ECP Model 220 Industrial Emulator
DESCRIPTION
The laboratory equipment used in this Lab is an ECP Model 220 Industrial Emulator which is a rotational motion control system designed for teaching purposes by Educational Control Products (ECP systems) company. The components of the system are shown schematically in Figure 5. The system is driven by a digital real-time controller with a digital computer (PC) as an interface. The ‘plant' is a rotational-motion assembly consisting of a motor-driven set of two turntables (called ‘disks' hereafter) which are coupled by means of selectable toothed belts and pulleys. Additional weights may be symmetrically added to the turntables thereby increasing their effective rotational inertia. Once the mechanical configuration (i.e. belts, pulleys, weights) of the system is selected, the ‘plant' constitutes of the motor/turntable assembly can be represented by the first-order rotational friction/inertia model described in experiment (1). The two turntables are provided with optical digital-encoders which measure their position, velocity and acceleration. The ‘control box' contains all the drive and measurement circuits required for actuating the system. A desktop PC, which contains a digital controller as well as ECP executive program, is used to complete the closed loop system. Controller and control-parameter selection, data entry, and output display selection are all made through the peripherals (keyboard/mouse/monitor) of the PC. Thus, for any given mechanical configuration, the remaining operations are entirely computer-menu-driven with a graphical-user-interface. Although the control signal is implemented through digital means ('sampled data'), it can physically emulate an analog system, which is the mode that is utilized for the purposes of this lab.
ROTATIONAL-MOTION ASSEMBLY
This unit basically consists of two belt-driven metal turntables (hereafter called ‘drive' and ‘load' disk) . The two disks are radially slotted, so that two or four symmetrically-spaced, brass weights may be attached in order to change (increase) their rotational inertia. In the lab setup, the positions of these weights are fixed at the maximum radius of each turntable . The two disks are coupled using toothed-belts and pulleys through an intermediate ‘speed-reduction' ( SR ) assembly. The SR unit has mechanisms to provide backlash, if required. Backlash (also known as “Dead Zone” or “ Deadband”) is a non-linearity which typically occurs in system transmission links such as gear trains and/or ‘rack & pinion' assemblies. The effect of backlash is seen as a short interval of inactivity (i.e. no output) whenever the direction of motion is reversed. It is a nonlinear effect. Since the experiments in this lab deal with ‘linear' systems, the backlash setting facility is not used in the lab setup.
Speed reduction ratios can be changed in the system by selecting interchangeable pulleys in the SR assembly. The equipment also permits a flexible belt to be used between the SR assembly and the ‘load' table, for advanced studies on “flexible-drive control systems”. If the flexible belt is not used and if the backlash set is ‘zero', then the rotational components are considered to form a rigid (as opposed to flexible) system. The drive disk is driven by a brushless DC drive motor through a 1:1 belt/pulley coupling. Another brushless DC motor, called the ‘disturbance motor', is coupled to the ‘load' disk through a 4:1 belt/pulley. Various disturbance signals such as step or sinusoidal (or other user-defined functions) can be introduced into the system through this motor. The disturbance motor also allows an equivalent viscous friction to be introduced at the load disk location.
The experiments described in this manual assume a rigid system with all inertias reflected to the drive disk axis at Encoder#1, with an overall reduction ratio of 4:1 (i.e. 4 revolutions of the drive disc at Encoder#1 result in 1 revolution of the load disk at Encoder#2). The encoder outputs are points of origin of feedback signals, so that a feedback loop may be ‘closed around' either of the encoders. In the lab experiments, the loop is closed around encoder #1 (at the drive disk), and all inertias and friction is ‘referred' to the drive-disk side of the rotational assembly. All the above components are mounted within a metal frame which can be oriented horizontally or vertically; however it is used in this lab with the rotational axes pointing in the vertical direction, so that gravitational torques will not be present.
The actual arrangement of the motional components is shown in Figure 3 .
Control Box
This unit contains the power supplies, servo amplifiers, interfaces, and other circuits needed to drive the two brushless DC motors and to process the encoder signals. It, also, contains the protection circuits which open the control loop under overload conditions. All circuits in the box communicate with a digital signal processor (DSP) located within the PC.
The front panel of the control box has two push button switches and a power indicator light. The black button is the ON switch and the red button is the ABORT ( OFF ) button.
Workstation
The PC system contains DSP hardware and software (i.e. the executive program labeled ECP32) through which the type of controller for the system can be specified. The DSP executes the control ‘laws' at sampling rates that are high enough to permit modeling of the system as either a discrete time or a continuous time system, although actual control is through discrete-time. Various controller configurations (see figure 6) can be implemented (either in continuous-time or in discrete-time) and the required parameters are set manually using the keyboard and mouse .
STARTUP SEQUENCE
The following sequence must be followed for startup of the equipment:
- Ensure that the protective plastic cover on the rotational motion assembly is in place and all mechanical parts are secure (not loose).
- Turn on the PC, login by the account given by TAs.
- Turn the control box ON by pressing the black button on it.
- On the desktop, click on ECP32 icon, ignore the pop-up windows for ‘User Account Control'.
SHUTDOWN S EQUENCE
The following sequence must be followed for shutdown. Failure to use this sequence may result in error messages such as “Controller not found ….”, etc at the next startup.
- First delete all your data files after you have recorded them on your USB key. Then click ‘Exit' from ECP ‘File' menu.
- From the ‘Start' menu, choose ‘Shut Down' . The entire setup including the control Box will be shut off when the computer shuts down.
If messages such as “Controller not found” or “ECP executive may be already running… continue anyway?” are seen, it is best to exit the program as above and start over.
Reference
[1] ECP Control Systems.
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