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What is a Closed Loop System? Pros & Cons

Although you probably didn’t know it, you’ve been using closed-loop systems in your everyday activities for many years. For example, one of the most common closed-loop systems is in your refrigerator, and it springs into action every time you open its door, causing the interior temperature to rise.

As a result, your refrigerator’s motor and compressor automatically start up to compensate for the temperature change. More refrigerant courses through your unit, creating evaporation, which cools the inside until the setpoint temperature is obtained.

Appliances like air conditioners, electric irons, and ovens exemplify everyday closed-loop systems, as does the automobile’s adaptive cruise control and emissions systems.

What is the Closed-Loop Control System?

A closed-loop control system, also known as a feedback control system, uses the concept of an open-loop control system as its forward path but adds one or more feedback loops between its input signal and output signal. “Feedback” means that some portion of the output is fed back, or returned, to the input to form part of the system’s excitation.

Closed-loop systems are designed to automatically achieve and maintain the desired output of the system by comparing it with the actual condition. It does this by generating an error signal, the difference between the output and the reference input. In other words, a “closed-loop system” is a fully automatic control system whose control action depends on the output in some manner.

It’s worth noting that a control signal can produce either negative or positive feedback. Negative feedback occurs when some function of the mechanism is fed back in a way that tends to reduce the fluctuations in the output and promotes a settling to equilibrium. In contrast, positive feedback typically leads to instability in the form of chaotic behavior or oscillation.

emergency stop control panelHow Does the Closed Loop Type of Control System Work in Industrial Applications?

Looking at manufacturing automation, such as CNC machine tools with a closed-loop system design, the machine provides a feedback signal to the interfacing software, ensuring each axis has completed its movement accurately before sending the following tool path to the machine.

For example, CNC machinery is comprised of complex systems supporting lightning-fast moves. A programming error can result in a cutting tool ‘jamming’ into the workpiece, creating a backlash in one of the machine’s axes. Unaware of the backlash, the CNC machinist runs the following tool path, which begins with a 5.00” move along the X-axis. However, because of the “play” in that axis, the table travels only 4.98”.

If your machine does not have a closed-loop industrial control, there is no feedback from the machine to the software. If your machine has that backlash on the X-axis, the software will not know that the X-axis only travels 4.98”. Without this real-time feedback, the software feeds the rest of the program to the machine.

Conversely, with constant feedback from a closed-loop control system, the software drives the X-axis motor to keep moving until the feedback on the machine tells the software that the table has now physically reached its 5” mark regardless of the play. After the feedback confirms that the X-axis is at 5”, the software will feed the following tool path into the machine. That’s the beauty of a closed-loop system, constantly “double checking” that the axis drives to the exact coordinate on the tool path before it continues to the next line of the program.

Although the closed-loop control system offers process control acting as a “security blanket” to ensure that your part is machined accurately, that doesn’t necessarily mean open-loop options on a CNC milling machine or CNC lathe is inferior. Thousands of CNC machines have defaulted to open-loop systems since the 1990s.

The open loop CNC mill or lathe is a low-cost solution for manufacturers who understand proper maintenance and adjustment of the machine, taking conservative feed rates on their tool path programs, and treating their CNC machine tools with the respect they deserve. Taking these steps with an open loop system ensures accurate parts with a resolution of 0.0002” and within a repeatability of 0.0005”.

Manual Machining – Hand Cranking on Closed-Loop

CNC Masters milling machines and CNC lathes can disable the axis motors for hand-cranking each axis, using them as traditional manual milling machines or lathes. This flexibility offers the best of both worlds: “CNC mode” or “manual mode” with these machines.

And with the closed-loop option added to your CNC Masters mill or lathe, manual mode means machinists have the added benefit of DRO (Digital Readout); the counters on the software screen will “display in sync” with the hand cranking of each axis. In addition, users can go back and forth from CNC mode to manual mode while retaining counter control through the CNC Masters software screen.

Our CNC machines are competitively priced and easy to learn and operate. They can run full-time throughout the week to meet your production needs, and now, with the option of adding the closed-loop system to your CNC mill or lathe, you will have an optimization of your industrial processes with a CNC Masters machine on your shop floor!

The CNC Masters Closed-Loop Option is Based on Mounted Scales Instead of Encoders at the Axis Motors

The benefit of our closed-loop option over other CNC machine manufacturers manifests in how they count the revolutions of the axis motor shaft. On such systems, if there is a backlash on an axis, the encoder at the motor will not pick up this physical error of the saddle or table’s movement.

With a CNC Masters closed-loop option, the MX scales are mounted on the machine and plugged directly into the machine’s control unit, meaning accurate controller output from the virtual table, saddle, or carriage positions on the mill or lathe.

