S TEPPER M OTOR I N T E R FA C E S
S TEPPER M OTOR I N T E R FA C E S
Stepper motor interfaces , as their name implies, are used in applications requiring control of stepper motors. Stepper motors are permanent-type magnet motors that translate incoming pulses, through a stepper translator, into mechanical motion. Stepper is a generic term that describes this type of brushless motor capable of making fixed angular motions in response to a step input.
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
S ECTION Components Special Function I/O and C HAPTER 2 and Systems
Serial Communication Interfacing 8
AB Marker
Channel A 1
Common
Channel B 3 Encoder
Pulses = Preset 7 + TTL
Output 8 – Device Pulses > Preset
9 + TTL Output
Power – Supply
12 +V –V
– 5 VDC
Power Supply
Encoder/Counter
Module
Figure 8-19. Encoder/counter interface connection diagram.
The motion of a stepper can be accelerated, decelerated, or maintained constantly by controlling the pulse rate output from a stepper module. The ability to respond to an input voltage (in the form of DC pulses) makes stepper motors well suited for incremental motor programmable control systems. Under controlled conditions, a stepper motor’s motion follows the number of input pulses. This ability to respond to a fixed input enables the system to operate in an open-loop mode, leading to cost savings in the total system. However, in high-response applications, closed-loop operation is generally required (using encoder feedback). Figure 8-20 illustrates a simplified block diagram of a stepper motor system.
Optional position loop feedback
Figure 8-20. Block diagram of a stepper motor system.
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
S ECTION Components Special Function I/O and C HAPTER 2 and Systems
Serial Communication Interfacing 8
A stepper interface generates a pulse train that is compatible with the stepper translator, indicating distance, rate, and direction commands to the motor. The motion induced can be rotational or linear, such as the forward or backward movement of a linear slide using leadscrews. Figure 8-21 shows a typical linear slide using a stepper motor that makes one revolution per 200 steps (resolution), thus yielding a 1.8 ° step angle (360/200 or 1/200th of a revolution). The stepper system shown in the figure provides a linear movement of 0.00125 inches per step because of the 4 threads per inch leadscrew. Example 8-2 illustrates how to calculate linear movement and step angle values.
Processor
Data
Stepper Module
Transfer
Encoder Module
FWD Pulses
4 threads/inch
0–10,000 pulses/sec
200 steps/revolution
X-Axis Scale
Speed Rate
Position
Figure 8-21.
A linear slide using a stepper motor.
E X AM PLE 8 -2
Referencing Figure 8-21, suppose that the 200-step motor is operating at half-stepping conditions (400 steps per revolution) and that the leadscrew has 5 threads per inch. What are the step angle and linear displacement per step used in the system?
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
S ECTION Components Special Function I/O and C HAPTER 2 and Systems
Serial Communication Interfacing 8
S OLU T I ON
To compute the step angle, divide the number of degrees in one revolution (360 ° ) by the number of steps required to turn the motor. Therefore, the step angle is:
360 ° Step angle =
with a resolution of 1/400th of a revolution. Linear displacement is the number of inches moved in one step. To calculate this, multiply the number of threads it takes to move one inch by the number of steps in
a revolution, since each thread requires one revolution (rotational-to- linear displacement). In this case, the leadscrew requires 5 revolutions to move one inch, and each revolution requires 400 steps.
1 " travel = ( 5 rev )( 400 steps/rev ) = 2000 steps
The number of outburst pulses sent to the stepper, which translates into linear or rotational units of travel, defines position displacement. Therefore, the number of pulses sent to the motor from the module determines the motor’s final position. The actual location also depends on the resolution of the stepper and the application, which defines the number of threads per inch of travel in the leadscrew.
The stepper’s movement includes both the acceleration and deceleration of the motor. The acceleration part of the move is the time required to achieve the continuous speed rate of the motor (in pulses/sec). The continuous rate is the final pulse/sec rate sent to the motor (frequency). This frequency may vary from 1 to 20 kHz (pulses/sec). Conversely, the deceleration part is the time required for the speed rate to decrease to zero (pulses/sec). Acceleration and deceleration, also known as ramps, are specified as a function of time (seconds).
Stepper motor interfaces operate in two modes: single-step profile mode and continuous profile mode . In single-step mode, a PLC processor sends indi- vidual move sequences to the interface. These sequences include the accel- eration and deceleration rates of the move, along with the final or continuous speed rate (see Figure 8-22). Once this move sequence is terminated, the processor may start another one by transferring the next move’s profile information and commands. The processor can store several single-step mode profiles and send them to the module through the PLC program control.
