Electronic control systems in Great Britain.

IT HAS TAKEN a long time to achieve the transition from mechanical and electromechanical to strictly electronic controls in the heavy industrial and agricultural sectors of engineering. There have been three reasons.

First, the expense of appropriate electronic equipment was prohibitive for such work when it was first available.

Second, industrial manufacturers tended to follow long-established, traditional designs.

Third, a breakthrough had to be realised in relations between electromechanical and purely electronics engineers, as nether was familiar with the other's problems. Frequently they were just not interested.

It was only wen the larger electronics manufacturing organisations had virtually saturated other and easier markets that they began to take any serious interest n the heavy industrial field. Digital electronics

Advances in semiconductor technology, including the development of the microprocessor, have led to an increase in the use of digital, as opposed to analog, electronics for control applications. Sequential controllers are now invariably electronic in operation.

An example of this can be found in the Babcock-Bristol Series 4 system, designed to start-up and shut-down industrial control systems and to perform on-line monitoring.

Typically employed for the operation of motors, valves and pumps, the system checks the condition of each step in a programmed sequence before allowing the program to continue. In the Series 4, glass encapsulated reed switches are combined with solid state timing circuits.

The design and installation of automatic control systems for pumps has hitherto been a complicated and expensive operation requiring relays, alternative start units, auxiliary contacts, control and alarm breakers or fuses, energy limiting barriers and so on. However, it is now possible to ake use of a solid state electronic pump controller that dispenses with most of the traditional hardware. Automatic alarms

Acutec controllers, supplied by CT (London) Electronics Ltd., taken the form of small sealed boxes, with input and output terminal blocks on the rear and switches and indicators on the front panel. Inside the unit is a printed circuit board in which semiconductors have replaced intrinsically safe barriers.

In fluid transfer systems, these units accept siignals from float switches, seal leak sensors and other sources, and automatically nitiate appropriate trigger alarms. An override manual faciliity is provided.

There is no limit to the size or capacity of pumps that can be controlled in this way. Such devices can form part of an energy saving pressure and flow compensated hydraulic system for tractors and the like. Substantial energy saving is possible, together with fuel economy, as pumping excess flow under load and recirculating full fluid flow while idling are eliminated. The combination of electronic control and hydraulic power has made possible systems that can be interfaced directly to microprocessor controllers or, if necessary, small computers. It must be borne in mind that, regardless of the system's specific application, the electronics are required to control a limited number of hydraulic functions. These are force and pressure, speed and positioning.

Positioning control is the most common and it may be either linear or rotary in action. Applications of such equipment are found in combine harvesters, pumps and irrigation machinery, crop drying equipment and prime movers, such as tractors.

Hydraulic actuators have proved themselves superior to other forms in applications where power exceeds a value of about 15W. Such units can accept signals from various kinds of displacement transducers, both linera and rotary, normally of the variable diffrential transformer (LVDT, RVDT) type although, for rotary positioning, synchros and servo potentiometers are sometimes employed as alternatives.

Such feedback transducers are usually mounted directly to the hydraulic motors or cylinders affixed to the moving load. In all examples of position feedback, rotation of the input shaft of the motor or cylinder through a flexible coupling causes the spools or valves in the hydraulic unit to be displaced axially.

Control signals produced by electronic transducers are not normally of sufficient strength to control the directly, so translation is effected by electrohydraulic servo valves that may operate in one, two or three stages. Transducer needed

These valves must not only respond instantaneously to electronic start signals but must contribute to the elimination of oscillations resulting from overshooting of the load.

In all cases, a transducer is required that is capable of producing an electronic signal proportional to its position, relative to a reference. Similarly, controlling the actuator or motor that moves the load is in no way dependent upon the load's nature.

Output drive signals are employed by hydraulic actuators to provide operating power for the load. One method of achieving this is by the linear amplifer is intended for applications where precise and rapidly responding positioning is required. The low power pilot motor rotation is converted by the linear amplifier at a predetermined transmission ratio into a highly amplified output motion.

Until recently, control of such hydraulic power systems normally incorporated a tacho-regulated DC motor as the pilot motor, but this type of unit is fast giving way to the more sophisticated stepper motor. This can progress its rotor very accurately through fixed angles at a high rate of repetition.

The main reason for this is that input signals are now frequently in digital form. Moreover, the stepper motor's electronic drive sytem can be made ad versatile as the end user wishes.

Since the advent of rare earth magnets, usually made from smarium-cobalt, new ranges of economically priced, highly efficient stepper motors have arrived on the market. One of the most interesting is the dual range (P30/P50) manufactured by Portescap.

These are of the permanent magnet type and make use of an unusual rotor design that is essentially a thin disc constructed from multi-polar, axially magnetized magnets. Each magnet on the disc's perimeter takes up a polarity opposite to that of its immediate neighbours.

This design imparts a number of advantages, among which are high acceleration, low inertia and excellent power-to-weight ratio. The Escap P312 operates at 60 steps per revolution, with no significant decrease in pull-in torque up to operating speeds of 600 steps/sec. It measures only 1.25-in. in diamater by 0.75-in. long and is continously rated at 0.5 W output power.

The alternative family is based on the Escap P532, which is a more powerful unit that operates at up to 100 steps per revolution and provides a peak output power of 30 W. Both can be driven by standard, low-cost drive circuits.

