Avoiding Common DRO Scale Problems

Capacitive DRO scales, such as Shahe, iGaging, AccuRemote, etc. have become a mainstay of DIY hobby digital readouts due to their affordability, availability and other beneficial properties. They have also earned somewhat of an undeserved reputation of being unreliable, flaky, and easily damaged. When used with their original reading head, capacitive scales are very stable and reliable. The problems arise when they are connected together in a two- or three-axis DRO setup. Fortunately it's relatively easy to avoid those problems following a few basic guidelines. By following the five steps outlined below before powering up the DRO for the firs time you will be much more likely to end up with a reliable digital readout.

1. Ensure That All Pins Are Properly Connected

Probably 80% of all scale reliability issues reported by TouchDRO users have been caused by improper scale connection. Capacitive scales operate on micro Amps of current and can often work with Vcc and/or Ground leads completely disconnected, but the reading will be extremely unreliable.Furthermore, the Absolute DRO+ scales provide their own clock and at some positions will produce a reading when the Data and Clock lines are reversed. Likewise the reading will be all over the place.

If you purchased a pre-made TouchDRO board with Micro-USB connectors already pre-installed, all you need to do is to ensure that the male plugs are inserted securely. Otherwise please follow your controller's documentation to ensure that that the pins are connected correctly and are attached securely.

2. Select a Correct Power Supply

Incorrect power supply can cause instability or worse yet, can damage your scales and the controller board. All versions of the TouchDRO controller use a DC power supply with center pin connected to the V+ and the barrel connected to Ground. All versions of TouchDRO controller have on-board voltage regulator(s) with 1V dropout. In other words the regulator requires voltage at least 1V higher than the voltage it will regulate. Therefore glass and magnetic controller requires at least 6V DC (5V for scales + 1V dropout) and all other controllers require 4.3V power supply voltage (3.3V for MSP430 controller + 1V dropout). The controller will function with lover voltage but the operation will be very unstable. Conversely, exceeding 13.8V DC will permanently damage the on-board voltage controller and will render the board unusable.

Many inexpensive "wall warts" are designed to output their nominal voltage at full load. Since TouchDRO controllers draw very little current (in 50-100 mA at most), the input voltage can be higher than nominal or fluctuate significantly. Therefore to ensure stable operation at least a 7.2V adapter should be used for 5V glass and magnetic scales and at least 4.5V should be used for all other scales. On the other hand using a AC/DC adapter with nominal output higher than 12V is risky and should be avoided.

Finally, when using the Mixed Scale TouchDRO controller, ensure that the correct scale voltage jumper is used. Providing 1.5V to 3V scales will make them extremely unstable. On the other hand connecting 3V to 1.5V scales will damage them in seconds.

IMPORTANT: When using a mix of 3V and 1.5V scales, their frames need to be isolated from each other. 3V scales connect Ground to their frames whereas 1.5V scales have their V+ connected. Failure to isolate the frames will likely lead to permanent damage to the scales and/or the DRO controller.

3. Ground the DRO Circuit

Any piece of wire can serve as an antenna, picking up noise from the electronic fluorescent light ballasts, VFDs, PWM motor controllers and other things. Strong enough noise on the data or clock line can wreck havoc with scale readings. Pulses on Ground or Vcc lines can cause scale resets that manifest themselves in random zeroing out of the reading. Pulses on the clock or data lines will cause position climbing or erratic reading that jump all over the place. In most cases the problem can be avoided entirely by properly grounding the circuit. Ideally TouchDRO controller board should be connected to the machine's body, which in turn should be grounded, and scale cables should have their shielding braid or foil connected on one side to the ground as well.

4. Eliminate Ground Loops

What are Ground Loops

Ground loops are just that, loops in the grounding circuit that are caused by too many grounding points. Issues associated with them are well known to home audio installers and audiophiles alike. In home audio systems they result in low-pitch hum through the speakers. In DRO setups ground loops can wreak havoc with stability and reliability, and can even damage the scales. To some people this is completely counter-intuitive.

Why Are Ground Loop Bad

To understand why too many ground connections can be as bad as too few, consider an average DIY digital readout setup with three iGaging scales mounted to the machine's frame via metal brackets. Since iGaging scales have the frames connected to the negative lead, which in turn creates a ground connecting between TouchDRO Ground net and the machine's frame. At the same time TouchDRO ground is connected to the mains neutral via the AC/DC adapter.

Furthermore, in most cases mains ground and mains neutral lines are tied together at the distribution panel. This creates a ground loop where one leg is the 6' of the scale cable and the other is several feet of wiring running to and from the distribution panel. The latter can cause serious problems for the scales. Consider the fact that at startup a typical single phase AC motor can create a current spike of up to 5-10 times its normal running current. For example a 5A motor can create 25-50A spike, which can cause short voltage jump in the mains neutral line (on the order of several volts). According to the Ohm's law Voltage = Current * Resistance. In this case the resistance comes from the copper wire on the order of 0.25 Ohm per 100 feet, resulting in around 6V spike in the neutral line. This creates a situation where for a brief moment where TouchDRO ground is at 6V instead of 0V, so the scale sees -3V rather than +3V Vcc. This can cause random scale resets and even damage.

How To Fix Grounding Loops

There are a few ways to deal with ground loops, especially the one caused by mains wiring. The easiest is to use mains-isolating power supply, which is a fancy name for an old-style wall wart with a heavy transformer. The two windings of the transformer are isolated from each other, so the DRO will use machine's ground as 0V reference. When a spike outlined above happens, it will not cause the Vcc reversal since the board can "float" up by 6V.

Another, more laborious, method is to isolate scale frames from the machine's body by using plastic mounting brackets or some other means. This way TouchDRO's ground voltage can fluctuate but it will be harmless since the scales won't anchor their 0V to the machine and thus the Vcc reversal won't happen either.

5. Properly Ground 3-Phase VFD

Three-phase Variable Frequency Drives convert 60Hz AC voltage to DC and then back to variable frequency pulse-modulated AC voltage. The modulation frequency, also called "carrier frequency" is usually in the 1 KHz to 7 KHz range and can create all sorts of noise in the wiring. A VFD will come with a manual that lists specific grounding requirements. If not grounded properly, it will almost certainly create havoc with the electronic devices in the shop, and even the whole house in some extreme cases.

At the bare minimum the frame of the VFD needs to be connected to the ground. It's also recommended to use a shielded 4-wire cable between the motor and the VFD. The shielding should be connected to the ground at one end, usually next to the VFD and the fourth wire should be used to tie motor's ground to the VFD ground. This is especially important when using an inexpensive Chinese VFD, which often lack proper LC filter circuitry.

Conclusion

Vast majority reliability issues with iGaging-type and other capacitive scales can be traced to faulty wiring, lack of proper grounding in the controller, ground loops, noise created by VFDs, or similar PWM speed controllers. Common symptoms include random resets, intermittent position jumping and overall instability. When facing such symptoms, the beast approach is to check the wiring first to make sure all lines are connected reliably and if not, correct the problem. Second, verify that you are using correct power supply and are connecting it properly. Third, ensure that any equipment that can produce noise is properly grounded. If after retesting the issue persists, make sure the board and the scale cables are shielded and properly grounded, but avoid ground loops, since they can create additional problems.