Introduction
During 1992 Photomec introduced the PIMOS control system as an option on its Integrated processors in order to improve electrical safety, provide more accurate machine control, make the machine easier to operate, make process data available to the operator, improve machine reliability and make the machine easier to maintain. At that time PLC’s were not widely used in film laboratories and our initial task was to ensure that operators would be comfortable with the new control system and its touch-screen control and display panel. In fact the first installations proved to be extremely popular with the operators and met everyones expectations of improved reliability and simplied maintenance. We soon made it standard equipment on our Integrated models and is now also used on our Compact and Modular machines as well as being at the very heart of the new ECN2 Minilab.
The PIMOS system is based on industrial standard programmable logic controller hardware using Photomec’s custom software.
But what is PIMOS and what can it offer you?
Automatic Control
Any control system can be divided into three constituent sections:
- Input
- Processing
- Output
Inputs
Various transducers that convert physical quantities into electrical signals provide input signals. Theses transducers may be simple push buttons, switches, thermostats or strain gauges, etc. They all transmit information about the quantity that is being measured. Depending on the transducers used, this information may be discontinuous on/off (binary) or a continuous (analog) representation of the input quantity.
Outputs
The control system must be able to alter certain key elements or quantities within the process, if it is to exercise control over the way that the process performs. This is achieved by using output devices such as pumps, motors, pistons, relays, etc. Which convert signals from the control system into other necessary quantities. A motor, for example, converts electrical signals into rotary motion. As with the input transducers, output devices can be simple on/off (binary) units or be continuously variable in operation between fully off and fully on (analog).
Programmable Logic Controllers (PLCs)
Introduction
The need for low-cost, versatile and easily commissioned controllers has resulted in the development of programmable control systems – standard units based on a hardware CPU and memory for the control of machines or processes. Originally designed as a replacement for the hard wired relay and timer logic to be found in traditional control panels, PLC’s have internal functions such as timers, counters and shift registers, making sophisticated control possible with even the smallest PLC.
A programmable controller operates by examining the input signals from the a process and carrying out logic instructions (which have been programmed into its memory) on these input signals, producing output signals to drive process equipment or machinery. Standard interfaces built in to the PLC’s allow them to be directly connected to the process actuators and transducers (e.g. pumps and valves) without the need for intermediate circuitry or relays.
Through using PLC’s it became possible to modify a control system without having to disconnect or re-route a single wire; it was necessary to change only the control program using a keypad or computer. Programmable controllers also require shorter installation and commissioning times as hardwired systems. Although PLC’s are similar to ‘conventional’ computers in terms of hardware technology, they have specific features suited to industrial control:
- Rugged, noise immune equipment
- Modular plug-in construction, allowing easy replacement/addition of units (e.g. input/output)
- Standard input/output connections and signal levels
- Easily understood programming language (e.g. ladder diagram or function chart)
- Ease of programming and reprogramming on-site
The PIMOS hardware generally consists of a powerful modular PLC together with a monochrome LCD touch screen.
The PLC monitors all aspects of the machine operation via switches, sensors, and operator intervention form the LCD touch screen. Based on the inputs to the system the PLC software controls various outputs to ensure precise and repeatable operation of the machine.
PIMOS Software Overview
The PIMOS software was designed and written by engineers at Photomec that had not only a thorough understanding of PLC’s but also of machine operator requirements together with needs of ease of maintenance and reliability.
All software for the PLC’s and for the LCD touch screens is written at Photomec and although there is a ‘standard’ version, most clients have special requirements so invariably the software is written bespoke for clients.
A small example of PLC programming code:
LD 253.15
MOV #0001 HR00
MOV #0350 DM0108
MOV #0350 DM0115
LD 000.00
OUT TR0
@INC DM0013
AND 000.06
@INC DM0030
CMP DM0030 #9999
AND 255.06
@INC DM0031
LD TR0
AND 000.05
OUT TR1
AND NOT 000.06
@INC DM0032
PIMOS Machine Functions and Descriptions
Machine Elevator Control
The elevator control is achieved using a DC motor controller with a low voltage feed back potentiometer, which controls the position of the elevators. The PLC receives inputs from micro switches to display and monitor elevator position and carryout any action required due to the incorrect elevator position (e.g. feed elevator at the top or bottom limit position.)
The control circuit is housed complete on a small PCB, which provides adjustment for elevator speed, ramp (very important for gentle handling of modern film emulsions), and overload current adjustment. The PCB has plug in connectors for ease of changing.
Machine Main Drive Control
A 3-phase AC motor provides the machine main drive. The motors used have mounted in-line with them a reduction gearbox, which is sealed from any form of additional lubrication for life. The 3-phase motor also has the advantage that it has no brushes and so the entire motor/gearbox combination requires no maintenance at all.
