Importance of Motion Control in Industrial Applications

In industrial applications, machines are the most important part. The efficiency of the machines lies on its control and the administration of the motion control system. The man function of the motion control system is to control the machines movement. WE can otherwise represent the importance of the control system by saying that they are responsible for controlling the heart beat of the motors. This is the reason why the motion controls for various machines vary as per their applications. For example the motion control system of a ball screw machine system will not produce accurate and precise results when applied to a motor driven tool.

In the normal working of the conventional servo drive system, the motion controller sends electrical pulse signals to the motor. The Servo motor uses a glass scale or rotary encoder to verify the motion and catch error signals. In case no error signals are identified, the encoder placed in the servo system sends back confirmation pulse signals to the motion controller to confirm the utilization of the motion command. In case the tool has glass scales incorporated in to the axis then positioning of the axis in the scale also needs to be verified. For this a signal is again sent from the motion control to the glass scale for the proper positioning of the axis. This is how a motion control system offers extensive accuracy without the backlash of the components of the servo system.

When a machine tool gets calibrated, the motion control checks the value that is entered to check the pitch errors as depicted by the encoders and the amplifiers. These errors occur when the input signals sent do not match the existing processing or motion of the unit. The motion controller has to play a major role in rectifying the error and check the value of the parameters to ensure the production of accurate movement. This is the reason why overshoot or undershoot of the tool is prevented from happening when the machine is moved to a high speed rate. 

A motion control system remains incorporated in almost all servo systems. They not only play an important role in controlling the motion of the machine tools but also prevent damage due to excessive load. By working on the action signals sent to it by the encoder, the controller works on it to ascertain if any error has taken place and accordingly commands the movement of the servo motor. In short it helps in decreasing the power consumption of the machine while increasing its efficiency.

Servo Amplifier

A servo amplifier is a part of the closed loop servo control drive system is designed to primarily control the performance of a mechanism. Basically they are four-quadrant, power amplifiers with regenerative features and equipped to supply power according to the control voltage to the overall motor. These servo amplifiers can supply energy to the load, as well as absorb energy from the load. They are also designed to convert the kinetic energy of the combined motor load into electrical energy while the load is decelerating.

The output of a servo amplifier is an analogue circuit. The circuit enables the current and voltage of the motor to be adjusted to control position, torque and velocity. The feedback and comparator stages though can have a mixture of analog and digital devices.

Few ideal characteristics that are desired in a servo amplifier are as follows:

• Constant Velocity – ability to maintain the commanded velocity. Tachometers are used to record the aberrations between the commanded and actual motor speed.

·         Infinite Acceleration – an ideal amplifier must be able to provide infinite acceleration which can be observed by abruptly starting and stopping the motor.

·         Linear response – the amplifier need to respond proportionally to the changes in the velocity input command.

·         Hold Position – the amplifier should drive the motor in such a way that the latter holds its position irrespective of any external force or disturbance.

·         Overload protection – a low resistance circuit should be there to protect the amplifier from excessive current.

There are a host of servo amplifiers in the market but one should be careful while choosing an amplifier. The compatibility of the servo amplifier with its servo is to be ensured. Some notable types of servo amplifiers are as follows:

• Push Pull amplifiers – one of the earliest designs of amplifiers designed primarily for two transistors to get switched on/off to share the current load for the motor.
• Chopper amplifier – another primitive DC-motor based amplifier that finds limited usage today.
• L Type (Linear Motion) Servo Amplifier – it is fitted with linear positioning function you can basically construct a linear positioning system using such amplifiers as a ball screw or rack & pinion.
• Digital Amplifier – it is usually used for higher machine performance as it promises improved reliability and repeatability through digital technology.
• R Type (Rotation) Servo Amplifier – it provides an additional rotation angle indexing function. Rotation angle index system can be constructed using this amplifier.
• DC servo amplifier – both brush and brushless amplifiers can be found in simple DC motors