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
S ECTION Components Special Function I/O and C HAPTER 2 and Systems
Serial Communication Interfacing 8
Continuous Rate
Continuous Rate
Continuous Rate
Accl. Decel.
Start
Move 1
Final Start
Move 2
Final Start
Move 3 Final
Move
Position Move
Position Move
Position
Position
Figure 8-22. Single-step profile mode.
In continuous mode, the motion profile is cycled through various accelera- tions, decelerations, and continuous speed rates to form a blended motion profile (see Figure 8-23). Rather than requiring additional commands for motion speed changes, an interface in continuous mode receives the whole move profile in a single block of instructions. The interface then performs the step motor control duty until the motion is completed and the processor sends the next profile. As in the single-step mode, the processor can store several continuous mode profiles in its memory and send them to the interface during the program execution.
Acceleration 2
Deceleration 3
Rate Acceleration 1
Continuous Rate
Continuous Rate
Figure 8-23. Continuous profile mode.
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
S ECTION Components Special Function I/O and C HAPTER 2 and Systems
Serial Communication Interfacing 8
Each stepper interface used to control a stepper motor controls an axis, since the motion generated causes a movement about either the X-, Y-, or Z-axis (see Figure 8-24). Depending on the PLC manufacturer, more than one axis may be controlled using several stepper module interfaces. When multiple- axis motions are required, the axes can be controlled either independently or synchronously (see Figures 8-25a and 8-25b, respectively). When controlled independently, each axis is independent of the other, executing its own single-step or continuous profile mode. The beginning and end of each axis motion may be different. When controlled synchronously, the beginning and end of the motion commands for each axis occur at the same time. A profile of one of the axes may start later or end before the other axes (see Figure 8- 25b), but the move that follows will not occur until all axes have started and ended their motions.
Processor and
Stepper
Stepper
Power Supply Module #1
Module #2
Stepper Module #3
Figure 8-24. PLC system using stepper modules to control three axes.
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
E S 2 C T N IO
Axis 1-X
Axis 1-X
Speed
a C Rate
In d Move 1
u Time s
Axis 2-Y
Axis 2-Y
e Speed x Speed
t& Rate w
Move 3 Time
x y t.c
Axis 3-Z
o Speed m Speed
Axis 3-Z
Move 3 Time In tio
te n c rfa
I/
End of Move 1
End of Move 2
End of Move 3 O in n a
(a) Independent
(b) Synchronous
Figure 8-25. (a) Independent axis control and (b) synchronous axis control.
S ECTION Components Special Function I/O and C HAPTER 2 and Systems
Serial Communication Interfacing 8
The use of a position/velocity feedback scheme (see Figure 8-26) can greatly improve the operation of a stepper motor control system, because this scheme provides closed-loop positioning control. The most common feedback field device used in a stepper control system is the encoder. In a position/velocity feedback scheme, the encoder is interfaced with an encoder input module to form a closed-loop stepper control system.
Position Feedback
Stepper Motor Driver or Translator
Absolute Encoder
Stepper Motor
Figure 8-26. Stepper motor with a position/velocity feedback scheme.
Knowledge of the load being driven is useful when applying a stepper interface in a stepper motor application. Loads with high inertia require large amounts of power for acceleration or deceleration; therefore, proper inertia matching is desired. As a rule of thumb, the load inertia should not exceed ten times the rotor inertia. The friction of the system should be examined to prevent the system from being underdamped (not enough friction) or from losing position accuracy (too much friction).
Coupling mechanisms connect a stepper motor to its load. These mechanisms include metal bands, pulleys and cables, direct drives, and leadscrews, which are used mostly for linear actuation. Figure 8-27 illustrates a diagram of a
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
S ECTION Components Special Function I/O and C HAPTER 2 and Systems
Serial Communication Interfacing 8
typical stepper motor interface connection with jog forward and jog reverse capabilities. During jog forward, the operator pushes the jog forward push button, which turns the motor ON for as long as the button is pushed. This allows for the load to be moved forward slightly, perhaps to place it in a specific position. The jog reverse push button performs the same task but in the opposite direction.
DC Power Supply
+V –V
FWD
Stepper Motor
Figure 8-27. Stepper motor interface with jog forward and jog reverse capabilities.