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).

Relays plc

Now that we understand how the PLC processes inputs, outputs, and the actual program we are almost ready to start writing a program. But first lets see how a relay actually works. After all, the main purpose of a plc is to replace "real-world" relays.

We can think of a relay as an electromagnetic switch. Apply a voltage to the coil and a magnetic field is generated. This magnetic field sucks the contacts of the relay in, causing them to make a connection. These contacts can be considered to be a switch. They allow current to flow between 2 points thereby closing the circuit.

Let's consider the following example. Here we simply turn on a bell (Lunch time!) whenever a switch is closed. We have 3 real-world parts. A switch, a relay and a bell. Whenever the switch closes we apply a current to a bell causing it to sound.

Notice in the picture that we have 2 separate circuits. The bottom(blue) indicates the DC part. The top(red) indicates the AC part.

Here we are using a dc relay to control an AC circuit. That's the fun of relays! When the switch is open no current can flow through the coil of the relay. As soon as the switch is closed, however, current runs through the coil causing a magnetic field to build up. This magnetic field causes the contacts of the relay to close. Now AC current flows through the bell and we hear it. Lunch time!

PLC EXAMPLE

Let’s start off with a simple example. Suppose you are making a controller for an overhead crane. The operator has a simple control box with four push buttons: "left", "right", "up", and "down". When the operator presses the "left" button (note that the "left" button is an input to the PLC), then the PLC turns on the appropriate output to the motor that makes the crane move left. The other three buttons would operate similarly. Sounds pretty simple – right?

Suppose it takes ten minutes for the crane to reach the full left position. Soon the operator’s fingers start to hurt (holding that button down for ten minutes at a time hurts), and they are going to beg / bribe / threaten you, the programmer, to latch that output on and add a stop button. Instead of having to press the "left" button for ten minutes, the operator wants to momentarily press the "left" button and the crane keeps moving left till the operator presses the "stop" button. So you reprogram the crane and now the operator picks up a 10 ton container, presses the "left" button, realizes he forgot to get a drink (of water), and knowing that the crane will be moving for ten minutes, goes off to get a drink. Or suppose the crane hits the operator and knocks them out. Who is going to stop the crane? There are some major safety considerations since you have a 10-ton container moving around with no one to stop it.

So you start adding safety light curtains and mats around the crane’s operational area, so that if anything comes into the crane’s operational area the crane automatically stops. You would also add Emergency Stop (E-Stop) buttons around the area so that anyone can press one of these buttons to stop the crane. You would want to add end-of-travel limit switches so that when the crane moved as far as it can go then the PLC would automatically stop the motor. You would also want to add some more inputs (feedback) to the PLC so that when a motor fault occurred the PLC would detect the fault, turn off the motor, and sound alarms. There are many other safety and diagnostic inputs you should add.

Do you see how a very simple application can grow in inputs and outputs very quickly? The good news is that by using a PLC for this application the PLC is very quickly and easily reprogrammed for the new inputs. Other wise you have to go get more relays and do a bunch of wiring for each new input and output.

Even More Complexity

We can extrapolate this simple crane into more complex systems:

• A "crane" that automatically loads or unloads 55 gallon drums onto pallets, containers on or off a ship, or adds a finite amount of reagent to a matrix of test tubes.
• Multiple cranes that have overlapping work envelopes and require collision avoidance and cooperative handling
• "Cranes" that work in three-dimensional space to store and retrieve items. Applications from electronics to pharmaceuticals show that automated storage and retrieval systems reduce errors significantly.
• Two axis controllers that move a video camera around for inspecting parts

The control systems engineer sees a lot of similarities in these different applications. All of these applications can use a PLC but these applications are just a tiny subset of all the control schemes that employ PLCs.