Pros and Cons of Capacitive Scales
It's not a coincidence that capacitive DRO scales have been a mainstay of do-it-yourself DRO setups for more than two decades. Even as the prices of glass DRO scales have come down, capacitive scales continue to occupy a prominent place in the hobby DRO ecosystem. That said, using capacitive scales as a machine digital readout requires some compromises.Therefore, it’s important to understand what the pros and cons of capacitive scales are, so you can choose the best setup for your needs.
Since magnetic scales are still relatively more expensive, the comparison here is made mostly with Glass DRO scales.
The main benefit of the capacitive scales is their relatively low cost. A set of Chinese scales for a small milling machine can cost around $100, while a lathe can be equipped for even less. Better yet, many of the scales now come with a remote display that can be mounted in lieu of a DRO unit initially used as a basic digital readout. Later, as the budget allows, the setup upgraded to TouchDRO for another $100.
Ease of Mounting
Capacitive linear scales are usually easier to mount than their Glass counterparts for a few different reasons. First of all, standard Glass scales are much bulkier than most capacitive scales. Besides being thicker along their whole length, they take up more vertical space as well since the reading head rides on top of the frame. Moreover, in order to protect them from dirt, the scales should be mounted with the slit pointing downward, posing additional mounting restrictions. Second, glass scales have a limited amount of flex in them and are thus sensitive to misalignment. Capacitive scales, on the other hand, have enough given to tolerate even moderate misalignment. Finally, capacitive scales can be easily cut to size, which makes them easily adaptable to almost any machine size.
Low Power Consumption
Capacitive scales are designed to run on a cell battery for months or even years. Although most TouchDRO users power their scales from the adapter directly, it's relatively easy to wire the scales to a separate power source. A set of 3V scales can be continuously powered from two AA batteries for well over a year, even when the TouchDRO adapter is powered off. This setup allows the scales to behave more like absolute scales (since they never need to reset and lose their position), eliminating the need to re-indicate the part before each machining session.
Data Retention on Brief Power Loss
Low power consumption offers one additional benefit: many capacitive scales retain their position for a few seconds after being powered off. For the ones that don't have an internal power reserve, a moderately sized capacitor can provide ample emergency backup. Combined with the fact that the TouchDRO application can take advantage of the tablet's built-in battery, brief power flickers usually don't result in loss of position as they would with a set of glass scales.
Variety of Models and Form Factors
Capacitive linear scales come in many different form factors and sizes, but the choices go well past just the linear scales. For example, many of the modern test indicators, thickness gauges, angle gauges, etc. use that same encoders and data protocols as the linear scales from the same manufacturers. This opens countless possibilities outside of just a simple 3 axis linear DRO.
The most obvious downside of the capacitive scales is their lower resolution. The maximum practical resolution offered by the better capacitive linear scales is about 10 microns; give or take, low-cost scales seldom resolve better than 0.001" or 0.01mm (in metric mode). This is more than adequate for smaller machines that lack rigidity to hold better tolerances but can become a limiting factor on larger more rigid machines.
Slower Refresh Rate
The refresh rate is another area where many models of capacitive linear scales fall short. With the exception of the iGaging 21-bit and Absolute DRO scales that can refresh at up to 50 Hz, many other scales have refresh rates of 3 Hz. This introduces significant lag into the system that is very noticeable even with slower traversal speeds.
Sensitive to EMI/Noise
Due to their nature, capacitive scales are inherently more sensitive to electromagnetic interference and voltage spikes. Making matters worse, the manufacturers don't put any effort into noise protection since the use in a multi-axis DRO is not the designed use case for these scales. As a result, these scales are very sensitive to ground loops and VFD noise that can manifest itself in random position flickers and, in extreme cases, resets or even scales damage.
As you can see, capacitive scales can offer a lot of benefits that make them an excellent option for do-it-yourself DRO setups. At the same time, they have drawbacks that should be taken into account. If you own a smaller machine, such as Mini Mill or Mini Lathe, capacitive "Remote DRO" scales will likely work well enough since the machine is not rigid enough to take advantage of 5-micron resolution. Furthermore, low cost, the option to cut the scales to length, and smaller profile work to their advantage. On the other hand, a larger machine will benefit from glass scales and will have more mounting options that negate the smaller size and flexibility of the capacitive scales. Ultimately, the choice will boil down to whether or not their flexibility and lower cost outweigh the lower refresh rate and resolution.