Servo drives interact with hardware axes, yet the methods for configuring motion vary widely. Input/Output (I/O) signals enable the connection of sensors and switches to facilitate functions such as homing and setting travel limits.
If you are new to servo motor drives, you will discover that they differ significantly from three-phase asynchronous motors driven by Variable Frequency Drives (VFDs); the fundamental distinction lies in the feedback mechanism. VFDs operate within an open-loop system, requiring an external controller to handle all feedback control; servo systems, conversely, feature integrated encoders-consequently, the motion parameters for the entire system must be defined *before* operation commences.
Previous articles have covered select examples of hardware axis configuration and demonstrated how to input parameters into the drive software (as illustrated in this tutorial); however, it is now necessary to delve deeper into the specific types of I/O signals required to execute various motion control tasks.
Specifically, three categories of input signals within a servo system warrant particular attention: limit signals, homing input signals, and general-purpose input signals.
Limit Switches
First, it is necessary to clarify the definition of the term: a limit switch is a type of hardware switch that is triggered by physical force applied to a lever or roller; it can be utilized in various scenarios involving object detection.
However, within the field of motor control, there exists a variety of switches used to detect whether a moving object has reached the end of its travel range; these switches are also referred to as "limit" switches. For most motion axes, a limit switch is typically installed at each end of the travel range-designated as the positive limit switch and the negative limit switch, respectively.
The limit switches employed on motion axes are not exclusively mechanical in nature; currently, many non-contact switches are available that utilize optical or magnetic properties to perform travel detection without generating any mechanical resistance.
Software Limits
Is it strictly necessary to use limit switches? The answer is no; in many situations, there is no need to rely on hardware limits.
Most servo drives support a software limit function: an encoder continuously tracks the axis's position in real time, and if that position exceeds a preset threshold, the controller prevents the axis from moving any further.
Homing Switches
The second category of digital input signals consists of "homing" input signals. Typically, a system requires only one homing switch, as the homing operation is executed-moving in a single direction-only when the equipment is powered up or upon receiving a specific command. When an axis is powered down, its current position information is erased (unless the system is equipped with a battery backup, as is common in robotic servo systems). The fundamental purpose of the homing process is to enable the controller to re-establish a "known starting point" for all subsequent movements.
Fixed-Position Homing
There are various homing methods in motion control; the most common (and easiest to understand) approach is as follows: the motion platform moves slowly in a specific direction until it triggers the homing switch, at which point that trigger point is designated as the "zero position."
There is also a subtle variation of this method: the platform first triggers the homing switch, then reverses direction-retracting slowly at an extremely low speed-until the sensor is no longer actuated; the position at that precise moment is then defined as the zero position. It is not difficult to imagine that contact-type limit switches equipped with lever arms may experience wear and tear after prolonged use, potentially leading to minute shifts (perhaps only a few tenths of a millimeter) in the homing position-this is one of the reasons why non-contact switches often offer distinct advantages.
Homing to the Center Position
The principle behind another homing method is as follows: the moving component first travels in a selected direction until it triggers a limit switch, then reverses direction until it triggers the homing switch. The premise of this method is that the homing switch is installed at the "center position" of the travel range, and-when the equipment is powered on-the system cannot initially determine which direction the motion platform needs to move in to reach the homing switch. Here, too, the use of non-contact switches is highly recommended, as the motion stage passes over the homing switch every time it traverses from one end of its travel range to the other.
Software Homing
There also exists a software-based homing method that relies entirely on software logic, requiring no hardware switches whatsoever. If the motion stage consistently comes to rest at a specific, known position whenever the equipment is powered off, the system will automatically designate that position as the "zero point" upon every subsequent startup. However, a critical caveat applies: if the position of the motion stage at the moment of power-off is not fixed-meaning it varies randomly-the system will erroneously designate this random position as the zero point upon startup. In such a scenario, if you subsequently command the axis to move to the 400 mm position, the axis may overshoot the end of its travel range, potentially resulting in a system fault.
Safe Torque Off (STO) Inputs
From a safety perspective, mechanisms must be in place to protect both the equipment and the operator in emergency situations. This functionality is implemented via two input signals: the Safe Torque Off (STO) signals (STO1 and STO2). These two signals must continuously receive a voltage input; otherwise, the drive unit will be commanded to shut down. Typically, these two signals are connected to an Emergency Stop (E-stop) circuit or to a safety relay equipped with dual normally-closed contacts.
General-Purpose Input Signals
The preceding discussion has covered most of the core functions associated with servo drive input signals; however, the capabilities of certain drives extend beyond merely driving a motor. Take, for instance, the SBD drive from CMZ Sistemi Elettronici: in addition to its servo drive capabilities, it features an integrated embedded PLC capable of executing structured test code.
As is the case with all PLCs, such integrated drives require a mechanism for receiving external signals-including process signals from sensors and inputs from user-operated buttons. These General-Purpose Input/Output signals (commonly abbreviated as GPIO) can be utilized to transmit any type of signal-much like those in standard PLCs-without being assigned any fixed, pre-defined functions. It should be noted, however, that not all servo drives are equipped with GPIO interfaces.
Servo Motion Control
Undeniably, configuring a servo system presents a significantly greater challenge than configuring a variable frequency drive (VFD). At the same time, however, a wide array of commissioning tools are now available to help streamline the configuration process and lower the technical barrier to entry. For professionals who have yet to venture into the realm of servo systems, it is never too late to start learning!