3 Point Speed Curves Feature
Contents
Overview
In most applications of acceleration and deceleration the rate at which a locomotive increases its speed forms a curve where the early speed steps do not increase the speed as rapidly as later speed steps. For most applications a simple 3-Point Speed Curve will be more than adequate to create the prototypical movement of most locomotives. To this end, this Speed Curve employs 3 CV’s, each defining the rate of speed of a given range of speed steps. Using the Start Volts, Mid Volts, and Top Volts, the speed steps from 1-128 are divided into three equal sections. Adjusting the values in the corresponding CV’s shifts the standard speed curve up or down (within the specific range) thereby creating an entirely new rate of speed from each speed step to the next.
Adjustment
To enable 3 Point Speed Curves, CV29 Bit 4 must be cleared. This is the default configuration in TCS decoders. To begin using and setting up the 3 Point Speed Curve, simply program values into CV 2 (Start Volts), CV 6 (Mid Volts), and CV 5 (Top Volts). The Speed Curve is defined in CV’s 2, 6, and 5 with each CV corresponding to approximately 1/3 of the speed range. Values may range from 1-255 and every value of 18 added to one of the Speed Curve CV’s adds (approximately) 1 Volt to the motor speed at the CV being adjusted.
NOTE: The 3-Point Speed Curve does not allow for parabolic speed curves. That is to say each consecutive CV in the curve must have a value higher than, or equal to, the CV before it on the curve. CV 5 must not be less than CV6, and CV 2 must not be greater than CV6 when CV6 is also in use. See below for the exact limitations on each CV.
NOTE: Note: CV2 may be employed independently of CV5 and CV6, and is always able to be used regardless of the configuration of CV29.
| |||||||||
NMRA CV 2 - Vstart (Start Volts)
CV 2 Adds an offset voltage to the motor so that the locomotive starts moving at a faster speed. This is useful for speed matching or for locomotives without BEMF. this setting is also useful for inefficient electric motors which have require a higher voltage to overcome "stall current." Some motors may lurch or repeatedly stop and start if the start volts are too low. Increasing the start volts can improve performance by increasing the low-end power. Vstart is also referred to as "boost" by some manufacturers.
CV2 is used in conjunction with CV6 and CV5 as part of the 3 Point Speed Curves feature
Excerpt from S-9.2.2 of the NMRA Standards and Recommended Practices:
"Vstart is used to define the voltage drive level used as the start voltage on the motor. The voltage drive levels shall correspond linearly to the voltage applied to the motor at speed step one, as a fraction of available rectified supply voltage. When the voltage drive level is equal to zero, there shall be zero voltage applied to the motor. When it is at maximum (255), the full available rectified voltage shall be applied."
| |||||||||
NMRA CV 5 - Vhigh (Top Volts)
CV 5 is used to specify the motor voltage level at maximum speed. This value is a fraction of available voltage. When CV 5 equals 255 or 0, the full available voltage will be applied at full speed, a value of 128 will apply half of the available voltage at full speed, and so on. Changes to CV5 will automatically adjust the entire speed curve to linearly scale from 0 speed to the new full speed unless otherwise affected by CV 2 and/or CV 6 .
CV 5 should be the first CV programmed when setting up "speed limits" in locomotives. In a TCS decoder, CV 2 and/or CV 6 do not need to be modified before modifying CV 5; however, if you have already set values in CV 2 and CV 6, this must be taken into account first before programming CV 5.
NOTE: CV 5 will not be "in use" as part of the speed curve calculation if programmed with a value of 0 or 1.
CV5 is used in conjunction with CV 2 and CV 6 as part of the 3 Point Speed Curves feature.
From S-9.2.2 of the NMRA Standards and Recommended Practices:
"Vhigh is used to specify the motor voltage level at maximum speed. This value is a fraction of available voltage. When CV 5 equals 255, the full available voltage will be applied. Values of 0, 1, or 255 indicate that Vhigh is not used in the calculation of the speed table. Vhigh requires no other CV programming to be activated."
| |||||||||
NMRA CV 6 - Vmid (Mid Volts)
CV 6 specifies the voltage drive level at the middle speed step (63/126). CV 6 is used to generate a performance curve in the decoder that translate speed step values into motor voltage drive levels, and is specified as a fraction of available rectified supply voltage. CV 6 is used to "slide" the curve in one direction or another, where the value in CV 6 will adjust the curve for greater or less sensitivity in the later half of the speed curve. CV 6 should be left unmodified when setting up "speed limits" in decoders unless you are unsatisfied with the default speed curve calculation.
NOTE: CV6 will not be "in use" as part of the speed curve calculation if programmed with a value of 0 or 128.
CV 6 is used in conjunction with CV 2 and CV 5 as part of the 3 Point Speed Curves feature.
From S-9.2.2 of the NMRA Standards and Recommended Practices:
"Vmid specifies the voltage drive level at the middle speed step. Vmid is used to generate a performance curve in the decoder that translate speed step values into motor voltage drive levels and is specified as a fraction of available rectified supply voltage. Values of 0 or 128 shall indicate that Vmid is not used in the calculation of the speed table."
