Using an ammeter

A load meter is a great accessory for machinery to get a sense of how near capacity a task is bringing the machine. Here a load meter is built for a CNC milling machine using a ammeter and a voltmeter. This approach measures apparent power instead of actual power so there is some error. While not perfect, it provides the feedback necessary to know if the machine is being overtaxed and if the machine has some reserve left in it.

The video describes the theory of the meter, using Fusion360 to design the panel, and the meters construction and use.

Reference material: Determining Electric Motor Load and Efficiency=

Using a power meter

A load meter is a great accessory for machinery to get a sense of how near capacity a task is bringing the machine. In a previous post, a load meter was built for a CNC milling machine using a ammeter and a voltmeter. This approach is not ideal because apparent power is being measured instead of actual power. Because the power factor changes with different motor loads, measuring power with a ammeter and voltmeter is not ideal.

Specialized meters are required for measuring Watts, or actual power. Murata has a power meter product that uses a microprocessor to correctly time the sampling of current and voltage waveforms. The current is inductively sensed from a load wire passed through a current transformer. The voltage is sensed via a fused connection to the mains.

The power measurements while drilling a 1/2″ hole into aluminum are compared to computed estimates of horsepower requirements. The two are surprisingly close. The disadvantage of the Murata power meters is their slow update of roughly 1 Hz, making them unsuitable for quick response. However, the power meters are likely the best method for measuring the load on the machine during steady-state operations.

Measuring tools

Various tools are tested on a CNC mill to create the same part using similar operations. Machine performance is monitored for each tool to begin building a catalog of tool parameters for Autodesk Fusion 360 to control a converted PM25MV milling machine.

Reporting from the motor drive

Could the DYN4 serial interface be used?

  • The serial interface may be preferred over all the other methods for controlling a spindle motor. The low baud rate and error checking should make serial communication pretty tolerant to noise. Commands are sent to the drive and the drive is trusted to execute them. Trust but verify.
  • I created a Python module to communicate with the DYN4. The code is available on GitHub. I have a DYN4 with older firmware but I implemented all the reads and the write commands that I needed.
  • I used the module to create a userspace LinuxCNC HAL component to command the drive. LinuxCNC prefers realtime control, which places constraints on programming. But with userspace components one has more flexibility including using Python and modules like PySerial.
  • There is a realtime serial component but this requires a hardware serial port. USB may not be practical to control with realtime constraints