Design and Implementation of a Truly-Wireless Real-Time Sensor/Actuator Interface for Discrete Manufacturing Automation - Networked Embedded Systems - page 720

always having to wait  ms for each signal to arrive would mean  s of additional cycle
time already (a % increase in a fast roundtable application!).
- Stay within time limits given by the application (e.g., a critical mechanically controlled
movement) which otherwise could mean destruction of the machine, tools in the
machine, or at least the produced product.
• Very high reliability to get a signal within a defined upper time limit (e.g., to avoid the
above described destruction).

In contrast to other wireless use cases like mobile operators and service, the very high reliability to
get a signal within a defined time limit is an absolute requirement.
Practical values for the error rate of not getting the signal in time depend on the industrial use
case assumed. A typical round table application can have easily  IO points, mainly sensors, of
which  are here assumed to be wirelessly transmitted in the following Table .. A round table is a
typical arrangement in factory, where a stepwise rotating table is used to do many diferent assemblies,
testing or loading/unloading steps in parallel. The substations on the table have additional relative
movements, so wireless is an issue due to D movements. Typically most of the IO signals are digital
and the events are typically separated in time (unsynchronized), resulting in many telegrams, worst
caseequalingtwotimestheamountofIOperproductioncycle(seeTable.).
In principal, the distribution of wireless latencies will qualitatively look like in Figure . in prac-
tical implementations. Sketched is a coexistence situation with other frequency users. If the wireless

TABLE . Events in a Roundtable Production

ProductionSteppers
Event per Second

 wireless nodes or IO points a ≥  telegrams per second

Per hour ≥ ,  telegrams

Per day ( shifts) ≥  mio. telegrams

Per year (w/o weekends) ,  mio. telegrams

Note: Less than one failure can be tolerated per year, resulting in a needed
packet error rate of

≥

 −

 .

≪

a

DigitalIOassumed,concentratingIOnodeswouldhavethesame
amount of telegrams, as only rarely IO points are simultaneously
triggered.

Probability Wireless system with

Direct
Cyclic

1
10 -1
10 -2
10 -3
10 -4
10 -5
10 -6
10 -7
10 -8
10 -9
10 -10
10 -11

Real-time
limit

Transmission

Probabilities
for latencies
larger than limit

0 5 10

ms latency

FIGURE . Typical distribution of latency measurements in wireless automation devices.