6.5.2 An introduction to closed loop control

This section describes the basic principles of closed loop control. If you are familiar with closed loop control go straight to section 6.6.1.

When there is a requirement to move an axis, the NextMove ES control software translates this into a demand output voltage. This is used to control the drive (servo amplifier) which powers the motor. An encoder or resolver on the motor is used to measure the motor’s position. Every 1ms* (adjustable using the LOOPTIME keyword) the NextMove ES compares the demanded and measured positions. It then calculates the demand needed to minimize the difference between them, known as the following error.

This system of constant measurement and correction is known as closed loop control.

[For the analogy, imagine you are in your car waiting at an intersection. You are going to go straight on when the lights change, just like the car standing next to you which is called Demand. You’re not going to race Demand though - your job as the controller (NextMove ES) is to stay exactly level with Demand, looking out of the window to measure your position ].

The main term that the NextMove ES uses to correct the error is called Proportional gain

(KPROP). A very simple proportional controller would simply multiply the amount of error by the Proportional gain and apply the result to the motor [ the further Demand gets ahead or behind you, the more you press or release the gas pedal ].

If the Proportional gain is set too high overshoot will occur, resulting in the motor vibrating back and forth around the desired position before it settles [ you press the gas pedal so hard you go right past Demand. To try and stay level you ease off the gas, but end up falling behind a little. You keep repeating this and after a few tries you end up level with Demand, travelling at a steady speed. This is what you wanted to do but it has taken you a long time ].

If the Proportional gain is increased still further, the system becomes unstable [ you keep pressing and then letting off the gas pedal so hard you never travel at a steady speed ].

To reduce the onset of instability, a term called Velocity Feedback gain (KVEL) is used. This resists rapid movement of the motor and allows the Proportional gain to be set higher before vibration starts. Another term called Derivative gain (KDERIV) can also be used to give a similar effect.

With Proportional gain and Velocity Feedback gain (or Derivative gain) it is possible for a motor to come to a stop with a small following error [ Demand stopped so you stopped too, but not quite level ]. The NextMove ES tries to correct the error, but because the error is so small the amount of torque demanded might not be enough to overcome friction.

This problem is overcome by using a term called Integral gain (KINT). This sums the error over time, so that the motor torque is gradually increased until the positional error is reduced to zero [ like a person gradually pushing harder and harder on your car until they’ve pushed it level with Demand].

However, if there is large load on the motor (it is supporting a heavy suspended weight for example), it is possible for the output to increase to 100% demand. This effect can be limited using the KINTLIMIT keyword which limits the effect of KINT to a given percentage of the demand output. Another keyword called KINTMODE can even turn off integral action when it’s not needed.

*The 1ms sampling interval can be changed using the LOOPTIME keyword to either 2ms, 500µs, 200µs or 100µs.

MN1928

Operation 6-13

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Baldor MN1928 installation manual An introduction to closed loop control
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MN1928 specifications

The Baldor MN1928 is a highly regarded motor designed for a variety of industrial applications, known for its durability and efficiency. This motor is part of Baldor’s extensive range of products, which are engineered to meet the demands of heavy-duty operations.

One of the key features of the Baldor MN1928 is its robust construction. Built with high-quality materials, this motor is designed to withstand harsh environmental conditions often found in industrial settings. The steel frame is not only resilient, but it also enhances the motor's cooling capabilities, enabling it to perform effectively over extended periods.

The MN1928 is equipped with advanced technologies that optimize its performance. One notable technology is the use of high-efficiency induction motor design. This reduces energy consumption significantly and contributes to lower operational costs. The motor is also designed with a continuous duty rating, making it capable of running for long hours without compromising its functionality or lifespan.

In terms of characteristics, the Baldor MN1928 features a reliable ball bearing design, which minimizes friction and wear, ensuring smoother operation and increased reliability. With a horsepower rating that suits a range of applications, it provides the necessary torque and speed to power various machinery effectively. The multi-voltage design allows for versatile installation options, accommodating different electrical systems while ensuring efficient performance.

Another important characteristic of this motor is its ease of maintenance. The design allows for straightforward access to components, making it simple for technicians to perform routine checks and maintenance. This is particularly beneficial in industrial settings where downtime can be costly.

Safety is also a priority in the design of the Baldor MN1928. Equipped with thermal overload protection, it prevents overheating, reducing the risk of damage caused by excessive temperatures during operation. Additionally, the motor complies with various industry standards, ensuring safe operation within diverse environments.

In summary, the Baldor MN1928 stands out as a reliable choice for industrial applications, offering a combination of durability, efficiency, and advanced technology. Its robust construction, high-efficiency design, and safety features make it a preferred option for many enterprises seeking dependable motor solutions.
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