Grounding, EMF, RF and the iGaging DRO
In our last article about the excellent iGaging DRO scales and some of the problems they encounter, we discussed sources of electrical noise generated by the mill and lathe machines. We also outlined some approaches to reducing the generation of the noise. A low noise environment is obviously the best case scenario. If there is no noise or interference to deal with, then the mitigation strategies are reduced or perhaps not necessary. While theoretically possible, it is unlikely that the majority of electrical and RF noise sources will be completely removed from the average shop.
Low voltage scales are susceptible to noise
Because the DRO scale operates at a very low voltage, small noise sources and ground loop issues can have a significant impact on the displayed position. The DRO scale uses digital pulses sent from the reading head back to the display unit. A tiny computer in the display unit counts those pulses to determine the direction of movement of the reading head and the number of ticks that the head has moved. The embedded computer then converts the pulses from the reading head into the selected display units, such as mm or thousands of an inch. The pulses coming from the reading head are extremely low voltage.
If the noise floor that the display head receives is significant compared to the digital pulses that are being sent from the reading head, the embedded computer in the display unit will have a difficult time determining the difference between a legitimate pulse from the reading head and a noise spike on the wire. The display computer may even interpret a noise spike as a pulse and show a change in position on the display. A series of noise spikes may also be seen on the display as the position flickering or shifting randomly.
One of the major influencing factors on the stability of the iGaging DRO measurement is the grounding of the DRO scale with the machine. Electrical noise often propagates along ground paths.
In this article we will address the electrical noise relative to grounding. The approach we take will be low cost from both a financial and time perspective. Each of the following mitigation strategies are within easy reach of even the most novice machinist on the smallest of budget.
Electrical Isolation of the DRO Scale and Sensor
The first thing you can do to improve the stability of the iGaging and similar DROs is to make sure that all parts of the unit are electrically isolated from the mill or lathe.
One way to do this is to mount the DRO to the mill or lathe using non-conductive blocks made of plastic.
Delrin is a type of plastic made specially for machining and prototyping. It makes an excellent and stable mounting platform for all parts of the DRO including the scale, the sensing unit and the display unit.
If you are unable to find Delrin, or it is not cost effective for your budget, there is an excellent alternative. Plastic kitchen cutting boards are made of sturdy, solid plastic that is cheap and can be cut, drilled and carefully machined. Like when machining Delrin, be careful not to overheat the tool bit or the plastic.
First, create a block for each end of the DRO scale and mount it to the machine (mill or lathe) in a way that allows for the sensing unit to move freely with the carriage or table whose position is to be measured.
Next, make a block that allows you to attach the sensing unit to attach to the lathe carriage or mill table. When attaching the sensing unit to the item whose position you are measuring, be sure to check that the table or carriage moves through its entire range without binding, moving or flexing the DRO scale.
After mounting the scale and attaching the sensing unit, route the sensor cable in such a way that it will not be caught by the motion of the machine. When routing the cable, do not route the cable in parallel with the machines power cables as this will transfer electrical noise from the machine to the DRO due to EMF transmission.
Electrical Isolation of the Display Unit
Mount the display unit in such a way that it is not electrically connected to the machine or to a common electrical ground.
Since the iGaging and similar DROs have aplastic display unit and a plastic mounting block, this step is pretty easy.
DRO stability issue solved!
By following the above steps, I have been able to remove ALL jitter from my iGaging DROs attaches to my mini-mill and mini-lathe. I am so happy with my iGaging DROs now that they no longer bounce the measurement!!!