STEP-TRANSMISSION SYSTEMS

All of the synchro units we have discussed operate on alternating current. In applications where alternating current is not available, there is a need for a system that can use direct current. The STEP-TRANSMISSION SYSTEM (sometimes referred to as the step-by-step system) is such a system.

A geared step-transmission system is often used to drive compass repeaters on naval vessels and merchant ships having dc power.

Although many variations are used, the simplified step-by-step transmission system shown in figure 4-3 is typical. In this system, a step transmitter and a step motor are used together to transmit angular data (information) between remote locations. When rotated, the shaft of the step transmitter periodically switches a dc excitation voltage from one pair of coils to another in the step motor. The step motor, which is the receiver in the system, responds to this varying excitation by rotating an amount that is proportional to the transmitter's shaft position.

Figure 4-3. - Step-by-step transmission system.

The stator of this step motor has six field coils spaced 60 degrees apart. The coils are connected in three groups of two coils each, with opposite coils connected in series. One end of each pair of coils is connected to one of the brushes in the step transmitter. The other ends of the coils are connected to the other side of the dc supply voltage through a common lead. As the rotor of the step transmitter is turned, the corresponding coils in the step motor are energized in sequence, producing a rotating step-by-step stator field. Thus, the motor rotates in abrupt increments or steps rather than smoothly.

NOTE: The theory of the step-by-step motor is similar to that involved in positioning the bar magnet (chapter 1, fig. 1-10).

For ease of explanation, we will replace the step transmitter with a battery as we discuss the operation of the step motor. If, as shown in view A of figure 4-4, we apply the dc battery voltage across the number 1 coils only, the armature will turn to the position shown. When we apply the voltage also to the number 2 coils, the armature turns to a position midway between the number 1 and number 2 coils, view B. If we now disconnect the number 1 coils, the armature turns until it lines up with the number 2 coils, view C. View D shows the number 2 and 3 coils connected and the armature rotated one step further. As long as this process is continued, the armature can be rotated through 360 degrees.

Figure 4-4A. - Step-by-step motor in various positions. VOLTAGE APPLIED TO NUMBER 1 COILS ONLY

Figure 4-4B. - Step-by-step motor in various positions. VOLTAGE APPLIED TO NUMBER 1 AND NUMBER 2 COILS

Figure 4-4C. - Step-by-step motor in various positions. VOLTAGE APPLIED TO NUMBER 2 COILS ONLY

Figure 4-4D. - Step-by-step motor in various positions. VOLTAGE APPLIED TO NUMBER 2 AND NUMBER 3 COILS

In actual operation, the step-by-step motor is driven by a step transmitter (rotary switch) as shown in figure 4-3. As the switch is rotated, it applies voltage first to coil 1, and then in sequence to 1 and 2 together, coil 2 only, coils 2 and 3 together, coil 3 only, and so on, until the complete revolution is made. As a result, the armature turns in 30-degree steps that follow the rotation of the rotary switch. The rotating arm of the switch can be turned mechanically to angles between zero and 360 in 30-degree steps. The actual angle through which the arm is rotated depends on the specific data to be transmitted by the system. Real systems may transmit data in greater or lesser steps than 30 degrees, depending on system design.

An important point in this type of system is that because the armature is soft iron, either end of the armature may turn and line up with the energized coils. For this reason, a hand reset control is provided on the step-by-step motor. This permits an operator to align the receiver with the transmitter each time the power supply is energized.

A step-transmission system is not self-synchronizing, and its limited number of steps does not permit data to be transmitted smoothly or where relatively small changes in data are required. This type of transmission system is cheap, rugged, and relatively powerful.

Q.2 Compare the power sources of synchros and step-transmission systems.
Q.3 A step transmitter is a modification of what electrical device?