The purpose of this laboratory experience is to provide an introduction to control systems and explore some of the parameters that will be further analyzed in detail in the upcoming lectures. By investigating the characteristics of a basic control system, students will have an overall view of the main components of the system.


The basic goal of a control system is to produce an output as a response to an input signal or command and keep it this way in the presence of external interference, disturbances, etc. Control systems can be basically classified as open-loop systems or closed-loop systems depending on how they are built. This introductory lab experience will explore both types of systems.

The laboratory equipment

This lab uses the Feedback MS150 Modular Servo System kit. This system has been explicitly developed for studies of control systems and tries to break solutions into different modules following the accepted and used "block model" when describing a physical system. Note that in real-life solutions, some of the modules will be integrated in order to maximize efficiency. However, because it would be difficult to visualize and measure the different signals involved in this process, for academic purposes we will use different blocks, having access to intermediate signals.

Each one of the blocks is fitted with a magnetic base that will be used to attach and secure them to the baseboard, providing a strong support for the whole system. Students are advised not to place magnetic- sensitive media (credit card, etc.) closer to the modular blocks.

The main power supplies for the Servo Units and the Motor Tacho Unit are fed through the cables terminating on octal plugs fitted to both the Motor and the Servo amplifier Unit. The lead from the motor should be plugged into the Servo amplifier and that from the amplifier into the power supply unit. These plugs can only be fitted in one direction to avoid misconnections.

Both, the Power supply and the Servo amplifier are fitted with 4 mm sockets from which ± 15V DC voltages can be drawn to operate all the other units for the system.

Power Supply Unit 150E This unit supplies unregulated 24 V DC 2A to the motor through the octal connection to the Servo Amplifier as it is the unit that controls the motor. On the front panel there are two sets of 4 mm sockets that provide stabilized ± 15V to operate smaller and provide reference voltages. When showing the schematics for the connections of different blocks, that connection for the octal plug that is located in the top part of the power supply will not be shown to simplify the schematic. However, students must remember to make this connection.

Motor Unit 150F The important thing for us about this unit is its low-speed shaft driven by a 30:1 reduction gearbox. A special push-on coupling can link the output to this shaft. We will connect the output potentiometer to this low-speed shaft. The power to the motor is attained from the Servo Amplifier unit by means of the octal plug. Again, this plug will not be shown on schematic diagrams.

Attenuator Unit 150B. This unit contains two variable 10k used to attenuate signals flowing between different modules. The proportion of resistance being selected is indicated by a dial graduated from 0 to 10. This unit can be used to either provide a reference voltage with it is connected to a DC voltage source, or as a control when connected to an amplifier.

Servo Amplifier 150D. This unit contains the transistors to drive the motor in both directions. The connectors in the front panel allow for different modes of control armature. The power to the servo amplifier is supplied by the octal connector at the lower end of the unit.

Operational Amplifier Unit 150A. This unit provides inverting voltage gain and a means of summing two or more signals as well as the circuitry for introducing compensation networks. The correct configuration of the for this experiment is in the 100 kΩ position.

Input and Output potentiometers 150H and 150K. These are rotary potentiometers used in experiments on position control. The 150H input potentiometer has ± 150° of motion while the 150K unit has no mechanical stops and consequently cannot be damaged by continuous rotation.

The input potentiometer generates the command signal. In this experience, the goal of the control system is for the output potentiometer to replicate the position of the input potentiometer as accurately as possible, without overshoots and without becoming unstable. The output potentiometer is connected to the low-speed shaft of the motor using the push-on coupling.

In a real control system, the input and output units will generally be different. For example, the output unit may be a parabolic antenna whose position is controlled by the input potentiometer that generates the control signal.

Pre-amplifier 150C This unit is used to provide the correct signals to drive the servo amplifiers. The two inputs can be summed, allowing two signals to be applied simultaneously, for example in the need of a reference voltage. A positive signal applied to either input causes the upper output (3) to go positive while the other output (4) will stay near zero. A negative input will cause the lower output (4) to go positive and the upper output (3) to stay near zero. This way, we can achieve bi-directional control drive.

A toggle switch allows use with either of two internal compensation networks on in the "normal" condition using neither. For this experience, the switch should be in the normal position.


The units in these experiences will be attached to the baseplate using their magnetic bases. Build the circuit in the figure below. You can use the different color wires to help you follow the connections in case you have to troubleshoot the system. Attach the octal connectors as:

- Connect the plug of the Servo Amplifier to the Power Supply octal socket. - Connect the plug of the Motor Unit to the Servo Amplifier octal socket.

Set the preamplifier switch to “normal” and the feedback of the operational amplifier to 100 kΩ.


Q1: Explain how this system works. Explain the signal path.

Q2. Draw a functional block diagram. You can consider the preamplifier and the servo amplifier as “black boxes” without having to explain how they work.

