Motion Control Tips | CNC Control

Simple motion control

Tryling, David P

In positioning applications that are repetitive and non-complex, PLC-supplied motion control hardware might be the answer

SOME READERS MIGHT view this article's title and say, "There is no such thing." And they may be right. But there are possibilities in motion control that tend to be overlooked. These could be added to your toolkit of ideas for solving motion and positioning applications.

There are numerous industrial applications that require the positioning of a part, the rotation of a tool, or the travel distance of a component. This is at the heart of what motion control is all about. In the simplest form, mechanical cylinders or actuators can perform any of the operations just described. However, when you add the complexity of making the distances, rotations, or other motions variable, it's fairly inevitable that an electronic device will be applied.

The rub, as they say, is always in the cost of the application. This is usually because the application of motion control is expensive in terms of hardware and complex in terms of effort. However, there are simpler ways of getting position control for these general applications.

Consider the application of programmable logic controllers, or PLC's, using pulse output capabilities. This is alternately referred to as Pulse Trained Output, or PTO. Many smaller and medium-sized PLC's are equipped with this capability, and they are easier to use than you would think.

Pulse output capability is the hardware and programming process where a chain of pulses is sent to a drive or power amplifier that moves a motor in a particular direction and for a specific amount. For example, if the PLC sends 10,000 pulses to a servo amplifier, the rotary action of the motor will move a specific number of rotations in relation to those 10,000 pulses.

There are two basic formats in which pulse output capabilities are available. First is the integrated output. This is most commonly found on smaller PLC's ("brick" type) where one particular output will be pulse-output-capable. When the pulse output is integrated to the PLC chassis, there is additional hardware related to that particular output. While in most cases the output can perform in a standard way, it also would be capable of providing a series of pulses related to an internal set of hardware. This hardware allows the output to operate independently of the PLC logic execution. Therefore, when a command is issued to send pulses to the output and then on to the power device, the PLC logic's scan time continues on while the pulse output processing hardware performs the pulse generation necessary as configured.

Obviously, there is a difference between PLC manufacturers when it comes to how these basic functions are programmed and executed. These will depend on the basic programming structure of the languages from the PLC manufacturer. Some manufacturers work with configured parameters, others may use block instructions that send data to the pulse output logic area and perform the configuration requirements. Regardless of the programming method, the manufacturers have similar capabilities. Listed in Table I are some of the parameters that are generally sent to the pulse output electronics.

PLC's are also able to perform some of the more common motion controller functions such as "jog" or "home" or "move to." These may be done using specific inputs that are tied to the pulse output hardware or by program parameters/bits.

The second type of hardware used for pulse output motion control would be a plug-in card. This is obviously used in PLC types that have racks for plug-in modules. Plug-in-type cards may have additional features that are not available from the manufacturer in their brick-type PLC's. These might include additional high-speed input lines for positioning and home sensing, overtravel, and other higher-speed functions called interrupts.

There are some important functions that the motion control hardware provides. Some bit functions that are usually available are shown in Figure 1. These can be used subsequently in ladder logic programming to control the flow of information to the motion hardware and for the balance of the program to know the status of the motion functions.

Just as in any other positioning system, accel and decel are important when thinking about and creating a positioning move. The pulse output hardware can provide accel and decel by changing the width of the pulses. It will be important, however, that the drive or power amplifier be capable of responding correctly to this. Through the use of accel and decel, a true motion profile can be created. (See Figure 2.)

One of the important planning details that will need to be established is calibration. As with any device or sensor, the motion will need to be calibrated. This is done by establishing the engineering units with which the machine will be operating and then determining the number of counts or pulses that equal the smallest increment of those engineering units, as shown in the equations at left.

A couple of things to keep in mind. First, while it is true that these motion control capabilities are extensive, there are things that this type of simpler motion control cannot perform. If motion needs to be coordinated between two devices or axes, if complex motion moves need to be performed, then this is not the hardware for your application. However, if your application requires configurable general motion, then this is a good fit.

Secondly, these motion devices operate in open loop format. What this means is that the control hardware in the PLC does not actually receive a position feedback signal from the power amplifier. The PLC hardware is sending a series of pulses to the power system, which interprets that and performs a movement. The PLC hardware assumes that this movement is completed, either from a signal back from the power amplifier or in some sort of timer. If the application has a high potential for slip or other position errors, you will need to look to different controller hardware.

By David P. Tryling, EA Electronics Editor