The main drive motor is controlled by a variable frequency industrial Inverter. The Inverter not only provides smooth quiet operation it also provides unsurpassed speed and current regulation. Ramp time, current overload, max speed is also adjustable. The connectors to the Inverter are plug in.
The speed command comes from the PLC in the form of a digital signal. The LCD touch screen indicates the true machine speed from a sensor mounted on to the machine output pacer.
Winch Motor Control
The winch motor is a 3 phase AC motor/gearbox combination and also has the advantage of being sealed for lift requiring no maintenance.
The Motor is, via contactors controlled by the PLC. The top and bottom limit positions are monitored by the PLC and also there are two infrared safety sensors which stop the motor should any obstruction break the light beams. The sensors look along the machine beams.
Dryer Temperature Control
A compact microprocessor-based controller carries out temperature control for the dryer. The controller has to it connected a Photomec Pt 100 standard sensing probe to measure the actual dryer temperature. The controller switches a solid state relay, which in turn switches the mains supply to the heater mounted in the dryer ducting.
The temperature controller is a true PID controller, which has its firmware parameters, programmed at Photomec. The controller has a ‘Pre-Tune’ and ‘Self-Tune’ facility, which provides accurate control to 0.1 degrees Celsius. Although the controller is independent to the PLC it does communicate with the PLC via a serial RS 485 link. This link allows the LCD screen to display the current temperature and allow changes to the controller’s set point also via the LCD screen.
Other Machine Functions
Film Break Detection
The PLC monitors the rotation of a roller mounted in the feed elevator. The sensor is a solid state proximity switch. If the pulses provided by the sensor stop for more than a given time period the PLC assumes a film break and carries out the necessary action and alerts the operator audibly and visually via the LCD screen.
Film End Detection
The PLC monitors the signal from a solid state sensor mounted in the film join box. The sensor is an optical InfraRed sensor, which has the advantage that is, does not have to come into contact with the film. If the signal from the sensor changes the PLC assumes that the film has finished and stops the input pacer and alerts the operator audibly and visually via the LCD screen. The sensor detects both 16/35mm film and 16/35mm leader.
Film Format Detection
The PLC monitors the signal from the two sets of sensors mounted in the feed elevator. The sensors determine whether the material is film or leader and whether it is 35mm or 16mm. This information is used to tell the machine operator what film is loaded on to the machine and also provides the information for the fully automatic replenishment system.
Wash and Rem Jet Water Detection
The PLC monitors the signal form the flow sensors mounted in to the machine plumbing. If whilst the machine is running the water supplies fail the PLC will audibly and visually inform the operator via the LCD screen. Water supplies are also controlled via solenoid valves controlled by the PLC.
Chemical Control Unit
The Chemical Control Unit is a stand alone unit but the PLC in the processing machine does monitor and control various functions of the control unit. A compact microprocessor-based controller controls all chemical solutions. The controller has to it connected a Photomec Pt 100 standard sensing probe to measure the actual solution temperature.
The controller switches a solid state relay, which in turn switches the mains supply to the heater mounted in the heat exchanger. The controller also controls a solenoid valve, which allows cold water to flow through the cooling coils to ensure accurate temperature control.
The temperature controller is a true PID controller, which has its firmware parameters, programmed at Photomec. The controller has a ‘Pre-Tune’ and ‘Self-Tune’ facility, which provides accurate control to 0.1 degrees Celsius. Although the controller is independent to the PLC it does communicate with the PLC via a serial RS 485 link. This link allows the LCD screen to display the current temperature and allow changes to the controller’s set point also via the LCD screen.
The circulation and agitation contactors have a signal fed through them, which is monitored by the PLC. If any contactor fails or overloads because of excess current being drawn by a pump motor the LCD screen indicates this and also informs the operator audibly. The PLC also switches the solenoid valves, which control the amount of replenishment that is fed into the chemical circulation systems. All chemical pumps and heaters are controlled by a fully automatic time switch, which can be programmed from the LCD touch screen. Agitation pressure switches can also provide warnings of pump failure of blocked spray bars causing reduced agitation.
Optional Features For PIMOS Controlled Machines
Modem Support
As an option machines can be supplied with a Modem which can be used for on-line technical support and software upgrades. The system operates with Photomec calling the remote machine and connecting it to a master computer at the factory.
Machine Parameter Printer
As an option machines can have a small 40-column printer supplied with them which can provide the following information:
- Machine Speed
- Date & Time
- Job footage/meterage
- Developer Temperature