Brushless Servo Drives

Brushless servo drives are automatic instruments that monitor the performance of any mechanism based on the continuous feedback that it receives. Basically, it is a closed loop mechanism that sends rectification signals if the performance of the machine / instrument is deviating from the commanded data. Now, the instrument that really monitors the performance is called the servo drive. It is just an electronic amplifier that gets linked to a servo mechanism to monitor its performance. Servo drives can be brushed or brushless. While brushed motors provide exchange via physical contacts, brushless motors provide electronic exchange without physical contact, with the help of sinusoidal or the trapezoidal drives. These trapezoidal drives can energize two motor windings simultaneously before proceeding to the next pair of windings. Sinusoidal servo drives, on the other hand, can provide do this to three motor windings simultaneously with the sinusoidal signals.

Brushless motors work quite like AC motors where the rotor movement is caused by a dynamic magnet field. Brushless motors are known to have predicable linear characteristics. Brushless servo drives are AC or DC types. It is basically the way the motor is powered gives it the name AC or DC.

Read more about ac servo drives.

Torque developed by brushless servo drives depends on the control technology used. Type of control is determined by the feedback scheme exercised. DC motors use Hall sensors for feedback, whereas AC motors use resolver or encoder for feedback. Speed accuracy is very high in fact with the more popular brushless drive, almost close to 0%. This is due to the presence of a digital encoder and the drive controller regulation of position. High torque to inertia ratios provides high acceleration and deceleration rates with dynamic response.  Controller bandwidth is almost 8 times higher than the Brush DC drive. The Brushless DC motors have good thermal characteristics too. Motor efficiencies range anywhere around 90-96 % and controller efficiency around 97% giving overall efficiencies far superior than brush DC systems.

The benefits of brushless servo drives are:

• Very precise average speed control over a very wide speed range
• High dynamic response ensuring precise instantaneous speed control
• Constant power factor meaning lowest possible input current
• Smaller in size than the brush type
• No regular motor maintenance is needed
• Feedback device (encoder) is fitted internally and thereby less subject to errors
• Higher overall efficiency

Brushless servo drives find their usage in CNC machines, blowmolding and a host of other industrial applications -

Understanding Motion Control Drives

Motion control is a term that describes a range of applications that involve movement in specific routes designed to precision performance. Earlier motion control applications require only an object to be moved from one place to another without paying too much heed to speed or change of motion. On the other hand, there are machine tool applications which require the exact coordination of all aspects of motion, including a high degree of synchronization for multiple simultaneous movements.

Motion control drives finds heavy usage in the robotics industry. Motion control drives are known to monitor the motion performance of the mechanism. The feedback and the subsequent rectification, if needed, makes it ideally placed to be used in the robotics field. These drives are connected to motors that can be AC or DC. Further categorization can be made of DC brushed, DC brushless, AC Induction, AC synchronization and Stepper.

The motor amplifier or servo drive is designed to takes commands from the motion controller. These signals are incorporated in the analogue version where the voltage with low current is converted to high current signals in order to drive the motor. Motor drives come in many various types and makes and are fitted when matched to a specific motor they need to run. Along with matching the motor technology, the drive needs to ensure a balance of voltage, a continuous current and the balanced correct peak current.

The feedback of the mechanism’s performance can be taken of different parameters like back EMF, current, velocity, acceleration, position and many more. It’s because the control of these parameters will ensure the overall control of the motion of the object. Some of the sources for these feedbacks are:

• Hall effect sensors
• Encoder
• Potentiometer
• Tachometer
• Acceleration specific sensor

A motion control drive’s range of applications is far more specialized than any other manufacturing applications. Motion control drives must be capable of:

• Zero-speed holding torque
• Quick start/stop cycles
• Repeatable velocity and torque profiles
• High accelerating torque
• Synchronization
• Precise speed control
• Positioning capabilities

Motion control can operate two levels – the linear or the rotational axis. A linear axis application like a vehicle in motion has a pre-defined traversing range with predefined end stops. The rotational axis has a vast and almost never-ending traversing range when applied. A rotary table on the other hand travels a pre-defined yet short distance. They have a selected path or direction for moving point to point.