© Arun Microelectronics Ltd 2015
3.6 Stepping rate resolution
The stepping rate is derived from a crystal-controlled clock frequency of 1.8432 MHz. The period of any given step
is generated by dividing the clock frequency by an integer value. From this it follows that the resolution of stepping
rates is better at lower speeds. The average error in the range 100 - 6000 is 0.08% and does not exceed 0.3%
Slow speeds below 10Hz are normally programmed as a single step command and a delay command within a program
loop which is executed once per step. The period of execution of a single step is the reciprocal of the starting
speed as defined in the acceleration command ( X ). To this must be added the time parameter defined in the hold
command ( h ) and the programmed delay to obtain the total period of the step. The SMD210's internal execution
delays are insignificant and the overall frequency accuracy obtained is close to that of the crystal.
3.7 Step division (Mini-stepping)
To control the motor at low frequencies, steps are divided into several smaller steps. The only reason for using
step division with vacuum compatible motors is the control of low-frequency resonances and reduction of vibration.
These application matters are discussed in section 4, below.
The table below gives the default settings.
Step Rate (sec
-1
) Subdivisions
500 1
200 - 499 2
100 - 199 4
5 - 99 8
The Mini-step command, M, can be used to change the transition frequencies. The default values are restored
every time the instrument is switched 'on' or is switched to manual control.
The current parameters may be examined with the V5 command. The parameters returned are those of the last M
command executed (which is not necessarily the last command sent to the SMD210), or the default parameters.
The SMD210 cannot maintain a motor in an intermediate position between complete step positions.
3:8 Use of the user inputs and limit switches
Connection of these signals to the SMD210 is via the Limits and User Connectors, described in appendix A.
All of the User and End of Travel inputs have bias resistors which define a "high" state when no external connection
is made to them. A contact closure between them and the "logic ground" connection on the "User" or "Limit"
connector to define the "low" state. The open-circuit voltage on these inputs is around +3.5 volts, the short-circuit
current is less than 1 milliamp, and less than 0.1 volt should appear across the switch when closed. Good-quality
gold-plated switches or NPN transistors with good saturation are required to achieve this. No voltages outside the
range 0 to +5v should be connected to these inputs, and there are none on the SMD210 Limits and User Connectors.
Switch "de-bouncing" is performed by the instrument, which results in a delay of 30 milliseconds between the switch
closing and the program taking notice. Several steps may have occurred between these events, even at moderate
speeds. If the switch can resolve a single step and this is desired then it should be approached at less than 20 steps
per second.
Cont.
(94 pages)
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