Servo Motor System
1. Objective:

Introduce the students to control the speed of a servo motor system. To test the motor in the directions of rotation with the functions of the speed control. To conduct a reference move on a servo system.

2. Equipment:

Festo Equipment Set TP 801: Basics of servo motor drive technology, including Roller Guide, Brushless Servo Motor, Smart Electromotor Controller, Limit switches. 24 V Power Supply

3. Theory:
A servo mechanism or servo is an automatic device that uses error sensing feedback to correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error correction signals help control mechanical position or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback which controls position the operator does this by observation. By contrast the car’s cruise control uses closed loop feedback, which classifies it as a servomechanism. A servomechanism is unique from other control systems because it controls a parameter by commanding the time based derivative of that parameter. For example a servomechanism controlling position must be capable of changing the velocity of the system because the time-based derivative (rate change) of position is velocity. A hydraulic actuator controlled by a spool valve and a position sensor is a good example because the velocity of the actuator is proportional to the error signal of the position sensor.

A common type of servo provides position control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magnetic principles. Usually, servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an “error signal”) is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. An entire science known as control theory has been developed on this type of system.

Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-wire systems in aircraft which use servos to actuate the aircraft’s control surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus. A modern hard disk drive has a magnetic servo system with sub-micrometer positioning accuracy.

3.1 Servo Motor Control Connections:

Figure 1 : Servo Motor Control Connections Scheme

4. Procedure:

Figure 2: Experimental Setup

5.1 Speed Control in a Servo Motor System:
Unmount the servo motor from the axes.
Important to power up the following
• Mains power
• 24V DC
• Vmemoc
• And power off in reverse sequence
Presetting the Testbox and Axis
• Set all switches on the Testbox is OFF
(Switches down)
• Set the Analog-IN 0.1 switch to Ground
• Set the Analog-IN 0 potentiometer to 0

Changing setting in Wmemoc
• Go to the Commands dialog box, and select
Motor speed control. No other boxes to be
• Select Set Point > Set point selection >
Selector A and click on “Analog input 0”.
Click OK.
• This ensures that the controller is checking
Analog input 0..
Limiting the speed of the axis
• Select Parameters > Device parameters > Set point scaling. • Select channel 0 tab and set the scaling to 200 rpm.
• This means that at 10V the system operates at 200 rpm. These values are activated immediately.
• Click the Auto offset calibration button.
• If you have drift on ground the motor turns without any input signal this compensates for it.
This is the default value and must be adapted during commissioning to suit an application!
• Click OK to close the dialog box.

Enabling the system.
• On the Testbox:
• Check the analog potentiometer is at 0.
• Switch Power Stage (Endst.) and Controller (Regler) to enable. • Switch analog switch from Ground to On.
Test experimental setup for 200 rpm, 3000 rpm, 6000 rpm.

5.2 Reference Move in a Servo Motor System:
Fit MTR-AC to the axis.
Fit and tighten the knurled screws.
Slide should always be in the mid position of the axis.
Important to power up the following sequence.
• Mains power
• 24V DC
• Vmemoc
• And power off in reverse sequence
Testbox Check
• Controller enable is OFF.
• Power Stage is OFF.
• All other switches OFF.
Go to the Commands dialog box and select Motor speed control, position control, positioning .
Select Set Point > Set point selection > Selector A and click RS 232/field bus. Click on the Reference move icon to open the Reference Position dialog box. In the reference Position dialog box, check that method 1 is selected as the mode.

Cover the controller enable switch on the SECC-AC with one hand ready to switch off if the system is noisy or movement is erratic.
Switch the testbox to “On” first.
Switch the Controller “On”.
In the Reference Move dialog box, start the reference move by clicking GO!

6. Results and Conclusions:
Part 1: Speed Control in a Servo Motor System:
Operate the potentiometer to control the speed and direction of the slide as follows, observing the reaction of the slide. Log your observations in the table below.

Part 2: Reference Move in a Servo Motor System:
Log your observations for the part 2 of the experiment for 5 different speeds
and reference points.

Work cited: the accounting circle Festo USA.