Motor Control with Meccano
Part 2b Direction of Rotation, cont
Limit Switches:
Limit switches are frequently added to the motion of some part of a model where the continued operation of the motor would cause a disaster, or at least be inconvenient.
Such a situation would be the trolley of the Block Setting Crane.
A limit switch at the end of the boom would prevent the trolley from being driven into the end of the boom producing stress on the system and stalling the motor.
Limit switches are not complicated.
In essence, the switch can be any type of easily activated switching device that will turn off the motor, or reverse the direction of the motor when it reaches the set limit.
Fig. 31 Manual Switch Operation of motor with limit switches.
The figure above shows a solution to the problem of applying limit switches to the use of a manual switch for the control of the motor.
Switch S1 is the direction control switch.It is a center-off type of toggle switch with a momentary contact to each side.
That is, there are three positions for the handle of the switch.
For the purpose of discussion, these will be called forward, reverse and center.
In the center position, no connection exists between the power source and the motor.
Thus, there is no motion of the motor.When the switch handle is pressed toward the right or forward position, the positive side of the power source is connected to the positive terminal of the motor, the negative side of the power source is connected to the negative terminal of the motor, and the motor shaft will rotate in a clockwise (forward) direction.
When the switch handle is pressed toward the left or reverse position, the polarity of the voltage applied to the motor is reversed causing the motor shaft to rotate in a counter clockwise (reverse) direction.
The springs in this momentary contact switch will cause the handle of the switch to return to the center (off) position when the handle is released.
The catalog description of this DPDT switch would be (on) none (on).
The parenthesis indicates a momentary connection.
This is a rather simple motor direction control giving good response to some task such as the control of the travel of the trolley on the boom of the Block Setting Crane.
But what happens if the operator continues to press the switch in the forward direction when the trolley is already at the end of the boom?
The motor stalls, the structure is subjected to unnecessary stress, or in other words, possible disaster.
The limit switch can prevent this disaster.
The simple solution to this would be to have a limit switch that will disconnect the motor when the trolley has reached the end of its travel.
With a switch connected in series with the motor, once it is activated and disconnects the motor, then the motor is stalled in the forward direction.
Something must be done so that the operator can back the trolley away from the end of the boom and continue to operate the crane.
The same problem exists for the other end of the boom, and so two limits switches are needed.
Examine Figure 31.
When the switch is pressed in the forward direction, the motor is connected to the source of voltage (the battery).
The motor will move the trolley in the forward direction until the forward limit switch is activated.
Notice that this switch uses the break contacts.
It disconnects the motor from the supply only when the switch S1 is in the forward position.
Changing S1 to the reverse position causes the motor shaft to rotate in a counter clockwise direction (reverse), an action which clears the Forward Limit Switch.
In this manner, each of the limit switches can prevent an overrun of the trolley in both directions without preventing the operator from bringing the trolley back from either end of its travel.
What kind of switch can serve as a limit switch?
Many of those which were discussed in the early parts of this series.
In the example used for Figure 31, the logical choice would be as small snap action switch using either a plain end or a roller end.
This switch would be mounted on the crane boom in such a position that as the trolley approached the end, a part of the trolley would contact the lever of the switch causing it to function.
The snap action switch is available as a 1C contact meaning that the switch can be used as either a make or break switch.
Another system which could be used as a limit switch is the Photo Gate which can be made using the components of the Electronics Kit.
Figure 32.
The Electronics Kit Photo Gate
This photo gate was used in a number of model plans using the lamp and photo cell of the Electronics Kit.
The lamp (with an end lens #608) would be used to shine a light on the photo resistive cell (#602).
The photo resistive cell would be wired in series with the coil of the Meccano Relay (#606) and a source of voltage (7.5 12 volts).
The contacts of the relay would than form the terminals for limit switch operation.
As long as the beam of light from the lamp to the photo cell is unbroken, the relay is energized.
However, when an object interrupts the light, the relay becomes reenergized and this causes the switch action in the relay contacts.
The components of the Meccano Electrikit can be used to construct a suitable limit switch.
A magnetic reed switch (Figure 13 14) can be used for a limit switch.
There is only one problem with the magnetic reed switch in that they are generally supplied only as a N.O. (normally open) switch.
Thus, some auxiliary relay circuit will become necessary for them to be used when a N.C. (normally closed) switch contact is required as in Figure 31.
However, it is possible to obtain a SPDT magnetic reed switch which is also shown in Figure 13 top switch.
No auxiliary relay circuit would then be needed.
If it is desirable to control the rotation of a shaft through a limit switch, a mercury switch (Figure 16 17) can be employed.
This can be mounted on a Bush Wheel or similar component arranged to tilt to break a connection when the shaft has reached its critical position.
There seem to be few limiting factors to the use of limit switches in motor driven models.
Considering the reported fragility of the plastic gear box on the M5 Motor, it would seem that the limit switch is cheap insurance against disaster.
Motor Control Systems frequently are just combinations of several basic control circuits.
Assume that it is desired to build a model for an exhibition which will operate continuously with a model tram traveling down a short length of track, stop at the end, reverse direction, than return to the starting point to repeat this action until either the exhibition or the patience of the viewers ends.
This back and forth motion is a job for the limit switch.
A block diagram of the total device would be as shown in Figure 33.
Figure 33.
Block Diagram for Oscillating Motor Operation.
The Relay Lockup Circuit shown in Figure 30 would be the basis for the limit switch detection system.
The two push-button switches shown in Figure 30 are the limit switches.
Note that S1 is a make (N.O.) switch while S2 is a break (N.C,) switch.
The arrow above the schematic symbol indicates the direction of action for the switch.
Snap action switches can be used for this function since they are provided with a 1C set of contacts which can be connected as either a make or a break switch.
The Relay Lockup Circuit controls the Motor Direction Control Device.
In this instance, the Motor Direction Control is accomplished through the use of a 2C contact for polarity reversal (Figure 22).
In this application, the 2C contacts would be the relay contacts of a DPDT relay.
Once the total circuit is visualized as a simple block diagram, then a final schematic diagram can be developed.
Of course, there are some refinements which will be added to that simplified block diagram to make it more practical.
The Block Diagram of Figure 33 can thus become the Schematic Diagram of Figure 34.
Figure 34.
Circuit for the
This circuit utilizes two DPDT Relays, Ry 1 and Ry 2.
Ry 1 is used as a polarity reversal circuit to determine the direction of rotation of the motor.
Ry 2 is the familiar Relay Lockup Circuit.
Ry 2 controls the action of Ry 1.
Switch S1 (DPST) is the master control for the entire circuit. It controls the voltage source for the motor as well as for the two relays.
When this switch is closed, the motor will begin turning in a clockwise direction (Forward).
Note that the relay Ry 1 is not energized.
When the Forward Limit Switch is moved in the direction of the arrow, then relay Ry 2 is energized.
Immediately the relay goes into its lockup mode and also energizes Ry 1 through its top set of contacts.
Energizing the relay Ry 1 reverses the voltage supply to the motor causing it to begin to rotate in a counter clockwise direction (reverse).
This will continue until the Reverse Motion Limit Switch is pressed in the direction of the arrow.
This breaks the lockup circuit for the relay, releasing both Ry 2 and Ry 1.
Once again the motor turns in the forward direction to repeat the cycle endlessly until switch S1 is opened.
The voltage sources B1 and B2 must be chosen to fit the requirements of their respective circuits.
If the motor to be controlled is the Meccano M5, then the voltage of B1 should be between 6 and 12 volts.
B2 must match the voltage requirements of the two relay coils.
The limit switches used can be any type which gives the desired switch action.