Månadsvis arkiv: oktober 2016

Stepper Drive Modes

This chapter has explained how to operate steppers by energizing one or two winding pairs at a tiem, but tere are a number of different ways to drive a Nema 23 stepper motor, and this discussion touches on four of them:

* Full-step (one phase on) mode – Each control signal energizes on winding.

* Full-step (two phases on) mode – Each control signal energizes two windings.

* Half-step mode – Each control signal alternates between energizing one and two windings.

* Microstep mode – The controller delivers sinusoidal signals to the stepper’s windings.

Full-Step (One Phase On) Mode

The simplest way to control a stepper is to energize one winding at a time. This is the method discussed at the start of this chapter. Figure 4.15 shows what the signaling sequence looks lide when controlling a stepper motor drive in this mode.

With each control signal, the rotor truns to align itself with the energized winding. The rotor always turns through the stepper’s rated step angle. That is, if a PM motor is rated for 7.5, each control signal causes it to turn 7.5.

Full-Step (Two Phase On) Mode

In the full-step (two phase on) mode, the controller energizes two windings at once. This turns the rotor through the stepper’s rated angle, and the rotor always aligns itself between two windings. Figure 4.16 illustrates one rotation of a stepper motor driven in this mode.

Figure 4.17 shows what the corresponding drive sequence looks like.

The main advantage of this mode over full-step (one phase on) is that it improves the motor’s torque. Because two windings are always on, torque increases by approximately 30%-40%. The disadvantage is that the power supply has to provide twice as much current to turn the stepper.

Half-Step Mode

The half-step mode is like a combination of the two full-step modes. That is, the controller alternates between energizing one winding and two windings. Figure 4.18 depicts three rotations of a stepper in half-step mode.

Figure 4.19 illustrates a control signal for a stepper motor driven in half-step mode.

In this mode, the rotor aligns itself with windings (when one winding is energized) and between windings (when two windings are energized). This effectively reduces the motor’s step angle by half. That is, if the stepper’s step angle is 1.8, it will trun at 0.9 in half-step mode.

The disadvantage of this mode is that, when a single winding is energized, the rotor turns with approximately 20% less toruqe. This can be compenstated for by increasing the current.

Microstep Mode

The purpose of microstep mode is to have the stepper turn as smoothly as possible. This requires dividing the energizing pulse into potentially hundreds of control signals. Common numbers of division are 8,64,and 256. If the energizing pulse is divided into 256 signals, a 1.8 stepper will turn at 1.8/256=0.007 per control signal.

In this mode, the controller delivers current in a sinusoidal pattern. Successive windings receive a delayed version of this sinusoid. Figure 4.20 gives an idea of what this looks like.

Using this mode reduces torque by nearly 30%, but another disadvantage involes speed. As the width of a control signal decrease, the ability of the motor to respond also decrease. Therefore, if the controller delivers rapid pulses to the stepper in microstep mode, the motor may not turn in a reliable fashion.

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Sychronous type AC Servo Motor

The stator consists of a cylindrical frame and a stator core. The stator core is located in the frame and an armature coil is wound around the stator core. The end of the coil is connected with a lead wire and current is provide from the lead wire. The rotor consists of a shaft and a permanent magnet and the permanent magnet is attached to the outside of the shaft. In a synchronous type AC Leadshine servo motor, the magnet is attached to a rotor and an armature coil is wound around the stator unlike the DC servo motor. Therefore, the supply of current is possible from the outside without a stator and a synchronous type AC servo motor is called a ”brushless servo motor” because of this structural characteristic. Because this structure makes it possible to cool down a stator core directly from the outside, it is possible to resist an increase in temeprature. Also, because a synchronous type AC servo motor does not have the limitation of maximum velocity due to recification spark, a good characteristic of torque in the high-speed range can be obtained. In additon, because this type of motor has no brush, it can be operated for a long time without maintenace.

Like a DC servo motor, this type of AC servo motor drive uses an optical encoder or a resolver as a detector of rotation velocity. Also, a ferrite magnet or a rate earth magnet is used for the magnet which is built into the rotor and plays the role of a field system.

In this type of AC Servo Motor, because an armature contribution is linearly proportional to torque. Stop is easy and dynamic brake wordks during emergency stop. However, because a permanent magnet is use, the structure is very complex and the detection of position of the rotor is needed. The current from the armature includes high frequency current and the high frequecy current is the source of toruqe ripple and vibration.

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The Input/Outpout System – Discrete Devices

In the first module, you learned about the basic architecture and operation of the Allen-Bradley Micrologix 1000, including a brief introduction to its I/O system. This second module goes into more detail about the I/O system of the Micrologix 1000 Omron PLC. It includes four sections:

1. Types of input/output devices
2. Input interfaces
3. Output interfaces
4. System and I/O power distribution wiring

Types of Input/Output Devices

A MicroLogix 1000 PLC uses its input and output interfaces to connect with field input/output devices. To review, all input devices provide a signal to the PLC, and all output devices receive a signal from the PLC. All I/O devices, however, do not send and receive the same type of signal. There are two different types of I/O signals and two types of I/O devices that use them. The two types of I/O devices are discrete devices and analog devices.

At the end of this section, you will know:

• the difference between the two types of I/O devices
• which type works with the MicroLogix 1000

Discrete Devices

Discrete devices are input or output devices that provide or receive discrete digital signals. A discrete digital signal is one that can report only two states, such as ON/OFF or open/closed.

A limit switch is an example of a discrete input device because,1at any given time, it is either open or closed. It sends a discretePLdigital signal to a PLC. This signal can have one of only two values, 0 or 1, indicating that the device is either OFF or ON, respectively (see Figure 2-1).

A pilot light is an example of a discrete output device (see Figure 2-2). It can only be ON or OFF. A discrete output deviceOFFDiscrete0receives a discrete digital signal from a PLC telling it to be in either one state or the other. A discrete output can never be in a state in between ON and OFF.Figure 2-2. A pilot light receives a discrete signal from a PLC.

Next article we will continue introducing Analog Devices. The following products are some hot sale OMRON PLC on our store.

cp1e e20sdr a OMRON PLC, CP1E CPU AC 100-240V input, 24DI 16DO Relay, USB port, Original brand new

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CP1H-XA40DT-D PLC OMRON, CPU 24VDC, input 24 point transistor output 16 point Original brand new


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USB peripheral port
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Transistor output (sinking)

CJ1W-ID211 PLC OMRON I/O 16 input point 24VDC Original brand new


High-speed input models are available, meeting versatile applications.
ON Response Time: 15μs, OFF Response Time: 90μs
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