Q3. Is this an open-loop or a closed-loop system? Why?

- The attenuator unit sets the gain of the system arbitrary units. Set the gain initially at zero. - Turn the power supply ON - Displace the input potentiometer 135º clockwise. - Slowly move the gain of the system until the output potentiometer reacts to the command input and starts moving. - As the output potentiometer nears the required angle, reduce the system gain so that the output potentiometer comes to rest nearly at the required point.

Q4. What happens as you try to get the output potentiometer rest at the required angle?

Q5. What happens if we cause a perturbation in the system, for example manually rotating the shaft of the motor?

Q6. Explain the advantages and drawbacks of this system.


With the power supply OFF and the system gain set to zero, build the system from the figure below. Make sure that the preamplifier switch is set to “normal” and the feedback for the Operational Amplifier set to 100 kΩ.

Q7: Explain how this new system works. Explain the signal path.

Q8. Draw a functional block diagram. You can consider the preamplifier and the servo amplifier as “black boxes” without having to explain how they work.

Q8. Is this an open-loop or a closed-loop system? Why? Compare your answer with your answers from Q3.

- Set the gain level of the system to zero. - Turn on the power supply. - Align the marks in the input and output potentiometers so they both rest at 0° - Set the input potentiometer to 135° either clockwise or counter clockwise. - Slowly increase the gain of the system until the output potentiometer reacts and starts moving. This is the minimum gain level that the system requires to operate.

Q9. Record the minimum level (marking on the attenuator plate) needed for the system to react. What’s the corresponding voltage?

In the following experiences we will measure the deadband for the system. The deadband is defined as the error between the desired response and the actual response for the system. Before anything else, ensure that both, input and output potentiometers are set at 0º

- Complete the following table. Set the gain control to level 1 and complete the following table. Start at 0º, move counterclockwise, return to 0º and proceed clockwise.

- If the changes in angle are too sudden, it is possible for the system to become unstable. If this happens, turn off the power supply until the motor stops moving. In any case do not let the current measured in the Power Supply unit to exceed 2A.

Level 1 Counterclockwise Clockwise

Input -150º - 135º -90º -45º 0º 45º 90º 135º 150º



- Repeat the measures now for a gain level of 2 and afterwards a gain level of 3 and complete the tables below.

Level 2 Counterclockwise Clockwise

Input -150º - 135º -90º -45º 0º 45º 90º 135º 150º



Level 3 Counterclockwise Clockwise

Input -150º - 135º -90º -45º 0º 45º 90º 135º 150º



Q10. What can you say about the relationship between the gain level of the system and the error between commanded input and response?

Q11. Describe how the system reacts differently between the three gain levels.

Q12. What happens if we try to repeat the measures for a higher level of gain, for example a gain level of 5?

The response of the system can be further evaluated by studying the error signal. This error signal is located at the output of the Operational Amplifier. In that circuit, Vout is the difference between a voltage proportional to the displacement of the input potentiometer and a voltage proportional to the displacement of the output potentiometer.

The goal of a control system correctly designed is to minimize the error signal taking into consideration the time it takes to carry out this task, possible overshoot and steady-state errors as we will study in the upcoming lectures.

In the next section we will measure two of the most important dynamic characteristics of a control system: its raising time (tr) and its percentage of overshoot.

- Set the gain of the system to zero. - Calibrate both input and output potentiometers to 0°. - Connect a digital oscilloscope to the cursor of the output potentiometer. This cursor is the terminal that is fed back as one of the inputs to the amplifier. This is the “position” feedback; the voltage at this terminal is proportional to the position of the output potentiometer. This is the signal that we will investigate in this section. - Set the gain of the system to level 1 - Change the input potentiometer to 90° while recording in the oscilloscope the voltage of the cursor at the output potentiometer. Because this is a single shot signal, not repetitive, you must configure the oscilloscope to capture this transient.

You should obtain a waveform similar to the figure below, although with different values for the time and amplitude axis.

- We define the rise time as the time that it takes for the output signal to raise from 10% of the final value to the first instance that reaches 90% of the final value. In the figure above, you have to measure the time at which the output signal reaches 0.1 V and the time at which the output signal reaches 0.9 V. The difference between these two times is called the rise time.

- We define the overshoot as:

− VV % Overshoot = max final x 100 V final

- Record these values (gain level = 1)

- Repeat the measurements for a gain level of 2 and a gain level of 3.

- Complete the following table:

Gain Level =1 Gain Level = 2 Gain Level = 3

Rise Time

% Overshoot

Q13. Comment on what happens to the rise time and the % of overshoot as the gain of the system increases.

- Record the waveform on the oscilloscope when the level gain of the system makes it unstable. What is the frequency of the oscillation?

Generate a laboratory report for this laboratory experience. Comment on the results that you obtained and how they relate to control system. Comment also on the experience itself.