CNC Masters MX scales are designed to work directly with the MX controller and MX software under real-world conditions, giving shops the security they need to machine their parts, with the added extra benefit of manually machining a simple piece on their CNC Masters milling machine or CNC lathe.

Please note that this option is not available on all CNC Masters machines. Be sure to check with us first if you are thinking of buying a CNC Masters machine that defaulted on open-loop but considering a purchase of a closed-loop kit at a later time. This option will only work with the MX control unit and MX software. If you purchase this as a kit to install yourself on your CNC Masters machine, please note that we will need your MX control unit returned to us so we upgrade the controller with the closed loop option. This option will not work with the older XU control unit on CNC machines.

The CNC Masters MX Scales

These scales are glass grating slideways that are lapped and come with JIS standard P5 grade ball bearings to achieve smooth and accurate movement and long working life. The scales come built with plastic seals that resist deterioration from oil and have high elastic recovery properties and durability that offer low slide resistance for CNC or manual mode jogging of each axis. The scales are designed to work with and plug into the CNC Masters MX Control Unit and Master MX Software.

Final Thoughts on Closed Loop Systems

Closed-loop control systems have several advantages over open-loop controllers. They can:

  • stabilize unstable processes
  • reliable and accurate by using feedback as input data.
  • continuous improvement streamlines the manufacturing process, saving time, operation costs, and raw materials
  • feedback systems monitor the output of the motors
  • able to make adjustments in position, set speed, and acceleration
  • PID controllers (proportional integral derivative) correct errors between a measured variable and a desired set point on some systems by using algorithms

For all their benefits, closed-loop systems can be costlier, more complicated, and have more parts that can fail, increasing maintenance expenses.

About Peter Jacobs

Peter Jacobs is the Senior Director of Marketing at CNC Masters, a leading supplier of CNC mills, milling machines, and CNC lathes. He is actively involved in manufacturing processes and regularly contributes his insights for various blogs in CNC machining, 3D printing, rapid tooling, injection molding, metal casting, and manufacturing in general. You can connect with him on his LinkedIn.

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29. Create a Peck Drilling Program in Circular or Rectangular Patterns
Using the Circular or Rectangular Drilling Wizards, you can program the machine to drill an un-limited series of holes along the X and Y planes. Program it to drill straight through to your total depth, use a high-speed pecking cycle, or deep hole pecking cycle. You can program the cut-in depth and return point for a controlled peck drill application to maximize chip clearance.

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20. Change up to 30 tools with compensation, and store your tool offsets for other programs
The MX supports…

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21. Use the optional ATC rack up to 8 tools for milling, drilling, and rigid tapping applications
The CNC Masters Automatic Tool Changer Rack and Tools (US Patent 9,827,640B2) can be added to any CNC Masters Milling Machine built with the rigid tapping encoder option. The tutorial will guide you through the set-up procedure using the ATC tools.

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22. Use the optional Rigid Tapping Wizard without the need for tapping head attachments
When you order your CNC Masters machine, have it built with the optional rigid tapping encoder. You can take any drill cycle program and replace the top line with a tapping code created by the wizard to tap your series of holes up to 1/2” in diameter.

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23. Use the optional Digital Probe to scan the profile and/or pockets of your fun/hobby type designs to write your tool path program and machine out a duplicate of your original design To “surface” scan an object, you can program the probe along the X or Y plane. The stylus will travel over the part starting on the left side front corner of the object and work its way to the end of the part on the right side. Depending on how the stylus moves, it will record linear and interpolated movements along the X, Y, and Z planes directly on the MX Editor.
To “pocket” scan an object containing a closed pocket such as circles or squares, the scan will start from the top front, work its way inside of the pocket, and scan the entire perimeter of the pocket.
Under the Setup of the MX software you will find the Probe Tab which will allow you to calibrate and program your probe. Your “Probe Step”, “Feed”, and “Data Filter” can also be changed on the fly while the probe is in the middle of scanning your object.

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24. Use work offsets G54-G59 for nesting applications
The work offsets offer you a way to program up to six different machining locations. It’s like having multiple 0.0 locations for different parts. This is very useful especially when using sub-routines/nesting applications.

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25. Create a Rectangular Pocket / Slot with our selection of Wizards to help you build a tool path program
The Cycle Wizards for the mill or lathe makes it easy to create a simple tool path without needing to use a CAD and CAM software.
On this Wizard, the Rectangular Pocket / Slots, can be used to form a deep rectangular pocket into your material or machine a slot duplicating as many passes needed to its total depth.

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26. Create a Circular Pocket Wizard
Input the total diameter, the step down, and total depth and the code will be generated.

