Stepping switch
Encyclopedia
In electrical controls, a stepping switch, also known as a stepping relay
, is an electromechanical
device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. It can step on one axis (called a uniselector), or on two axes (a Strowger switch). Stepping switches were invented by Almon Strowger in 1888. The major use for these devices was in early automatic telephone exchange
s (commonly called Strowger or step-by-step exchanges or steppers) to route telephone call
s. Later, they were often used in such equipment as industrial control systems.
A basic stepping switch is an electrically-operated rotary switch with a single (typically input) terminal, and multiple (typically output) terminals. Like other typical rotary switches, the single terminal connects to one of the multiple terminals by rotating a contact arm, sometimes called a wiper, to the desired position. Moving from one position to the next is called stepping, hence the name of the mechanism. Using traditional terminology, this is a single-pole, multi-position switch.
While some stepping switches have only one pole (layer of contacts), a typical switch has more; in the latter case, all wipers are aligned and move together. Hence, one input with multiple wires could be connected to one of multiple outputs, based on the receipt of a single set of pulses. In this configuration, the rotating contacts looked, in general, somewhat like the head support arms in a modern rigid ("hard") disk drive. Multipole switches were common; some had perhaps as many as a dozen poles, but those were less common.
Most switches have a bank of stationary contacts extending over half a cylinder, while some have only a third of a cylinder. The typical "half-cylinder" switch has two sets of wiper contacts opposite each other, while the "third of a cylinder" type has three sets, equally spaced. For any given level, both or all three wipers are connected, so it makes no difference which of the two (or three) is connecting.
An electromagnet advances (steps) the wipers to the next position when fed with a pulse of DC. The magnet's armature (spring-loaded) operates a pawl that advances a ratchet. When the pawl reaches its full stroke, it blocks the ratchet so it and the wipers will not overshoot. When power to the coil disconnects, the spring retracts the pawl. Another pawl pivoted on the frame ensures that the wipers do not move backward; contact friction keeps them in place.
In most applications, such as telephony, it is desirable to be able to return the wipers to a "home" position; this is at the beginning of rotation, at one end of the array of fixed contacts. Some switches have a cam attached to the wiper shaft. This cam operates a set of contacts when the wiper is at home position, which is at the beginning of the span of rotation. Other circuit designs used one level (pole) of the contacts to home the wipers, so the separate homing contacts weren't needed.
Typical stepping switches have contacts directly operated by the stepping magnet's armature; these contacts can serve to make the magnet cycle ("self-step") and advance the wipers as long as power is applied. The external control circuits remove power when the wipers reach the desired position; that could be the home position.
Most stepping switches rotate the wipers in only one direction, but some are bidirectional; the latter have a second magnet to rotate the wipers the other way. A third variety "winds" a spring as the wiper steps progressively, and a ratchet holds the wipers from returning to home position. When the circuit is no longer needed, another electromagnet releases the holding pawl; the spring then returns the wipers back to home.
Stepping switches were quite noisy in operation (especially when self-stepping), because their mechanisms accelerated and stopped quickly to minimize operating time. One could compare their sound to that of some snap-action mechanisms. Nevertheless, they were engineered for long life, given periodic maintenance; they were quite reliable.
So far, this description refers only to single-axis stepping switches, sometimes known as uniselectors.
For a different view of an array of stepping switches, see the Panel switch
article; look for the photo labeled "Panel GCO District Sender Selector Frame". Here, you are looking into the bank of fixed contacts and the rotary wiper contacts; the stepping electromagnets are also in view. Note the silver-colored dials that show the current position of the moving contacts.
or two motion selector in Britain). Typically, a single compact group of wipers could connect to one of 100 different fixed contacts. When the switch was idle, the wipers were disengaged from the fixed contacts. The wipers moved up and down on a vertical shaft, and rotated into the contact bank to make a connection. Gravity returned the wipers to their home position at the bottom.
This type had two stepping coils with pawls and ratchets, one to raise the wipers to the desired banks of contacts, and one to rotate the wipers into the banks. These were commonly used in telephone switching with ten banks of ten contacts. The coils were typically driven by the electrical pulses derived from a rotary telephone dial. On a two-motion selector, as a digit was dialed, the wipers would step up the banks, then automatically rotate (self-step) into the selected bank until they found an "unused" outlet to the next switch stage. The last two digits dialed would operate the connector switch (final selector in Britain). The second to last digit would cause the wipers to move up and the last digit would cause them to rotate into the bank to the called customer's line outlet. If the line was idle then ringing
voltage would be applied to the called line and ringing tone was sent to the calling line.
