Is it time to abandon your analog controls?

To further aid in identifying the culprit, consider this: design of the board is analog — meaning that the circuitry that provides the means of varying the speed or controlling the process is primarily a combination of discrete devices such as resistors, capacitors, op-amps and other non-integrated electrical components. Likewise, its feedback is primitive. It may have tachometer feedback, or possibly no feedback at all. Its process regulation may be completely open loop or include only armature or “Back EMF” feedback. No pulse or networked encoders will be used here. There are also a small number of parameters that can be changed on an analog drive — usually acceleration, deceleration, gain, span, dead band, torque and maximum speed, and all must be manually adjusted by single or multi-turn potentiometers. 

Another characteristic of analog controls is that the command reference signal is also analog, meaning that it is either a 0-10 Volt DC or a 4-20 mA current signal. This reference usually originates from the drive or controller itself and is proportioned using a 10K ohm potentiometer mounted externally. The analog reference signal may also come from another source, such as a PLC or closed-loop controller with an analog output that is wired to the reference analog input terminals.

Analog limitations

Analog controls are susceptible to environmental conditions because they are composed of many potentiometers, resistors, capacitors and transistors. Because resistance changes with temperature and sometimes humidity, analog systems may drift and have to be tuned for control one way in the summer and another in the winter. I have seen poorly conditioned injection molding facilities that needed hourly adjustments to their equipment in the spring and fall seasons because ambient conditions could swing wildly throughout the day.

Older analog controls do not offer the same flexibility as their digital counterparts. Because there are only a few adjustments, there are only a few things that can be optimized. As quality demands are ever increasing, variations in operating conditions often can’t be addressed. Maybe your six-sigma analysis dictates that you need to control the addition of soy milk to your gluten-free cake batter in 0.01 liters per minute and the pump you are controlling can only get you 0.1 on a good day. Certainly, there are other factors, but an analog drive may not have the resolution you need to meet your goals. Along these same lines is the lack of quick-changeover capabilities. Because the analog controls must be manually adjusted rather than programmed, your setup time may be relegated to hours rather than minutes. Think about having to reset five individual timers on a filling machine rather than having a recipe that loads what you need into PLC registers before you can say “Big Ben.”

If a single board analog control fails, it may need to be sent out for repair or be replaced. Maybe that’s no big deal if you have a 1/2 hp motor controller that you might even keep on your shelf. But if you have a 300 hp pump for a wastewater system, changing out that behemoth is going to be a daunting task. Newer digital drives typically have separate power and control boards that can be swapped out at a fraction of the cost.

Old stuff means old and difficult-to-source parts, which will likely result in extended downtime or increased cost to fix and maintain. Ever been down and had to find an obsolete board on eBay or one of the many third-party resellers of surplus or reconditioned items? No need to elaborate on this horror story.

Get on the digital bandwagon

Process manufacturing expertise today is all about communication. PLCs communicating with drives. Drives communicating with each other. Smart sensors that can be automatically set to see a blue registration mark on your labeler in the morning and a red one after lunch. A PLC that talks to remote I/O over Ethernet I/P and uses a gateway to send torque limit parameters to digital vector drives over ProfiNet. IP addresses everywhere. If you haven’t brought the digital revolution to your facility, it is time.

Ethernet, EtherCAT, ProfiNet, ProfiBus, CAN bus, and more. These communication protocols, readily available on today’s digital controls, can communicate over simple or sophisticated networks. Modern VFDs and DC drives have hundreds of parameters that can be controlled, monitored and reported back to the PLC and/or an up-stream data collection system. Parameters related to catching a spinning load, adjusting PID values on the fly or changing a drive’s carrier frequency can all be used to optimize system performance.

Factories are becoming more and more intelligent with devices and sensors that are connected to the internet and have an IP address, but how can that data be harnessed for greater manufacturing efficiency? Manufacturing intelligence allows connected devices to communicate with a central server that monitors processes and keeps managers informed as to what the system is doing. Is the control system running at peak processing speed? How often is the line running? What is the scrap rate? How much downtime versus uptime is happening? With the use of historians and data collection software, this information can be captured, saved, reported, analyzed and sent — all functions that are unavailable with analog controls.

Taking it to the next level, preventive and predictive maintenance are attainable. Trending a bearing temperature and setting an alarm for when it is out of a pre-defined range can allow a motor to run at peak performance with less downtime because the equipment’s operating information helps detect and predict problems. The bearing can be scheduled to be changed before it seizes unexpectedly and causes a bigger issue.