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27. Do Thread Milling using a single point cutter Wizard

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28. Cut a gear out using the Cut Gear Wizard with the optional Fourth Axis

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19. Disable the axis motors to manually hand crank each axis into place
Easily de-energize the axis motors by clicking [Disable Motors] to crank each axis by hand, and then press [Reset Control] to re-energize the axis motors.

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30. The MX interface can easily be interchanged from Mill Mode to Lathe Mode
Use this interface for your CNC Masters Lathe. It contains all the same user-friendly features and functions that comes in Mill Mode. Simply go to the Setup page and change the interface.

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31. Use Tool Change Compensation or the optional Auto Tool Changer Turret if your application requires more than one tool in a single program
You can offset the length and angle of each tool and record it under Tools in your Setup. The program will automatically pause the lathe’s movement and spindle allowing you to change out your tool, or allowing the optional ATC Turret to quickly turn to its next tool and continue machining.
On the MX interface, you also have four Tool Position buttons. Select your desired T position, and the auto tool post will quickly turn and lock itself to that position.

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32. Use the Lathe Wizard Threading Cycle to help you program your lathe’s internal or external threads in inches or metric

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33. Use the Lathe Wizard Turning / Boring Cycle to help you program simple turning and boring cycles without having to go through a CAM or writing a long program with multiple passes

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34. Use the Lathe Wizard Peck Drilling Cycle to help you program your drill applications or for face grooving

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35. Facing / Grooving / Part Off Cycle Wizards – with Constant Surface Speed
These cycles can be used with Constant Surface Speed allowing the spindle speed to increase automatically as the diameter of the part decreases giving your application a consistent workpiece finish. With CSS built into the wizard, there is no need to break down the cycle into multiple paths and multiple spindle speed changes.

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36. This is our list of supported G and M codes which can be found under Tools > G Code/ M Code List in the MX
If you plan to use a third-party CAM software to generate your tool path program, use a generic FANUC post processor and edit it to match our list of codes. As an option, we also sell Visual mill/turn CAM software which comes with a guaranteed post processor for our machines to easily generate your tool path programs based on your CAD drawings.

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37. Our pledge to you…

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10. Run each tool path independently to study its movement
1. Run the machine on Trace mode. You can run each tool path independently, one line at a time to study the tool path movement on the machine to verify the position of the application and if any fixture/vise is in the way of the cutter’s path.

2. You can also verify your program by clicking on the Trace and Draw buttons together. This will allow you to view each tool path independently one line at a time in the Draw Window.

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2. Clutter Free Interface
The MX is engineered for the CNC MASTERS machine so you do not have to fiddle with a detailed complicated configuration that can be overwhelming. Just load in the MX and start machining!2. Clutter Free Interface
The MX is engineered for the CNC MASTERS machine so you do not have to fiddle with a detailed complicated configuration that can be overwhelming. Just load in the MX and start machining!

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3. Features Tour and Tutorials Included
The Features Tour will give you a quick run-down on all the features the MX can do for you. The Tutorials are easy to follow even for the first time CNC machinist.
Feel free to download the MX on any of your computers. We recommend downloading the MX along with your CAD and CAM software there at the comfort of your office computer to generate your tool path programs. You don’t need to be hooked up to the machine either to test your program in simulation mode.

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4. Navigate and Edit Your Program through the MX interface with Ease
With a few clicks of the mouse or using touch screen technology, you can easily navigate through the MX interface importing saved programs into the Editor from the File drop down menu. Using standard windows features to edit your program you can then lock the Editor Screen to avoid accidental editing, and if you need to insert a line in the middle of a program, just click on [ReNum] to re-number your tool path list.
You can create a program or import CAM generated G-code tool paths into the Editor
The X Y and Z W arrow jog buttons are displayed from the point of view of the cutter to avoid confusion when the table and saddle are moving. You can also adjust your spindle speed and coolant control while jogging each axis.

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5. Feed Hold – Pause in the Middle of your Program
Feed Hold lets you pause in the middle of a program. From there you can step through your program one line at time while opting to shut the spindle off and then resume your program.
You can also write PAUSE in the middle of your program and jog each axis independently while your program is in pause mode.

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6. Hot Keys
Hot Keys is an alternative method to easily control your machine using your hard or touch screen keyboard. One can press P to pause a program, press S to turn Spindle On, G to run a program, Space Bar to Stop, J to record your individual movements one line at a time to create a program in teach mode.

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7. Pick Menu – for conversational mode programming
Write FANUC style G-codes directly into the Editor or select commands off the [Pick] menu and write your tool path program in conversational mode such as what is written in the Editor box. You can even mix between conversation commands and G-codes in the same program.

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8. Pick Menu List of Options
Use commands such as MOVE, SPINDLE ON/OFF, COOLANT ON/OFF, PAUSE, DELAY, GO HOME…. to write your tool path programs in conversational mode.