Another variant of the two-axis switch was the Stromberg-Carlson
X-Y Switch which was quite common in telephone exchanges in the western USA. It was a flat mechanism, and the moving contacts moved both sidewise, as well as to and fro. It was quite reliable, and could be maintained by people with minimal training.
could be constructed. Or by feeding the stepping contact with an endless pulse
train via a relay
, and controlling the relay from the switch's own output, it can be made to automatically hunt for the first unpowered line (or powered, depending on whether the relay is normally open or normally closed). They could also be used as a demultiplexer, so that two input lines could control a number of output devices. One input line steps the switch until the correct device is selected, and the other then powers that device. Many other applications are possible.
Such switches were used in a series of Japanese cypher machines during World War 2
: CORAL
, JADE
, PURPLE (the names were American). Some of the equipment used to break the Enigma machine
code also used many such switches, which some observers called the "Machine Gun" for the loud noise.
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
, is an electromechanical
Electromechanics
In engineering, electromechanics combines the sciences of electromagnetism, of electrical engineering and mechanics. Mechanical engineering in this context refers to the larger discipline which includes chemical engineering, and other related disciplines. Electrical engineering in this context...
device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. It can step on one axis (called a uniselector), or on two axes (a Strowger switch). Stepping switches were invented by Almon Strowger in 1888. The major use for these devices was in early automatic telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
s (commonly called Strowger or step-by-step exchanges or steppers) to route telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
s. Later, they were often used in such equipment as industrial control systems.
Single-axis stepping switches (uniselectors)
Stepping switches were widely used in telephony and industrial control systems (among related applications) when electromechanical technology was paramount.A basic stepping switch is an electrically-operated rotary switch with a single (typically input) terminal, and multiple (typically output) terminals. Like other typical rotary switches, the single terminal connects to one of the multiple terminals by rotating a contact arm, sometimes called a wiper, to the desired position. Moving from one position to the next is called stepping, hence the name of the mechanism. Using traditional terminology, this is a single-pole, multi-position switch.
While some stepping switches have only one pole (layer of contacts), a typical switch has more; in the latter case, all wipers are aligned and move together. Hence, one input with multiple wires could be connected to one of multiple outputs, based on the receipt of a single set of pulses. In this configuration, the rotating contacts looked, in general, somewhat like the head support arms in a modern rigid ("hard") disk drive. Multipole switches were common; some had perhaps as many as a dozen poles, but those were less common.
Most switches have a bank of stationary contacts extending over half a cylinder, while some have only a third of a cylinder. The typical "half-cylinder" switch has two sets of wiper contacts opposite each other, while the "third of a cylinder" type has three sets, equally spaced. For any given level, both or all three wipers are connected, so it makes no difference which of the two (or three) is connecting.
An electromagnet advances (steps) the wipers to the next position when fed with a pulse of DC. The magnet's armature (spring-loaded) operates a pawl that advances a ratchet. When the pawl reaches its full stroke, it blocks the ratchet so it and the wipers will not overshoot. When power to the coil disconnects, the spring retracts the pawl. Another pawl pivoted on the frame ensures that the wipers do not move backward; contact friction keeps them in place.
In most applications, such as telephony, it is desirable to be able to return the wipers to a "home" position; this is at the beginning of rotation, at one end of the array of fixed contacts. Some switches have a cam attached to the wiper shaft. This cam operates a set of contacts when the wiper is at home position, which is at the beginning of the span of rotation. Other circuit designs used one level (pole) of the contacts to home the wipers, so the separate homing contacts weren't needed.
Typical stepping switches have contacts directly operated by the stepping magnet's armature; these contacts can serve to make the magnet cycle ("self-step") and advance the wipers as long as power is applied. The external control circuits remove power when the wipers reach the desired position; that could be the home position.
Most stepping switches rotate the wipers in only one direction, but some are bidirectional; the latter have a second magnet to rotate the wipers the other way. A third variety "winds" a spring as the wiper steps progressively, and a ratchet holds the wipers from returning to home position. When the circuit is no longer needed, another electromagnet releases the holding pawl; the spring then returns the wipers back to home.
