold­ ing contact appearing in the same circuit as the relay coil. Re­ turning to Fig. 13·11, we see that CR 1 is an off-return MEMORY. Direct substitution then provides the correct circuit (Fig. 13.13f). Note that when signal B is absent, the MEMORY should turn off ; this is accomplished through the NOR at the OFF input.

We then proceed to the next line (Fig. 13·14a). Examination indicates that CR2 is a three-input AND consisting of inputs CR 1, LSl , and . ( NOTE: A CR2 contact does not appear, and therefore a MEMORY is not involved.) The NOR equivalent is shown in Fig. 13·14b.

If we proceed to the next line (Fig. 13·14c), we see that CR3 is also a three-input AND consisting of inputs CR 1, LS2, and CR2. Figure 13·14d shows the NOR equivalent.

Consider the next line (Fig. 13·14e). SV1 is the output de­ vice; therefore we simply draw the symbol for an output ampli­ fier and show that the input is CR2 (Fig. 13·14/). The same is true for SV2 (Fig. 13·14g).

The next step consists of first putting the circuit pieces together , making sure that all like “labels” are connected together (i.e., connections are made from CR 1 to all CR 1 points, from LSI to all LS1 points , etc.) as shown in Fig. 13·15, and then reducing the circuit to its simplest form. At this point , we want to connect a signal to NOR b  and a signal to NOR d , but these signals do not appear to be present. The versatility of the NOR shows to great advantage here, as NORS can be eliminated to redu cethe circuit to its simplest form. To eliminate NORs , it is necessary to understand the complementary principle.

Figure 13·16 illustrates the complementary principle with a one-input NOR. A 0 input gives a 1 output. An X input gives an X output, and a CR3 input would give a CR3 output. The output of NOR b is CR3, and therefore a line can be run directly to the

CR3 connection that feeds SV2. The same can be done with NOR d for the CR2 that feeds SVI. Therefore, NORs b and d can be eliminated. Notice too that NORs a and c produce a output (i.e., the complement of CR 1). The NOR MEMORY has a output available; hence NORs a and care not necessary. The final circuit is shown in Fig. 13·17. With a little practice, circuits can be reduced mentally as the conversion is made, but it would be best to work by straightf orward substitution until one becomes more experienced.

The second method is to convert conventional logic to NOR logic. If a conventional logic diagram is to be converted to NORs, the following steps should be taken. Step 1: Replace each NOT

with a NOR (Fig. 13·18a). Step 2: Replace each AND with a NOR and determine the NOR input by taking the complement of the AND inputs (Fig. 13·18b). Step 3: Replace all off-return MEMORYS with the NOR MEMORY (Fig. 13·18c). Step 4: Make all circuit connections, adding NORs only where complemen ta ry signals are needed. Step 5: Examine the circuit for redundancies (Fig. 13·18d) and remove unnecessary NORs.

Figure 13·19a provides an example of an English logic circuit. Figure 13·19b covers steps 1 to 3. If we make circuit connec­ tions (CR 1 to CR 1 points, CR2 to CR2 points, etc.) according to step 4 and examine the circuit for redundancies (step 5), the final circuit is as shown in Fig. 13·20.

The third method is to convert word description to NOR logic. Due to the newness of solid-state control and its associated sym­ bols, this is probably the most difficult method, especially for those accustomed to thinking in terms of relay circuits. However, as the use of solid-state control increases, more circuits will be developed by this method , since it is, in reality, the pure ap­ proach. Depending upon one’s background, two variations are possible. The designer can convert word description to English logic and use the second method for conversion to NOR logic, or he can convert word description directly to NOR logic, as follows. First, try to get an overall picture of the general applica­ tion. Visualize the machine as it goes through the sequences. If possible, actually view the machine in operation. From the word description, notice key words such as and , or, not, time delay, and momentary (suggesting MEMORY). Attempt to break up the description into pieces, and then, part by part, work out the complete logic circuit.

For an example, consider the machine that consists of a planer table which moves back and forth by means of a reversing motor. Momentary action of limit switches located at each end of the bed initiates the reversing contactor which causes the table to oscillate. The protective controls desired are: extreme operating limit switches on each end of the bed, motor overload protection, interlocking with the coolant supply, and minimum lubrication pressure.

What is required to provide a ready-to-run position? With minimum lubrication pressure present, the coolant motor operat­ ing, the drive-motor overload relays incorpora ted in the circuit, and the emergency switches normally closed, it is now desired that the machine either run continuously or jog.

We should pause at this point to visualize a table that oscil­ lates back and forth; its movement is controlled by a limit switch at each end of the table, and emergency limit switches are mounted near the operating limit switches to prevent the table from traveling too far in the event the initial limit switch fails. Of equal importance is the fact that some type of logic must be performed to put the table into operation and also to stop the machine by either manual or automatic operation if predeter­ mined conditions are not met.

The ready-to-run circuit could be controlled by a six-input AND. In general, all forms of start-stop circuits consist of the off-return MEMORY. In this case, the inputs to the AND circuit are coming from pilot devices, and therefore the AN D circuitry can be accomplished by merely connecting these signals in series. The complete ready-to-run circuit would appear as shown in Fig.13·21a.

Consider the work cycle of the machine. When the machine is in the ready-to-run position, the operator presses either a right traverse or left traverse button. For example, if the right traverse button were pressed, the table would move to the right unt il momentary engagement of the right limit switch. When the right operating limit switch is actuated, one desires the right traverse contactor deenergized and the left traverse contactor energized

so that the table is driven to the left until it hits ·tie left operating limit switch. Actuation of the left limit switch . . mses the revers­ ing contactor to reverse the motor and drive the table to the right. In some applications it would be desirable to have the table stop at one end of its travel in order to blow off chips (i.e., a time delay is involved).

Examination of the above information reveals that the ready­ to-run circuit and either the limit switches or the traverse buttons cause the machine to operate. In addition, it is a momentary action of the limit switch or momentary closing of the traverse buttons that causes action. Therefore, a memory circuit is involved. A decision must be made on whether an off-return MEMORY or retentive MEMORY should be used. The machine builder in this instance might insist on having the equipment start up where it left off after power failure, in which case a retentive MEMORY should be used. The work-cycle part of our circuit is shown in Fig. 13·21b. The combined circuit is shown in Fig. 13·22.