practicalities of digital control a survey. the overall system the individual controllers the...
Post on 20-Dec-2015
215 views
TRANSCRIPT
Practicalities of Digital Control
A Survey
The Overall System
• The individual controllers
• The interconnection
• The central computer or computers
• The transducers and actuators
The individual controllers
• P L C
• General-purpose controllers
• Purpose-built
More on the P L C
• Probably still ‘ladder logic’ -- good for on-off control but cumbersome for analogue --but ...
• Can incorporate analogue I/O
• Can often include p.i.d. controller blocks and routines in high-level languages
• Most modern PLC types can be interfaced to a SCADA system
Traditional example -- Mitsubishi F1/F2
• Basically ladder logic
• Can do analogue quantities, but awkward
• Good at on -- off control
• An analogue ladder example follows
• More modern networkable PLCs will be described in a later lecture
General-purpose Controllers
• Most common type is PID but other strategies possible
• Parameters can be changed/downloaded by/from SCADA system
• Self-tuning types becoming more popular but care is still needed in their use
• Less good at on-off than the PLC
• Better at ‘analogue’ control
DSP Possibility
• Can be based on DSP Chips
• Very fast micros optimised for multiply-and-add ... the sums of digital control
• Some can operate in floating-point
• Serial interface to I/O
How fast shall we sample ?
• Shannon/Nyquist Theorem -- we must sample at least twice the highest frequency present if we are not to lose information
• Actually we need to sample faster than this to avoid aliasing and because of noise
• 10 - 20 x highest frequency of interest is usual
Why ?
• Less than 10 x means it is difficult to produce an effective anti-aliasing filter
• More than 20 x leads to a double penalty ...
• We have to do the sums faster ...
• ... and more accurately if they are to work !
• But what is this aliasing thing ?
Suppose we sample this signal every 14 s
0 20 40 60 80 100-10
-5
0
5
10
x
x
x
x
x x
x
“I’ve got aliasing, doctor.”
• We have ‘found’ a sine wave of much lower frequency than the actual one.
• The system may be able to respond to the lower frequency one ...
• ... even if the original was too fast for it to respond to.
“I’ll give you a prescription for ...”
• Some effective screening (the high-frequency signal is likely to be the mains or Radio 1/2/3/4/5/etc)
• An analogue low-pass filter on the inputs
The anti-aliasing filter
• Has to be analogue (it would itself be at the risk of aliasing if it were digital !)
• It must not appreciably affect signals within the normal frequency range of the controller but it must effectively remove everything above half the sampling frequency.
• The faster we sample, the easier it is to remove the aliasing signals.
A Sampling-rate Example
• Controller 0.1s + 1 +2/s
• We have already digitised with a sampling interval of 0.05 s
• We will see what happens with 0.25 s ...
• ... and 0.01 s.
• Using the simple substitution.
We obtain with Ts = 0.25 s ...
• 1.9 - 1.8z-1 + 0.4z-2
• -----------------------
• 1 - z-1
• This one causes very serious degradation of performance -- if not actual instability
What is happening ?
• The problem is that by sampling we are producing a Transport lag
• We remember from Analogue Control that a Transport Lag is a pure time delay ..
• ... and that it reduces system stability by increasing the phase lag in the loop.
• We introduce one by sampling ...
What is happening -- Continued
• ... because an event happening during a sampling interval is only detected at the next sampling instant.
• So the delay in detecting it can be anything between zero and a full sampling interval ..
• .. so it is Ts/2 on average.
• This is the effective extra transport lag introduced by sampling.
...So let us use Ts = 0.02 s ...
• 11.02 - 21z-1 + 10z-2
• ---------------------------
• 1 - z-1
• If we do not do the sums very accurately, we entirely lose the integral term !
Ts = 0.02 s .. A Consequence
• We will lose the integral term entirely if we use 8-bit arithmetic.
• I will do the sum ...
• ... in which we are only allowed integer numbers between 0 and 255 (or probably between -128 and +127 in practice)
Interconnection
• Multi-line bus (VME etc)
• Parallel or serial
• Two-wire (FIELDBUS etc)
• Systems often combine hardware and software
Analogue Interfaces
• Voltage ranges (often 0 -> 10 V or -10 -> 10 V)
• Current loop (usually 4-20 mA)
• So e.g. for 8-bit, 4 mA converts to 0 and 20 mA converts to 25510
• What would the values be for 12 and 16 bits ?
Arithmetic
• Now normally floating-point within the controller
• Fixed-point arithmetic is still used in some low-cost (often mass-produced) equipment
• It saves hardware cost but incurs extra development time
• Input and output are still fixed-point
Precision
• 8-bit I/O restricts us to 0-255 decimal
• 12-bit often used in ‘good’ systems
Supervisory Control -- SCADA
• Central computer (or network) connected to local controllers, PLCs and data loggers
• Data recording as well as control -- often now with an economic process optimisation overtone
• Central control of parameters and setpoints but the local controllers and PLCs do the actual controlling
SCADA Continued
• Often able to do statistical analysis on the data collected
• Especially ‘trending’ to see if quantities are changing when they should be constant (or vice versa)
SCADA Continued
• Upmarket PCs often used now instead of minis/mainframes/workstations
• Examples follow ....
First -- just a PLC !
• Canal-lock control panel
• Controlling two sets of gates and ..
• ... two sets of paddles.
• Needs to detect gate position and water level on each side (done via pressure)
• Hydraulics to operate gates and paddles
Again not SCADA -- Disk Head Drive
• Linear motor plus drive electronics
• Must be fast, so DSP chip used
• Position feedback from format track pattern on disk
A Glassworks
• Central Computer -- high-spec PC (duplicated)
• “Hot End” -- GP Controller for zone temperatures and feed + PLC for batching
• “Cold End” -- PLC (mostly on-off)
• Transducers -- mostly of the “on-off” type apart from temperature
Transducers
• Analogue then A - D ...
• or...
• ... direct to digital
Example -- Position or Angle
• We can use a potentiometer or LVDT
• to give a voltage dependent on the position or angle to be measured
• then digitise it
Position or Angle -- Continued
• Or we can use a Gray-coded disc or strip to give a digital reading directly.
Precision
• The control is only as accurate as our measurement of the quantity being controlled
• Our transducer must be accurate enough to fulfil the specification
Timing
• Interrupts
• Real-Time Clock
• Watchdog Timer
Real-Time
• Operating System or Language ?
• Hierarchy of interrupts
• Solves the sampling-interval problem
• May need an arrangement for immediate action in the event of problems during an interval
• Local or central ?