HBX CPU-1000 HVAC Controller
Version 1.38
© HBX Control Systems Inc. 2012
Page 19
Pulsed Modulated Injection Theory (PMI)
PMI uses all 3 (proportional, integral, derivative) functions of a PID Control algorithm to open and close 1 of 2
relays housed within the CPU-1000. The control can choose to use PMI on either a pump (PMIp) terminals 26, 27
& 28 or a valve (PMIv) terminals 15 & 16.
In the example of a pump, the PMI logic will determine how long the pump needs to be ON for and how long to
be OFF for to maintain adequate injection rates to stabilize the target temperature in the mixed (radiant) heat
emitter. In the case of pump injection, the pump is being held on (pulsed on) e.g. full speed, for a calculated period
of time and then held off (pulsed off) e.g. zero speed, for again a calculated period of time. The ON pulse and the
OFF pulse, time is continuously being monitored and compensated for by the Controls internal algorithm using PID
application theory.
* It is important to view this as a long on pulse or off pulse as opposed to a rapid series of pulses. The
duration of each pulse will likely be several seconds or even minutes before the beginning of a new cycle
length. It is also important to recognize that this control principle cycles the device on and off more frequently
than is customary. This has been taken into consideration and several safety features are built into the Control
to alleviate any potential adverse effects on the Control, the electrical environment, pump motors and valve coils.
In the case of PMIv or valve injection the theory is exactly the same. With regard to the slower and varying response time
of the valve the control PID will compensate and respond with timing changes accordingly.
HBX has tested this injection principle both in the lab and in extensive customer site locations with a variety of
manufacturers’ valves and pumps with exceptional control characteristics and no detrimental effects on either valve
motors/coils or pump motors etc.
The main advantages to employing this control strategy with mixing are:
i. Single Phase pumps up to 10 amps (1HP, 120VAC) can be used for injection
ii.
Extra Controls do not need to be wired into the main Control
iii. The option exists for future mechanical system upgrades or modifications without changing
your primary controller
iv. Greater injection control using valves
PMIp is not recommended with split-phase motors. Premature failure of the start winding
and/or centrifugal starting contacts may occur.
PMI - Theoretical Representation
0 min 1 min 2 mins 3 mins 4 mins 5 mins
On
120V
P I T P
O
V
A. Typical Short Cycle Runtime B. Typical Long Cycle Runtime
* In both examples, actual cycle time may be longer or shorter then shown
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