H
Haude Daniel
- Jan 1, 1970
- 0
For the umpteenth time I'm thinking about a way of replacing horrenduously
expensive commercial piezo motor drives that we use with our STMs with something
cheap and simple.
The motors are driven in a slip-stick fashion using a waveform like the one
shown here:
http://www.nanoscience.de/group_r/members/dhaude/waveform_walker.png
(To reverse the direction of motion the polarity needs to be inverted).
The S-shaped part of the curve isn't terrible critical. What is
critical is the steep slope between the slow parts. With a 20n capacitive load,
this is almost 1A of current at 44V/µs
The obvious approach to this would be to follow a low-voltage waveform generator
with some big-ass HV amplifier. I've played around a bit with stuff along the
lines of Fig. 3.75 in AoE but never got it anywhere near the needed performance.
Besides, the thing needs to be short-circuit proof in both polarities. The split
supply is a requirement because I need ~400V swing but a maximum voltage of
~200V across the piezos.
So I thought if it weren't possible to exploit the fact that we know exactly
when the steep rise is supposed to happen, and to add a "yanker stage" to the
output of a slow and cheap HV amplifier like this:
+250V supply----+--V.Reg.------+---+ +220V
| | |
| D 1uF
| +-G |
| | S GND
| | |
+------+ +----+---+ | |
|wavefm| | | | |
|gen. +---------+ HV amp +---R1-|---+---R2----+
+---+--+ | | 1k | | 100 |
| +----+---+ | | |
| | -+ | CLoad
trig _____|_ Gate__/ | 1n..20n
| | | Drv. \ D |
+--L.Shift--+ | ---G GND
| S
| |
-250V supply-----+--V.Reg.------+--1uF--GND
-220V
The slow amp needs to be isolated from the yanker in some way, otherwise the
yanker would probably upset the feedback loop. For this I added R1.
The idea is that once the rising slope comes along, the top MOSFET essentially
shorts the output against some power rail whose voltage is adjusted to the peak
of the amplified waveform. The FET is kept on for some time until the slow amp
has caught up and then turned off, so that the HV amp can take over for the slow
part. A mismatch between the yanker supply voltage and the amp peak output of a
few volts wouldn't matter.
There could be intrinsic short-circuit protection by foldback-limiting the
yanker supplies to the few mA of average current that this thing actually needs.
In case of a short circuit the voltage of the 1uF storage caps would collapse,
and the HV amp would protect itself.
Before I start thinking about actually implementing this beast I'd like to hear
if anybody thinks that there's anything fundamentally wrong with it, and if
there are better ways to solve the problem.
And an adjustable amplitude would be great as well... nah, let's leave it.
--Daniel
expensive commercial piezo motor drives that we use with our STMs with something
cheap and simple.
The motors are driven in a slip-stick fashion using a waveform like the one
shown here:
http://www.nanoscience.de/group_r/members/dhaude/waveform_walker.png
(To reverse the direction of motion the polarity needs to be inverted).
The S-shaped part of the curve isn't terrible critical. What is
critical is the steep slope between the slow parts. With a 20n capacitive load,
this is almost 1A of current at 44V/µs
The obvious approach to this would be to follow a low-voltage waveform generator
with some big-ass HV amplifier. I've played around a bit with stuff along the
lines of Fig. 3.75 in AoE but never got it anywhere near the needed performance.
Besides, the thing needs to be short-circuit proof in both polarities. The split
supply is a requirement because I need ~400V swing but a maximum voltage of
~200V across the piezos.
So I thought if it weren't possible to exploit the fact that we know exactly
when the steep rise is supposed to happen, and to add a "yanker stage" to the
output of a slow and cheap HV amplifier like this:
+250V supply----+--V.Reg.------+---+ +220V
| | |
| D 1uF
| +-G |
| | S GND
| | |
+------+ +----+---+ | |
|wavefm| | | | |
|gen. +---------+ HV amp +---R1-|---+---R2----+
+---+--+ | | 1k | | 100 |
| +----+---+ | | |
| | -+ | CLoad
trig _____|_ Gate__/ | 1n..20n
| | | Drv. \ D |
+--L.Shift--+ | ---G GND
| S
| |
-250V supply-----+--V.Reg.------+--1uF--GND
-220V
The slow amp needs to be isolated from the yanker in some way, otherwise the
yanker would probably upset the feedback loop. For this I added R1.
The idea is that once the rising slope comes along, the top MOSFET essentially
shorts the output against some power rail whose voltage is adjusted to the peak
of the amplified waveform. The FET is kept on for some time until the slow amp
has caught up and then turned off, so that the HV amp can take over for the slow
part. A mismatch between the yanker supply voltage and the amp peak output of a
few volts wouldn't matter.
There could be intrinsic short-circuit protection by foldback-limiting the
yanker supplies to the few mA of average current that this thing actually needs.
In case of a short circuit the voltage of the 1uF storage caps would collapse,
and the HV amp would protect itself.
Before I start thinking about actually implementing this beast I'd like to hear
if anybody thinks that there's anything fundamentally wrong with it, and if
there are better ways to solve the problem.
And an adjustable amplitude would be great as well... nah, let's leave it.
--Daniel
