Servo motor control using PLC

i have been working with servo motor for quite sometimes. anyhow, it is also good to have training from the original equipment manufacturer.





difference between linear servo system and motion control system:

linear servo system: PLC will supply pulse input to the motor drive (digital type) then the motor driver will give power through PWM to the AC servo motor. feedback will be given by encoder to the driver and driver will do the error counter.

motion control system: PLC will supply pulse to the motion control unit. then motion control unit will give analog input 0-10V to motor driver. the motor driver will give power through PWM to the AC servo motor. feedback will be given by encoder to the driver and driver will give the error signal to the motion control unit to do the error counting.

i will start explaining on the linear servo system. basically, there are two common ways to control the direction of the motor which is using the CW/CCW pulse mode or using the pulse + direction mode. u need two pulse outputs from the servo motor to do this. make sure u know the setting from your plc of how the pulses are being output. please check with the manual of the pulse timing diagram for better understand. it is important also to know the response time of the pulse.

make sure the wiring is correct. basic outputs from the PLC to the servo driver (amplifier) are the pulse outputs (u need two outputs here, if you are using the pulse + direction, one output will supply the pulse whereas the other output will tell the servo amplifier the direction. if u are in CW/CCW mode, one output is for clockwise pulse and the other is for counter clockwise pulses).

u also need to output the servo RUN signal to on the motor and Reset signal to reset any alarm. just for extra information, not all alarms can be reset from the reset signal, for some alarms, u need to reboot the servo driver.

for the input to the PLC from the servo driver, u need the feedback from the AC servo motor encoder. if you are using open collector, one feedback input is enough. it will count the Z phase from the encoder and pass it back to the plc to do the pulse count.

u also can tap into the PLC the 'pulse output complete' signal from the servo driver. this is important to make sure all pulse have been supplied and the motor is ready for next move.

here is a sample of ladder diagram on how u can jog your motor backward, tell the motor to count how many pulses has it moved backward, and use the same amount of pulses to go forward. since what i use is pulse+direction mode, when i moved backward, the encoder gave me a negative-signed pulses. i need to make it positive so a little arithmatics has been done so that the amount of pulses use to move forward, is the same amount of pulses supplied to the motor to move backward. it is also worth noting here that the PULS instruction is to tell the plc what is the amount of pulses to be supplied, the SPED instruction is to tell the frequency of the pulses, and this will determine the speed of the servo system, and the PRV is the instruction to tell the PLC to receive the pulse count from the Servo driver output ( input to the PLC).
click HERE for the ladder logic.

long term timers

Example 1:
Long-term Timers
The following program examples show two ways to create long-term timers
with standard TIM and CNT instructions.
Two TIM Instructions
In this example, two TIM instructions are combined to make a 30-minute
timer.


TIM and CNT Instructions

In this example, a TIM instruction and a CNT instruction are combined to make a 500-second timer.TIM 0001 generates a pulse every 5 s and CNT 0002 counts these pulses. The set value for this combination is the timer interval × counter SV. In this case, the timer SV would be 5 s x 100 = 500 s. With this combination, thelong-term timer’s PV is actually the PV of a counter, which is maintained through power interruptions.

for more information, go to yourplctrainer.com

two stage counter

When an SV higher than 9999 is required, two counters can be combined as shown in the following example. In this case, two CNT instructions are combined to make a BCD counter with an SV of 20,000.

Planning your project

It is very important to plan your work in designing your PLC projects. identify your project requirements, inputs and outputs involve etc.

Today i will give you a free PLC programming on how to do a PLC controller for a water tank system. This is a typical exam question for PLC beginner.

The inputs involve are 1 start button addressed at 0.00 and 2 level switches addressed at 0.01 and 0.02 respectively. The one address at 0.01 will turn ON whenever the tank is empty thus it will switch ON the water valve (output) which is address at 10.00. The other level switch will turn ON whenever the tank is full and consequently will turn the water valve OFF. By doing this control, the water tank will not run empty. The water will fill into the tank whenever the empty switch is ON. Address 14.00 and 14.01 is just internal relay address to sequence the programming.

Click HERE for your ladder logic

For more information, go to http://yourplctrainer.com

PLC tutorial and guides for automation

hi there, this blog is dedicated to provide you with PLC tutorial specifically for industrial automation and robotics.

There are two types of PLC available in the market, which is relay type and transistor type. the major difference between these two is the response time. The response for transistor type PLC can go as fast as 0.5 microsecs compared to those of relay types which have response time around 10 ~ 20 millisecs.

The ability of transistor type PLC to do fast switiching makes it possible for it to have pulse outputs and high speed input to control AC servo motor.

for more information, you can visit yourplctrainer.com