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9. Draw the Tool Path to verify it before pressing Go
Hit Draw to view your tool path program drawing, check out its run time, or even simulate the tool path in 3D mode. This can be helpful to quickly verify your program before running it. You can also slow down or speed up the drawing or simulation process.
You can also hit Go within the Draw Window itself to verify the cutter’s position on the machine. The current tool path will be highlighted and simultaneously draw out the next path so you can verify what the cutter will be doing next on the program.

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MX Software – Easy to Use, Easy to Learn – Included with your machine purchase
The MX software is designed to work seamlessly with your CNC Masters machine. It is made to work with Windows PC – desktop, laptop, or an all in one – on standard USB. Use it on Windows 8 or 10 64-bit operating systems.
No internal conversion printer/serial port to USB software or additional conversion hardware is used with the MX.

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11. Counters display in Inches or Millimeters – Continuous Feed
1. When running a program, the counters will display a “real-time” readout while the machine is in CNC operation without counting ahead of the movement.
2. The current tool path is highlighted while the machine is in operation without causing slight interruptions/pauses as the software feeds the tool path to the machine. The MX internally interprets a program ten lines ahead to allow for “continuous machining” avoiding slight interruptions as the machine waits for its next tool path command.
3. “Run Time” tells you how long it takes to run your tool path program.

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12. Use the “Go From Line” command to start in the middle of your program
If you ever need to begin your program from somewhere in the middle of it, use [Go From Line] which you can find under Tools. The Help guide will walk you through how to position the cutter without losing its position on the machine.

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13. Exact Motion Distance without over-stepping on an axis while jogging
Use “Relative ON” to enter a specific coordinate to jog any of your axes to an exact location without having to write a program. It’s like using “power feed” but easier. You can jog an exact distance on any of the axes without needing to keep the key pressed down and mistakenly over-step the movement releasing your finger too slowly off the jog button.
Let’s say you need to drill a hole exactly 0.525” using the Z. So you enter 0.525 in the Z box. Next, adjust the JOG FEED RATE slider for the desired feed rate. Then “click once” on the +Z or -Z button to activate the travel. In this case you click once the -Z button first to drill the hole exactly 0.525”. Then click once on the +Z button to drive the axis back up 0.525”.

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14. Teach Mode – Jog Input
You can create a tool path program by storing each point-to-point movement by simply jogging an axis one at a time. Click on either of the Jog Input buttons to store each movement on the Editor Screen. You can then add Spindle ON, feed commands, and press GO to run the new program as needed. This is a great feature to help you learn to create a program by the movements you make on the machine without necessarily writing out an entire program first.

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15. Override on the fly to adjust the Jog Feed to Rapid or the Spindle Speed during the middle of a program
1. Jog Feed and Rapid with Override: You can adjust feeds using the slider from slow minimum 0.1″ per minute to a rapid of 100″ per minute of travel. You can even micro-step your jog as low as 0.01”/min. The [-][+] buttons allow you to fine tune feeds in 5% increments while the program is in motion.
2. Spindle Speed with Override: You can adjust speeds using the slider from a slow minimum RPM to the max RPM according to the machine setup. The [-][+] buttons allow you to fine tune feeds in 5% increments while the program is in motion.

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16. Adjust Counters using Pre-Set if you cannot begin the program from 0.00
In a situation where you cannot begin your cutter at it’s 0.00 location, you can “Pre-Set” directly into the counters by typing in your beginning coordinate. You can press Go from here to run your program. You can also “zero all” or “zero” your counters independently. With one click of the [Return to 0.0] button, all axes will travel back to its respective 0.0 on the machine.

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17. Set and Save your 0.00 position for future runs
Set and save your 0.00 position on the machine. These coordinates will be recorded as the first line of the program in the Editor Screen. Should you desire to return to this program at a later date, you only have to click on the Set Zero Return button. This will command the machine to automatically jog each axis to its saved “set” 0.00 position according to the recorded coordinates at the first line of the program.

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18. Create a “Home” position to clear your application and run multiple times
Let’s say you need to machine one application times 100 pieces. This usually requires a jig to retain that physical 0.00 position. But in this case, you want the program to end with a clearance of the axes to easily switch out the next piece of stock and start again. With Save Home, you have the ability to save this offset (home) position while still retaining your Set Zero position where the machine will mill your part out. Pressing [Save Home] will record this new position under the Set Zero line in your program.
Pressing [Go Home] will jog your axes back to your “saved home” position where you originally pressed the Save Home command. You can also input GO_HOME from the Pick Menu as its own tool path in your program. At the completion of your program the axes will end at your Home position. Replace your part, then press [Return to 0.0] button to allow the axes to return to its zero position, and press Go to start your next run.

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