Stepping switches were quite noisy in operation (especially when self-stepping), because their mechanisms accelerated and stopped quickly to minimize operating time. One could compare their sound to that of some snap-action mechanisms. Nevertheless, they were engineered for long life, given periodic maintenance; they were quite reliable.
So far, this description refers only to single-axis stepping switches, sometimes known as uniselectors.
For a different view of an array of stepping switches, see the Panel switch
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...
article; look for the photo labeled "Panel GCO District Sender Selector Frame". Here, you are looking into the bank of fixed contacts and the rotary wiper contacts; the stepping electromagnets are also in view. Note the silver-colored dials that show the current position of the moving contacts.
Two axis stepping switch
Slightly more complicated was the two axis stepping switch, (also called Strowger switchStrowger switch
The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger...
or two motion selector in Britain). Typically, a single compact group of wipers could connect to one of 100 different fixed contacts. When the switch was idle, the wipers were disengaged from the fixed contacts. The wipers moved up and down on a vertical shaft, and rotated into the contact bank to make a connection. Gravity returned the wipers to their home position at the bottom.
This type had two stepping coils with pawls and ratchets, one to raise the wipers to the desired banks of contacts, and one to rotate the wipers into the banks. These were commonly used in telephone switching with ten banks of ten contacts. The coils were typically driven by the electrical pulses derived from a rotary telephone dial. On a two-motion selector, as a digit was dialed, the wipers would step up the banks, then automatically rotate (self-step) into the selected bank until they found an "unused" outlet to the next switch stage. The last two digits dialed would operate the connector switch (final selector in Britain). The second to last digit would cause the wipers to move up and the last digit would cause them to rotate into the bank to the called customer's line outlet. If the line was idle then ringing
Ringing (telephony)
A ringing signal operates a bell or other ringtone generator in a telephone, to alert the user to an incoming telephone call.Telephone bells or ringtones are rung by impressing a 105-volt RMS 20-Hertz sine wave across the tip and ring conductors of the subscriber line. This signal is produced by a...
voltage would be applied to the called line and ringing tone was sent to the calling line.
Another variant of the two-axis switch was the Stromberg-Carlson
Stromberg-Carlson
Stromberg-Carlson was a telecommunications equipment and electronics manufacturing company formed in 1894 as a partnership of Alfred Stromberg and Androv Carlson. Along with four other companies, it controlled the United States national supply of telephone equipment until after World War...
X-Y Switch which was quite common in telephone exchanges in the western USA. It was a flat mechanism, and the moving contacts moved both sidewise, as well as to and fro. It was quite reliable, and could be maintained by people with minimal training.
Applications of stepper switches
As well as the decoding of pulses from telephones, stepping switches could be used for a variety of purposes, depending on how they were wired. By connecting several in series with the highest output of one going to the stepping contact of the next, a counterCounter
In digital logic and computing, a counter is a device which stores the number of times a particular event or process has occurred, often in relationship to a clock signal.- Electronic counters :...
could be constructed. Or by feeding the stepping contact with an endless pulse
Pulse (signal processing)
In signal processing, the term pulse has the following meanings:#A rapid, transient change in the amplitude of a signal from a baseline value to a higher or lower value, followed by a rapid return to the baseline value....
train via a relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
, and controlling the relay from the switch's own output, it can be made to automatically hunt for the first unpowered line (or powered, depending on whether the relay is normally open or normally closed). They could also be used as a demultiplexer, so that two input lines could control a number of output devices. One input line steps the switch until the correct device is selected, and the other then powers that device. Many other applications are possible.
Such switches were used in a series of Japanese cypher machines during World War 2
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
: CORAL
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
, JADE
JADE
JADE was the codename given by US codebreakers to a Japanese World War II cipher machine. The Imperial Japanese Navy used the machine for communications from late 1942 until 1944...
, PURPLE (the names were American). Some of the equipment used to break the Enigma machine
Enigma machine
An Enigma machine is any of a family of related electro-mechanical rotor cipher machines used for the encryption and decryption of secret messages. Enigma was invented by German engineer Arthur Scherbius at the end of World War I...
code also used many such switches, which some observers called the "Machine Gun" for the loud noise.