Choosing the right controller for a machine can be a difficult and, at times, stressful process. There are constantly new products being released into the marketplace and it can be a challenging task to simply compare two controllers, much less identify the right controller for your application. It can take a knowledgeable engineer in tandem with a savvy sales rep to find the right combination of price, function, performance, and other important factors to satisfy the application’s requirements.

Without sufficient consideration paid to balancing requirements and capacities, the result can be what is seen in far too many machines in the field; a system that is overkill for the application, a solution that meets the bare minimum performance requirements (sometimes not even that), or a system that uses the wrong technology for the task at hand. Even with an ideal sales rep, it’s important to know which questions to ask to ensure that the control solution you create meets your requirements and expectations. The following points highlight what can be generally considered the most important factors for choosing a PLC for machine control

Horsepower
Well, processing power, that is. Machine control lives and dies by its ability to compute the required logic program in an accurate and timely manner. If a bit that’s required to activate an output cannot be driven within the expected time frame in any given situation, at best the result will be less than optimal performance and at worst could cause damage to the product, machine, or operators. In many cases, when an underpowered controller is used in a system, engineering time is required to reduce machine performance so as not to outpace the controller’s ability to control. In the present environment, there can be little, if any, excuse for selecting a controller that doesn’t have enough processing power. Of course, it’s easy to select a controller that has an unreasonable gap between the required and specified performance. Finding the sweet spot in between takes strong communication between engineering and sales, but it can be generally considered smart to err on the side of being slightly over-specified. The alternative of being under-specified is not worth the time and headaches that will eventually arise.

Scalability
How many machine makers do you know that make, and have only made, a single machine design? Not many take this approach and it would be foolish to do so in a harsh financial environment that is still weeding out those who do not innovate. Selecting a controller for a single machine can be a difficult process and doing so for every new machine design and configuration would be ridiculously redundant. Choosing a controller or control platform that is able to adapt and expand both functionality and performance to meet expanding (or contracting) machine needs is vital to long term sustainability. A controller platform that has options for discrete I/O, positioning, motion, analog I/O, temperature, communication and other types of control allows for the best match of cost and application-specific performance.

Communications
Whether it’s for getting data from a sensor or setting the parameters of a servo amplifier, having availability on a standard network connection is essential in today’s factory environment. A machine controller needs to effectively control most, if not all, of the operations in a machine, either directly, or as recent trends would indicate, remotely. Direct wiring and exotic fieldbus cables are being replaced by something that everyone is familiar with; Ethernet. This standard medium for connection is not only easily obtainable from countless outlets, but also enables simple implementation of distributed control without the traditional headaches of creating cable terminations or wiring into terminal blocks. Having an Ethernet port built into the controller makes this simple and cost-effective. While there may be countless interpretations of an “Ethernet” network based on which vendor you look at, it looks like the physical medium, at least, is here to stay.

Lifecycle
Hardware failures can mean a black mark against your company, but not being able to efficiently and inexpensively replace components can mean the end of a relationship. The benefits of product quality can be problematic to quantify, especially for some machines where the expected lifecycle of other more costly components can necessitate an upgrade or replacement machine well before the controller will likely fail. In simple terms, the controller should not be the weak link in the machine. Some failures are inevitable and a controller that can exchange or even hot swap components with a minimum of downtime can go a long way in maintaining customer confidence. The warranty provided from a vendor can speak volumes about how confident they are in the quality of their product.

Cost
Going with a $15,000 controller system in a $25,000 machine versus a comparably capable $3,000 controller is generally not a smart fiscal decision. Difficult decisions may need to be made to bring the price level of a machine to the point where it can actually be sold. In a machine where the controller only makes up 5% of the total cost, a 50% or more increase in cost could very well be within reason.

These factors impact the vast majority of machine controllers, but there can always be that X factor, that killer feature that puts all other controllers out of contention. While having an exclusive technology in a machine can be a very attractive, and in some cases necessary feature, it’s always important to have multiple options available. This provides a stronger position to negotiate from and more reassurance that the machine design will be translatable into future designs.