I BASICS OF INDUSTRIAL SERVO DRIVES Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved 1 The What and Why of a Machine Servo The control user should be familiar with servos. The user will understand what a servo is and why it is required in so many applications. This discussion will answer these two basic questions: What is a servo? Why use a servo? Any discussion of servos will have to employ the term ‘‘feedback.’’ Thousands of times every day we require information to be ‘‘fed back’’ to us so that we can perform normal activities. When controlling a car down a highway, feedback is provided to our brain by the gift of sight. How terrifying it would be if we were traveling at 70 mph and we lost the ability to see. Our brain, which is the center of our control system, would have little feedback to help it decide what corrective actions need to be taken to maintain a proper path. The poorer feedback channels still available would be the senses of hearing and touch, which would allow us to ride the shoulder. The result would be a lower speed, poorer control, a very irregular path, and a greater chance for an accident. Inferior feedback on a machine blinds the operator or the control just as it does a driver. When using numerical control and servos, poor feedback can result in inferior parts, poor productivity, and high costs. Essentially, feedback is the retrieval of information about the process being controlled. It verifies that the machine is doing as commanded. There are two types of feedback—negative and Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved positive. Positive feedback, used for instance in radios, is not discussed here. Negative feedback, required to make a servo work properly, subtracts from commands given to the servo so that a discrepancy or error between output and input can be detected. This discrepancy initiates an action that will cause that discrepancy to approach zero. A perfect example of a negative feedback system is a wall thermostat and furnace, as depicted in Figure 1. If, in a 658F room, we set the thermostat for 728F, then 728F can be considered the command. The 658F of the room feeds back, subtracts from the 728F command, and results in a 78F discrepancy or error that instructs the furnace to supply heat. The furnace supplies heat until the negative feedback is sufficient to cancel the command so that no discrepancy exists and no further heat is required. A servo or servomechanism is a system that works on the negative feedback principle to induce an action to cause the output to be slaved to the input. Our thermostat/furnace example was one of a servo which induced the generation of heat. Any servo has two basic elements. These are a summing network and an amplifier. The summing network, as shown in Figure 2, is simply a device that sums the negative feedback (F) with the comma nd (C) to generate an error discrepancy (E). Our driver’s summing network (Figure 3) was his brain. The command would be to keep the car in the right-hand lane. If he was straddling the center line, the feedback through his eyes to his mind would so indicate. If he subtracts this feedback information from the Fig. 1 Thermostat example. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved command, he will deduce that an error exists, which can be corrected by moving 2 ft to the right. A machine feed axis drive example (Figure 4) would be to have a command of þ10 in. If, as the machine was executing the command, we took a snapshot of the summing network when the machine reached þ9in., we would see a command of þ10 in., a feedback of þ9 in., and a resulting error of þ1 in. The error would be the amount of further movement required for the feedback to equal the command. The second main element (see Figure 5) is the amplifier. This is simply a power device that takes a small error (E) and multiplies it by an amplification factor, which is a measure of the muscle or power available to drive the output and thus the feedback device (F). The amplification factor is what is normally referred to as the gain of the system. Fig. 2 Summing network. Fig. 3 Feedback. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved Our driver (Figure 6) mentally computed an error (E) requiring that the car be moved 2 ft to the right. This instruction traveled from his brain through nerves to the muscles in his arms and hands, which turned the steering wheel and with the muscle in his power steering caused the car to move into the right-hand lane. In the machine feed axis example (Figure 7) the error of þ1 in would be in the form of a small voltage, which would cause the servo motor to turn, and axis positioning motion would result. If we connect the two elements together, as shown in Figure 8, a basic closed- loop servo system results. A new command will generate an error (E), which will activate the muscle until sufficient movement has caused the feedback (F) to be coincident with the command (C), at which point the error (E)is zero and motion is no longer instructed. The term ‘‘closed loop’’ suggests that after entering a command, signals traveled around the loop until equilibrium is attained. Fig. 4 Summing network for positioning. Fig. 5 Amplifier. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved Our automobile and driver (Figure 9) are now a servo system with a command to stay in the right-hand lane. The servo is shown above and the position of the car to the centerline is shown below. When the car is actually straddling the white line, an error is mentally computed, which instructs the driver’s muscle and the car’s muscle to take a corrective action. When this corrective action is taken, the position of the car corresponds to the command, no further error is detected, and no further action is taken. Our snapshot of the machine axis servo feed (Figure 10) showed a command of þ10 in., from which the þ9-in. feedback was substracted. The þ1-in. error instructed the axis to continue moving. As the axis continues on, the error will get smaller and smaller until the feedback indicates that the axis is in position with no resulting error. The earliest axis feed systems used the operator as the summing network that closed the servo loop. The command was located on a part drawing, and the operator’s mind was the summing network. The operator read the scale on the machine, subtracted it from the desired command, and Fig. 6 Human muscle. Fig. 7 Servomotor. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved came up with an error. The operator instructed the machine’s muscle by shifting the appropriate levers to engage motion. As the machine neared the commanded point, the error indicated the need to slow down to one or more intermediate geared speeds. Creep speed was used to accurately reach final position. A repeat performance on a second and third axis would complete the operation. Fig. 9 Car example. Fig. 8 Closed-loop servo. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved JUST WHAT ARE SOME OF THE BENEFITS OF A SERVO SYSTEM? Here are six benefits of a machine axis feed servo drive. 1. Shorter positioning time: The servo operates at maximum positioning rate until the ideal time to decelerate, at which point it slows down uniformly to the end point with no hesitation at intermediate feeds. Since it dynamically searches for zero error, variations in machine conditions are compensated for. Positioning time is thus minimized. 2. Higher accuracy: A servo continually homes to the final position so that on January mornings it will continually strive to push the axis toward the end point, whereas on the Fourth of July when the machine might have a tendency to overshoot, the servo error will reverse and force the machine back into position. 3. Better reliability: An outstanding feature of servos is the ability to control acceleration and deceleration so that the mechanical hardware will hold its specification tolerance much longer. 4. Improved repeatability: Repetitive moves to a particular com- manded point will show much better consistency. The result is more consistency of parts that are intended for interchangeability. 5. Coordinated movements: Since all axes are closed-loop servos, they are continually responding to the command at all feed rates. Coordinated movements thus require the generation of coordi- nated commands through employing interpolators with the control. 6. Servo clamping: There is no longer a dependency on mechanical clamps for servoed axes because of the continuous position- Fig. 10 Closed-loop position servo. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved holding capability of a servo. The stiffness of the servo must be relied on for any contouring movements requiring that both axes be in motion. Properly designed, the servo can also hold the axis very stiffly at a standstill. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved . I BASICS OF INDUSTRIAL SERVO DRIVES Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved 1 The What and Why of a Machine Servo The control. basic questions: What is a servo? Why use a servo? Any discussion of servos will have to employ the term ‘‘feedback.’’ Thousands of times every day we require