6 Features to Look For in a Modern CNC Motion Controller
6 Features to Look For in a Modern CNC Motion Controller
If you have had to work around the limitations of legacy parallel port controllers, you already know that the controller is to blame. Jerky motions, intermittent disconnections and a limitation in the max machine performance are all controller induced and not due to skill.
Step pulse frequency not limiting your motors
This is the number one setting that is overlooked prior to troubleshooting issues with the machine’s performance or when the machine simply will not go faster than it is currently running.
The step pulse frequency is how often the controller is able to send a signal to the stepper motor driver measured in kilohertz. The controller can only send a signal as often as the driver is able to receive it. This means that if your controller can only send 25kHz to the driver, the driver is only able to move 25,000 steps per second regardless of the driver’s capability.
Legacy parallel port controllers can generate approximately 25kHz, while the more modern USB implementations can be pushing 100kHz to 200kHz. Microstepping is another factor that uses the step pulse frequency. 1/16 microstepping is very demanding on a 25kHz controller, while the same microstepping is much less demanding on a 100kHz controller. With this kind of performance, you get smooth, accurate and fast motion while also enabling you to use higher feed rates.
If the spec isn’t listed somewhere obvious, take that into account. It probably isn’t nearly as high as you were led to believe.
Opto-isolated inputs as standard
When you start to hear about EMI, it often sounds like something you only hear about on fringe TV channels. However, EMI is actually electromagnetic interference that can cause your CNC to behave erratically, and is the culprit behind ‘phantom’ missed steps.
Spindles emit tremendous amounts of EMI, specifically at higher power ratings. Your spindle motor isn’t the only suspect, with your stepper drivers also contributing to the problem. Without isolation, EMI can invade the delicate electronics of your controller or driver, resulting in erratic behavior and missed steps that can be difficult to troubleshoot.
Opto-isolation is used on inputs to keep out EMI, and works by having a small phototransistor on one side of a miniscule physical gap, and a light emitting diode on the other. When voltage is applied to the LED, it will be detected by the phototransistor on the other side. Because the only medium crossing the gap is light, any EMI will be blocked from crossing the gap, protecting the electronics of the controller.
All professional controllers have opto-isolated inputs, including limit switches, probe, e-stop and other peripherals. If you are using a spindle over several hundred watts, you’ll also want that capability on your controller to protect the other delicate circuitry. It will save you a considerable headache when you are troubleshooting an issue that looks like a problem with the firmware when it’s EMI causing the issue.
True 4-axis support, not cloned axis
Many 3-axis controllers have a 4-axis option that is available, but that typically means that they have cloned one of the existing axes (the Y) to use a second driver for a gantry configuration.
This is not a true 4-axis CNC, as all it does is have two drivers for the Y-axis and both move at the same time to create an X-Y plane. There’s no independent movement or rotating axis such as A-axis.
True 4th axis support is what is needed to allow for independent rotation of the workpiece or another axis. The 4th axis can be a rotary style A-axis, or a more straight forward independent X-axis and is used for 4-axis machining such as wrapped engraving, rotary indexing mills or 3D workpieces that have undercuts and need rotation to access them.
As such true 4-axis machining requires a true 4th axis, a separate driver for the axis or driver capable of driving multiple axes at once.
If you intend to make a machine and possibly want to make it capable of 4-axis machining sometime later, you need a 4-axis controller. The Rabbit Board 4-Axis is a true 4-axis controller with USB support that allows for independent axis rotation such as A-axis rotation.
Onboard buffering over USB
One issue with early USB controllers is that the PC would hiccup, causing the data stream to drop for a moment resulting in the machine stopping mid-cut. This is much less of an issue, except when it still happens and you have no idea how to fix it.
Some controllers buffer commands internally, so that only a few milliseconds’ worth of information are affected when this hiccup occurs. This allows for a much smoother experience and prevents the machine from stopping dead in its tracks due to a momentary PC hiccup.
This works by having the processor in the controller buffer a set of command for a period of time. This way, if the USB connection stutters, it won’t immediately halt the machine. The buffer is then refilled after a period of continuous USB transfer.
When looking at a USB controller, you need to know whether it is buffered or not. If it has command buffering, it is able to withstand minor stutters or hiccups without causing the machine to hitch. If not, it will stutter.
Spindle speed control output
A controller that cannot modulate a spindle, but rather turns it on and off, is not taking advantage of a spindle’s capability, is inconvenient when you want to change speeds, or is wasting time when you have to stop the machine to go adjust it.
A controller should be able to support either a PWM signal or 0-10V analog signal to control your spindle’s speed, with the VFD or spindle driver handling the modulation for the spindle. Being able to modulate spindle speed is part of optimizing your tool paths, where different materials and tool diameters can require different surface speeds to keep the tool from wearing out too fast or from cutting too roughly. Not being able to modulate speed may lead to you keeping a consistent spindle speed, or manually changing speeds between tools since changing speeds would need you to stop the spindle and change a setting.
Modern spindle inverters and VFDs are capable of taking a PWM signal, but older ones still need an analog 0-10V signal to adjust the spindle speed. Either option is preferable to no option, but you should know what is required.
Dedicated peripheral outputs for coolant and accessories
Not enough people think about how they want to manage their coolants. Turning flood coolant on and off, switching to mist between certain operations, switching to a brief air blast at certain times; it’s all controlled by an M-code in your g-code or manually done by you.
It is good to have an M-code to control it, but that doesn’t help much if your controller cannot turn it on and off automatically. This is where a dedicated output from the controller to control a relay comes in.
It’s the same with all your other accessories and peripherals. You need the dust gate to close during operation, but open when you finish to clear the workspace. You need the air assist to come on when you fire the laser, or your fixture lights to come on while the machine is operating. Having a controller with enough outputs to control what you need when you need it can automate the whole process for you. If you don’t have a controller with those outputs now, you have to get one that does or risk having to manually turn on and off your accessories.
When you are shopping around, these outputs aren’t always going to be there. Make sure to look for them in the specs.
