Little black boxes

The second approach, also still in use today, is the standalone drive, also known as a smart amplifier. In this approach the controller is a “box,” and is usually rack or rail mounted. The drive either plugs into the wall, or is fed with a DC bus voltage. This architecture is shown in Figure 2.

There are many variations of how such stand-alone drives are controlled. Most of them can be controlled by a PLC (programmable logic controller) using digital inputs, and pre-programmable locations. More modern variations include the ability to download programs into a on-board memory, so that each drive can execute an autonomous sequence of actions such as, “start the motor at speed x, when signal y goes high then coast to a stop....” In addition to variations in how they are programmed, standalone drives are also available in multi-axis configurations. Stand-alone drives such as this work well when the behavior of each axis is fairly simple, and more or less autonomous. Using this approach it would be difficult to synchronize two or three such drives to follow a precise multi-dimensional curve, but it is easy to repeat a basic motion, or track an incoming encoder signal and execute master/slave electronic gearing or an electronic camming.

Compared to motion cards, the advantage of standalone drives is that wiring is simplified. Since the connections between the calculation portion and the amplifier portion of the controller are internal to the drive, all of the wiring used to interconnect a motion card to an amplifier is eliminated. Another advantage is that drives can be located essentially anywhere in the machine, saving cost and improving reliability by shortening cable distances. The central disadvantage, at least historically, of stand-alone drives is that they tend to be big and expensive, particularly for multi-axis control. This is because using older technology, packaging a profile generator, an amplifier, and a AC to DC or DC to DC power converter meant it had to be big.