Motion and Logic Controllers Product Theory

We must talk about basic logic control in order to understand motion control, as motion control is a subtype of logic control itself.


Logic Control

Logic control was initially implemented as automation in factories and consisted of relays, counters, timers, and other components. Because early logic control had to be wired up on a per-process basis, they could not be easily rewired to complete another task, and often would suffer under more harsh industrial environments. Programmable logic controllers were developed as a solution to these problems, replacing the growing number of parts required and to allow for repurposing and reprogramming later. The first PLC was developed in 1969 by Bedford Associates for General Motors, and was originally named the Modicon (Modular Digital Controller) until the universal term “programmable logic controller” was coined.


The programmable logic controller replaced electrical relays with a solid-state programmable device that could perform logic functions in the same way that the relays could, without requiring the physical relays themselves. A PLC could be thought of as a simulation of those same relays. In order for PLC’s to be successfully implemented into any system of interest, the engineers who created the PLC designed it in such a way that the coding language was similar to a ladder logic diagram, which most technicians at the time were familiar with. Ladder logic refers to a specific type of diagram that is used to represent industrial control logic systems. Nowadays, PLCs are programmed using five different programming styles as defined by the international standard IEC 61131-3: Ladder diagram, function block diagram, sequential function chart, instruction list, and structured text. Ladder diagrams are still one of the most popular forms of coding PLCs today. For more information about ladder logic programming and PLC function in general, please refer to the dedicated PLC page here.


Since the invention of the PLC, logic controllers have evolved to meet more specific needs and processes. Some of these advanced controllers are devices known as programmable automation controllers (PACs), industrial PCs (IPCs), micro-PLCs, and more. Each type of controller is explored in the Product Selection Guide page here.


Motion Control

As mentioned above, motion control is an application of logic control. Motion control is the use of a logic controller to control the motion of a motor or actuator. Motion controllers are used in many different engineering applications, such as robotics, automation, assembly, machining.


Due to the rather complex nature of some motion control applications, sometimes a PLC alone is not enough to get the job done. PLCs are fine and capable of handling motion in one direction, however, devices such as industrial computers are used for their increased processing power and ability to handle motion in, say, two or three axes.


Components of a Motion Control System

The four main components of a motion control system are the motion controller, the electric drive, the motor, and the feedback device. Depending on the system, and what type of feedback the system requires, there may or may not be a feedback device included.


Motion Controller

The motion controller is the PLC, PAC, or IPC of the system. It is the heart and brain of the system. The motion controller takes information from the system, processes the information, and sends out commands to the other parts to function as designed. Motion controllers are selected with the chosen application in mind, and the complexity of motion, response time, and analog-digital resolution are a few of the things considered when deciding what kind of motion controller to use.


Electric Drive

An electric drive is what connects the motion controller to the motor. The drive takes the signal from the controller and determines what voltage and current needs to be sent to the motor in order for it to function as intended. Different types of electric drives include digital, analog, linear, switching, stepper and servo, each with their own applications.



The motor is the arm and legs of a motion control system. Motors are what perform the actual motion by converting electrical energy into mechanical energy. All motors are either powered by alternating current (AC) or direct current (DC).


Feedback Device

A feedback device is typically a sensor placed in the circuit to collect information about how the system has performed a function. This could be an encoder on the motor, ultrasonic sensor measuring distance, or a thermocouple measuring a change in temperature on a part. The motion controller receives information from a feedback device and adjusts its performance accordingly. Feedback devices are only included in systems that utilize closed-loop feedback. Open-loop systems do not utilize feedback, and therefore do not require a feedback device.


A keen eye might recognize that a motion control system could also be described as something like a servo or stepper motor connected to a controller. A servo motor itself contains a drive, motor, and feedback device, and the only thing separating it from being a full fledged motion control system would be the addition of a controller to process information and control the servo. More information about servo motors themselves can be found in the Servo Motor pages here.