A good example of a rotary switch actuator is the Velec unit, a single phase actuator for operating heavy duty rotary switcheswitches or valve assemblies. Available from Hornet Electrical, these units are offered as multi-position drives for four-, six- or eight-way operation (typically up to 250 A rating) or as two-position bistable units for 60 degree switching action, typically up to 600 A.

In its multi-position version, the actuator comprises a stepper motor with a built-in electronic drive, logic and DC control supply for interfacing directly with manually or electronically controlled systems.

While stepper motors hold the ascendancy for the time being, the eventual apperance of electronically controlled commutated motors currently under development may change the situation. The new type of motor follows the construction of a conventional AC motor, except for the use of a permanent magnet rotor, and of an electronic controller that makes the motor virtually programmable in operation. Variable speed operations

The application of variable speed drives is a specialized area, and each one tends to be customised to a specific requirement. Some systems call for a highly stable constant speed, while others require several drives to run simultaneously at the same or at slightly differeing speeds.

Yet other uses call for the motor to run to a predetermined program of different speeds, sometimes wth controlled acceleration or deceleration.

Both AC and DC motors can be employed in variable speed operations. While the AC variety is essentially a fixed speed device, variable speed can be provided by varying the voltage and/or the supply frequency.

The DC motors are amenable to control over a wide range of speeds. By making use of electronic control techniques, speed regulation can be easily maintained as the load varies, especially on motors of the shunt wound type. Series wound motors allow high torque to be maintained at low speeds, making them an attractive proposition for traction drives.

The high overall efficiency of DC variable speed drives is facilitated by the low voltage drop in the thyristor semiconductor device that is invariably incorporated. The thyristor is established as the principal solid state device used in phase converter circuits to control field or armature voltages applied to DC motors. Variation of armature voltage is achieved by controlling the firing angle of thyristors incorporated into rectifier bridges.

The type 550 DC thyristor controller, manufactured by SSD Ltd, is a four quadrant unit, capable of full power regenerative operation up to 400 hp and featuring a separate forced ventilation duct to allow for "dirty" air cooling. All of its control circuitry is housed in a separate enclosure and the use of circuit cards simplifies maintenance procedures and permits economy in stock holdings.

Adaptive control circuitry provides these controllers with exceptional performance characteristics, particularly at low torque settings. Digital firing of the thyristors permits operation in any location, given supplies within the range 45 to 65 Hz.

The Stardrive range of DC drives from Thorn EMI Automation covers thyristor controlled adjustable speed units up to 575 kW, both non-regenerative and fully regenerative. Units for single phase operation are available for powers up to 9.2 kW. The operational state of these drives is indicated on LED indicators that form part of a self-monitoring system.

One example of the modern AC motor controller, the VF085 manufactured by Renold Power Transmission Ltd, is constructed with interface electronic circuitry to provide automatic motor speed adjustments for temperature or flow rate sensitive systems. These fully isolated controllers are available for power outputs up to 1.5 kW and an optional interface is available for external reference signals. Digital feedback

Development engineers working with Simplatroll Ltd have devised a new system for controlling the speed holding factor of variable speed motors. Their new microprocessor based Speedlock unit, by making use of a digital feedback system, is unaffected by heat (for all practical purposes) and is claimed to overcome problems of drift. The unit locks on to a speed reference datum and will hold the set speed of a master and follower motor drive to within [plus-or-minus]0.05% indefinitely, from start-up.

The design is based on the use of a digital pulse or shaft encoder, which provides a pulse train whose repetition rate represents the speed of the motor. The internal circuits then compare the measured rate with a reference to produce an analog error signal at the output, directly controlling a four quadrant motor drive system.

Digital pulse encoders of the type employed can produce up to 5000 pulses/sec. They can also be obtained in linear versions.

Due to the growing popularity of distributed as against centralised control in industrial systems, sequential controllers, with their attendant timers and counters, are being replaced in many applications by efficient, stand alone electronic programmable controllers (PCs).

Small and economical PCs are useful in many different roles for either sequential or continuous operations. The user makes his choice of the type he requires after deciding how many input/output signals need to be interfaced, how much memory is needed, and whether future expansion of the system should be allowed for.

A PC can be defined as a digitally operated electronic system that employs a programmable memory for the internal storage of working instructions for implementing specific functions to control machines or processes.

One of the latest PCs to emerge in the United Kingdom is the TPC 9000, manufactured by Tempatron Ltd. This is a medium range unit capable of interfacing with up to 256 input/output channels. It can be operated from batteries or by mains electrical supplies.

It can be easily programmed, using a hand-held program loading device or a standard input keyboard terminal. Fibre optics

Electronically based control systems are now beginning to benefit from the concept of fibre optics as a communication medium. The use of fibre optic cable, which has lately become very much more cost effective, confers on the control system freedom from electromagnetic interference, while offering the advantage of intrinsic safety in harzardous areas.

Research into this and many other areas of the use of electronics based equipment continues. Work is proceeding, for instance, on a low cost data acquisition system, based on the use of microprocessors, which when installed on a tractor can be used to memorise data on the effects of speed and depth on the draught of various types of implements, including chisel and mouldboard ploughs and a field cultivator.

Electronically controlled robots are being provided with crude vision facilities with a view to their being used for crop picking, and there have recently been reports of laser control being applied to earthmovers.